NOT a typical Hitachi carb issue...?

Okay, been lurking the forum for the whole spring while building my XJ750S into a caferacer (I know, I didn't start the project, but I'm finishing it)

Anyways, I have gone through chacals carb posts with a fine comb and still can't get these stupid HSC33 carbs to work properly.
When I got the bike I immediately noted that these carbs are... let's say interesting and checked everything. "Luckily" for me the previous owner already did something to the bike since the carbs had 43 pilots and 112 main jets installed, but as far as I know it had been run like that prior disassembly. Unfortunately all the original plastics, including the airbox were missing.

Welp here comes the part most already guessed: pods.
Since there was no airbox and this was a cafe build the only option was to put pods onto the carb rack. Opted for foam pods instead of true metal pods since apparently Yamahas really dislike the metal net pods...

Since I wrongly assumed the jetting was stock (now know it doesn't match to anything chacal's info really) I threw on a 120 main jets and after wet setting the floats and syncing the carbs I needed to switch the pilot from 43 to 45 to get the mixture screw around the 2-3 rotation area. Now the idle is spot on and synced.

The issue is the main jets and middle area. I always assumed that there was gonna be some dead zone between the transfer from pilot to needle circuit, but no matter the jetting or shimming the needle there's an severe burple/rich zone at 3k rpm. After that it pulls well, but I'm down to 112 main jets again and it still feels extremely rich at WOT and middle area.

To further confuse me after test pulls the 1 and 3 spark plugs are completely sooted up BUT 2 and 4 seem good/a tad lean...how? What's happening? Can the YICS cause this or what am I missing?

 

Have you done a compression and valve check? Little things can add up.

 

Yup, compression is good and valve clearances within spec (although I did check these before running the engine after it had sat for a few years). At the same time I had all covers off and checked the internals. Also checked the cylinders with a borescope and everything seemed spotless (engine only ran for 50tkm)

Today I lowered the floats about an 1mm (wet level is now about 2-3mm) and I'm down to 110# mains and from cold it seemed to run extremely well, but still blubbers rich when warmed up, but it's frustatingly close and way better anyways.
I'm fairly confident the pilot circuit is spot on as the idle and 0-10% throttle is about perfect. I plan to further adjust the pilot circuit with a lambda sensor after I get the carbs otherwise set.

After the test run the plugs are the same: 1 and 3 sooted up while 2 and 4 remain almost more to the lean side, which makes no sense to me...

I also noted that it couldn't hurt to inprove the air flow into the carbs by tossing the uni filter collars and currently printing a velocity stack style mounting collars for the filters. Pulled the dimensions outta my behind but let's see...

 

Forgot to mention that the unifilter mounts I'm manufacturing will remove any harsh angles in front of the carb intakes and will provide a total of 6cm of stack style tubing of which half sits inside the foam filter.

In my head this should inprove the operation of the carbs and in theory might lean the mixture enough to suit the current jetting...? Easy test anyways.

 

Update:
Went down to 108# mains and tried the unifilters with and without the self made "laminar flow" tubes between the filters and carbs. Tried lowering the floats down to 5mm wet level, and did a test run between every change and basically it went worse.

What throws me off is the fact that this thing is supposedly close to it's stock jetting with pod filters and it's still running extremely rich ..what gives?

 

Aaand took the carbs to church with all steps this time and found out an issue with the enrichment circuit, which is basically leaking. Lapped the plungers and wells and plugged the extra breathers on there and made sure the plungers aren't leaking by sitting brake cleaner in the wells.

Slapped everything back on the bike and...it's still drowning in gas and randomly picking up revs at idle. Tried it with new sparkies and now change. Can't find any intake leaks, vacuum systems have all been deleted and even the idle is all over the place. Real freaking weird...

Maybe it's time to let this one sit for a while because I'm just stumped at this point.

 

Picking up revs at idle sounds like an intake air leak. Check that your head-to-carb boots arent cracked, & that the clamps aren't worn out & actually hold.

Once verified, do a full carb sync for a really smooth idle.

Also pull your plugs and verify they aren't fouled or running too lean.

https://turbododgeparts.com/content/faqs/how_to_read_a_spark_plug.html

Also, poor running with stock jetting and pod filters is what we expect. Stock jetting on pods will never run right.

Pod filters permit a different amount of airflow than the stock airbox does, and so, one must jet the bike to match the pods.

When I got my XS, the airbox was gone and someone installed pods with stock jetting, which ran like shiat. I went on ebay, got a used/correct airbox and now she runs awesome..

Pods and bobs and bling and all of that is about how the bike looks, or maybe how you look on the bike... Personally, I only really care about how it runs and these bikes all run best with the stock airbox.

 

Thx for the input but it was mostly unnecessary if you had read the previous posts...

Sure the original airbox would be nice, but not worth sourcing especially now when the whole bike is built to a specific look/style.
Someone else started this project and has done a bunch of misc. crap on top of losing most of the stock parts - including switching the jetting into "something" which I now realize is from a generic rebuild kit. Pilot was 43# and main 112# which I mistook as being stock and replaced them with 45 and 130 and have since come back down by steps until it's now 45 and 108 AND still running rich /.

All possible intakes have been ruled out with an overpressure smoke test and none found. Also visibly gone through everything.
That said I did found out the culprit for the wandering high idle rpm: it was the throttle axle seals. The outermost seals were already switched into double o-rings and all the rest were.those proper V-rings BUT I took another blunder by checking them and deeming them ok by visually checking them and determining they wete soft and tight on the outside seats...what I missed was that they were all almost 1-2mm loose on the axle itself. -> replaced all the rest with suitable fat o-rings and no more wandering idle + they synced great.

All this done the bike runs richer than ever. Apparently Hitachi carbs don't require as dramatic up-jetting than Mikunis and I now have way too large jet on the pilot at least. According to data I can find and crosscheck these euro spec XJ750's should have 40# as pilots and 104# mains on the outer two and 106# on the inner two cylinders.

After my holiday the trial and error continued...

 

For most of the rest of the world, a combo like that is stock for the 650cc carbs.

The HSC33 carbs (if that is truly what is on your bike) used much leaner (smaller) mains and pilots but the air jets were changed and the main jet needle was massively richened up (probably to compensate for the much leaner fuel jets). The emulsion tube was also different than the HSC32 series, as well as the vac piston spring (which probably contributed to a greater and/or quicker rate of rise of the vac pistons and hence the richer main jet needle...........).

It's actually unusual to have any of these HSC series carbs run rich in stock form, unless:

a) too large jets or incorrect main jet needle
b) mixture screw is turned far out from proper position
c) choke plunger valves are not fully seated, or, there is a flaw (gouge, etc.) in the choke plunger valve seat machined into the carb body.
d) fuel levels in the bowl set way too high
e) some combo of the above.
f) someone has drilled out fuel jets to a larger size or installed one of those "performance" jet kits (which typically also involve drilling the vac piston air bleed holes to a larger size). NOTE: aftermarket "performance" fuel jets may not be marked with the correct Hitachi "size" designations. Hitachi fuel jets (but not the air jets) are marked in millimeter sizes: for example, a #120 fuel jets should have the drilled hole diameter or 1.20mm, a #130 should be 1.30mm diameter, etc.......a 340 pilot jet would be 0.40mm diameter). Aftermarket mfgs. might inscribe "43" on a pilot fuel jets, but measurements show it to actual be a size #50 (for example).

TOO LEAN is the issue most commonly encountered.

Maybe so.....and certainly not part of your current rich-condition issue.....but I would strongly suggest that you eventually replace those o-rings with the correct v-seals.

 

Hey chacal, I truly appreciate the reply!

Yeah, this thing quickly turned from an easy project to an headache caused mainly by the carbs.

The bike is without a doubt an XJ750S euro model, with the HSC33 carb rack.
No idea about the real model year since my country designates model year by registration and it's registered 1986, but it still has the same color carbs. Fuel inlet is between the 2/3 carbs, breathers between 1/2 and 3/4 and the extra breathers at the mixture screw base on every carb, which makes me certain these are HSC33.

The air jets on each one are 225# and 70# which should be stock. Now I also replaced the fuel jets into 41# pilot and 108# main since my own research points to the Hitachi's not being THAT sensitive to pod filters.
It seems to be way better mixture wise on the 41# pilots as it's no longer eye stinging rich...

That said, the throttle axle seals were a miss. Throttle is sticking bad and keep experiencing random intake leaks, although can't pinpoint the location. At the moment I can't proceed with adjustmens since it's all over the place, one second it's choking and the next it's pulling 4k while sitting on it's own.
Only thing I am sure is that it's not leaking from the boots or YICS...

And yes the enrichment plungers are still causing issues by not seating correctly. No idea how to fix that issue other than blocking them off completely for now. I might need to abandon these Hitachi's and get something else to get this thing working properly...

 

Update on the current plan:

So I know I could have get the correct seals from chacal, but living where I do those seals won't get to me for this riding season so....

I did an inquiry run around my city and found a set of 8x12x3mm actual axle shaft seals to go. I plan to install these onto the carbs "wrong way around" to keep air out rather than fluids in. In my head these proper shaft seals can't be worse than the original "V-seals" right...?
These seals are a bit too fat with that 3mm thickness, but the excess is on the outside lip which will poke out of the carb body -> exacto knife will take care of that ~ 0.7mm excess that would be binding on the shaft spacers.

On top of that I wen't ahead and bought a new arsenal of lapping compounds and will take my time lapping the enrichment plunger seats on each carb + thoroughly test them after that they absolutely do not leak.

These will be the final steps I'm willing to take with these carbs and if the enrichment plungers still leak these carbs are scrap anyways...

 

This is now turning into a project thread, but I need to vent...

So 2 out of 4 carbs now done. Took my time with each one, completely dismantling everything and spending about half an hour per enrichment plunger of lapping. Tested the plungers with penetrating oil (the extra breather nipples on HSC33's came handy on this) and neither one I lapped had any kind of leaks even with air pressure. So the choke/enrichment circuit shouldn't cause issues after I get the other two worked on.

Also took my time to soak and clean everything again, just to be sure. Got the new shaft seals on the carbs, which did need some modifying since the seats on the carb bodies are actually 11.5mm and the seals outer diameter was for 12mm hole, but damn they should be tight now. Also polished the throttle axles and rounded off any sharp edges on them before installing them on the carbs with plenty of grease.

I have to say I'm pretty confident on these seals working like a charm and the axles now move with basically zero friction...

 

Update:

Got all of the carbs "refurbished" so to say. For some reason the 4# carb enrichment plunger was leaking really bad when I originally thought the 1# was the worst with the couged plunger seat. On 4# it took about four times the work to get it lapped and sealing properly but got it done.

Finally bench synced the carb rack and took the effort to reposition the throttle flaps so that they have the exact same opening on the lower edge + exact same amount of pilot orifice showing.

Now I need to find the motivation (and time) to get the rack back on the bike, synch them and get on the road to see if the needle and mains still need work...

 

Wet set float levels before putting them back on the bike.
Just to make certain all 4 floats are shutting off

 

I've done this for half a dozen times already, mainly because there's contradicting info on what the levels on this spec bike should be. Tried the 1mm wet level first which was just ridiculously rich and now I have them wet set at 5mm(+/-1mm).
Took dry measurements before I dismantled everything and verified that the levels were the same while rebuilding... But you may be right, I might wanna wet set them again - again just to be sure...

Luckily this is a cafe bike now so it's pretty straight forward taking the carb rack out and back in again. Done it so many times in the last month I lost count...

Speaking of the float valves: does anyone have concrete info if the HSC33 should have rubber tipped needle valves or brass? Because this thing has the rubber tipped ones and again, contradicting info if these are original or some BS aftermarket ones...

 

Hitachi float valve needles were originally rubber-tipped; many (but not all) of the Mikuni carbs from that era used metal-tipped needles. Thru the aftermarket, you can purchase metal-tipped needles for the Hitachi HSC32 / 33 series carbs, and that's actually what is recommended for people who live in an area that has fuel (gasoline) with a high (10%-up) ethanol (alcohol) mixture as the available fuel (this is a very common mixture in North America). The reason for the use of metal-tipped needles is that they are far less prone to seizure within their seat, which is much more likely to occur with a rubber-tipped needle.....especially in the presence of alcohol (which will cause most types of rubber to swell a little or a lot, depending on the type of rubber used to make the tip).

So the metal-tipped needles greatly reduce the chances of fuel delivery issues due to a needle that gets seized in the closed position (no fuel!) or seized in a partially or fully open position (flooding).

 

Thanks @chacal, that confirms that everything inside the carbs should be correct. Clearly the previous dude bought some kinda kit for the carbs since needlevalves, seats etc. were new/spotless. Shame that he didn't bother to replace the seals while at it, but it is what it is...
Fortunately we still have 5% ethanol gas still available and that's what I use on all my gas guzzlers for obvious reasons. (Daily drivers are all EV, so never need to buy any other type either)

Got some motivation finally to test the current setup and determined that I'm a dum-dum...

Screw me sideways but after installing the newly sealed carb rack on and getting the engine warmed up the erratic extremely lean condition came back...LUCKILY I had switched into clear gas tubing and realized that my forward tilted tank(to allow adjusting) was low on gas and the rear mounted petcock was running dry...filled it up and repositioned the tank more vertical to continue adjusting. /end of dum-dum

That said the idle is now THIS CLOSE to being spot on but still a little bit lean while the screws being at 4.5 rotations open, any more open and it just chokes out so guess that's the end of adjustment for these screws. No matter, they shouldn't be that far open anyways.

Just ordered 42 and 43 pilots and will test with the smaller first. Didn't bother to test drive it for the full throttle or middle area because of the limping idle and living in the city, but stationary/against the clutch it seemed to be pretty peppy and revvy.

 

Where are you ordering Hitachi Pilots, jets, etc? My xs850 has the HSC34 (running stock airbox and jets). MikesXS has carb kits for the HSC34, but I've never found a source for the individual jet sizes... anyone?

 

Well I was having a hard time since Hitachi jets are apparently non-existent nowadays...

Literally just happened to test some Dellorto 5mm jets, since they look the same. The thread pitch is not perfect as Dellorto has 0.75 pitch and Hitachi has 0.8 pitch...but they do fit without hassles and the jet sizes are the same.

 

While gathering motivation and rejetting the carbs, I decided to check the intake boots again - just to be sure. The boots I had never taken off because the bolts were extremely stuck. I tried the 4th boot upper bolt and it immediately snapped and I had to drill it out. Didn't touch the rest of them since...

Well decided to do this properly for once and got a smoke machine aaaand...check the first picture. ALL of the boots were leaking from the gasket except cyl 4 which I already replaced...
Glad I did it before trying the carbs back on though. Even more happier when 2 and 3 boots came undone without hassle. Of course cyl 1 boot BOTH bolts snapped no matter how delicately I tried to warm them up and drench them in penetrating oil...(Second pic)
Luckily they were on the first pot so easy to drill out...in theory until my extra hard Magnum bit snapped in the lower bolt...fun times let me tell you.

Ultimately got them out and all of the boot gaskets replaced. Boots themselves were original but surprisingly completely fine (we need the 80's rubber and rock back!)

Finally checked again with smoke and found no further leaks. Next throw the carbs back on and see how far off the jetting is now since there were hiding intake leaks again...

 

Very clever testing method and very typical results (including the sheared bolts). Keep us posted, great work.

 

Well smoke generator is a must have tool in the automotive world, so borrowed one from work. Really need to get my own in the carage...

YICS channels made the smoke test very straight forward as I only needed to feed the smoke into one intake and block the rest (ping-pong balls happen to be perfect blockers for most bike intakes).
Funny how all of the boots had basically a pinhole leak exactly at the center bottom of thw gaskets. And of course none of these reacted in any way to spraying with pilot while running...

We'll see how this thing ends up. Still quite baffled how I managed to do almost 140km of test runs and determine it was running damn rich with all these issues found since. Other than the leaky enrichment plungers nothing really explains that - it should have been extremely lean all through this.

I'm hoping that tomorrow is the day my family doesn't need me for anything and I'll get to take my time with the carbs and hopefully get this thing into proper driving condition.

 

Status update:

Got the carbs back on, got it running and... it's freaking running so lean that it just continuously pops from the exhaust. Now seriously what the...

I upped the pilots, managed to find an correct the remaining intake leaks and still it's ridiculously lean with the pilot screws 4,5 turns out on the #43 pilots. It is not pulling extra air through the carb rack or inlet boots. Now I'm seriously baffled about what is happening and lost all motivation again...

 

Again monologuing here, but after a small beer fueled thinking-over I still might have few things to check:

1. Cause of the conflicting data available the float wet levels might be too low. No real way to test this other than wet setting them again for the +1mm level (goddammit)
E: Yup this isn't it, just re-checked and wet levels are exactly 1mm while on the bike.

2. The open headers are causing part/most/all of this. Need to try with some temporary baffles and order some kind of mini mufflers (open pipes are honestly starting to annoy me too)

3. I have synced the carbs with proper manometer but I didn't block the YICS channel. What I found out online was that while generally the blocking tool is unnecessary it can cause that false lean floating rpm issue if you have bad luck... let's just assume this concerns me at this point.

4. And my personal favorite: all of the above

 

Okay, after another few hours I verified all the carbs wet levels being at 1mm on the center of the bowl while on the bike, this should be correct right?

Open headers were infact causing reversion, and the popping went away after I made baffle flutes on the collector outlets. Hoped restricting the open 38mm tips down to ~18mm would have also helped with the mixture but no dice. Still having the rpm hover...

Tried to re-sync the carbs with YICS blocked. Managed to find perfect diameter rubber hose that sat extremely tight in the channel when I screwed a 5mm threaded rod through it. No noticeable difference on manometer readings with YICS blocked, won't bother with that again...

Guess I need to fiddle with the jetting again.
What I can't comprehend is how in the world was this thing running/idling so ridiculously rich with all those previous air leaks and with #45 pilots and now starting over from #41 to 43 AFTER fixing several intake leaks it's still so lean?
Literally no difference with the #41 pilot with those small boot leaks compared to these #43 pilots, no leaks and baffled up exhaust. How?

 

Couple of thoughts:

a) does your carb set (assuming you have verified that they actually are HSC33's) have the 4 or 5 brass "nipple" ports coming off the side of the carb bodies, just to the left or the pilot mixture screw block. These would be facing rearward at a 45* angle, horizontal, tilted towards the rear of the bike? (on some models, the #2 carbie had one of those ports AND and up-towards-the-sky brass nipple port in the same general area). NOTE: these are not the bowl overflow t-fittings between the #1/2 and #3/4 carbs.

b) have you super-verified that the AIR JETS (living under the vacuum piston rubber diaphragm) are not reversed in their positions (they can be physically interchanged). The smaller "size" (engraved onto the top of the jet) is the MAIN air jet, and should be located in the "center" hole (i.e. directly above the main FUEL jet below it), while the larger "size" jet is the PILOT circuit air jet, and should be located in the forward-most of the 3 holes (towards the front of the bike). Reversing these 2 jets (or having wildly different sizing than stock) will cause the bike to run very rich.

c) have you verified the main jet NEEDLE size (and verify that the needle has not been disturbed within its small white plastic "head cap")? For your bike 1986 XJ750 air-cooled---perhaps a 41Y model?---I believe they should use the size Y-14 model needles (and I'm not sure of that, as some Euro bikes have used some very obscure sized jetting and needles to meet specific country emission requirements, and we do not have a comprehensive list of those variations). These needle "sizes" are (most of the time) inscribed onto the shaft of the needle, just below that head cap (note: on some models, Hitachi did away with the "Y-xx" inscription, and instead used a series (single, double, triple) circumferential engraved "rings" in the same position on the shaft....a really dumb things to do, in my opinion, but certainly easier to do from a manufacturing viewpoint).

Original main needle size configurations are as follows:

Y-10: used on all USA market XJ650 (except Turbo) models, 1982-84 XJ650 Canadian bikes, and 1982 XJ650 Euro (model 14R) carbs.

Y-11: used on all 1980-81 XJ650 Euro (model 4K0 and 4K1) and XJ650 Police (37G) carbs.

Y-12: used on all Canadian 1980-81 XJ650 Maxim, Midnight Maxim, and Canadian 1982 XJ650RJC Seca (non-yics) models.

Y-13: used on all USA market XJ750 (1981-83) models.

Y-14: used on all non-USA market XJ750 (1981-84, except XJ750RL and XJ750E-II) models and XJ750 Police (24L and 37H) models.

Y-17: used on all XJ750E-II models.

Y-18: used on all XJ750RL Seca models (1984) only.

Y-19: used on all XJ700 air-cooled models (Canada)

Y-20: used on all XJ700 air-cooled models (USA)

All of the following size needles are used on Hitachi HSC40 series carbs, and are much longer (and do not interchange with) the shorter HSC32/33 series needles above.

Y-22: 1981-82 XV920 RH/RJ models and 1981-83 XV750 models

Y-23: 1981-83 XV750 models

Y-24: 1982 XV920 J models

Y-25: 1982 XV920 J models

Y-29: all 1984-85 XV700 models, 1986 XV700SS on the #1 cylinder, and 1988 XV750 California models

Y-31: 1983 XV920 K models and 1983 XV920 MK models

Y-32: all 1986-87 XV700 models on the #1 cylinder

Y-33: all 1986-87 XV700 models on the #2 cylinder, 1983 XV920 K models, all 1984-85 XV1000 models, and all 1986-87 XV1100 models

Y-34: all 1984-85 XV1000 models

d) other than that, I'm out of ideas!

 

Truly appreciate the cents chacal.

a) Don't know how to further verify these being 33's other than measuring the throats. I have went with the thought that none of the 32's should have these extra features that these carbs have, like those extra nipples you mentioned. Check the pictures, every carb has that extra breather in the base of pilot mixture screw that I have capped off.

b) I have verified that the air jets are okay and they even correspond to your list of variants possible. As a super-duper verification I went and took a picture. 70# main air jet is in the middle and 225# pilot air jet in the front.

c) Now this is where it gets tricky. Previous owner said he had "replaced all the brass" in the carbs and when I tried to get more info he couldn't elaborate. When I first tore everything apart I was pretty convinced he only replaced both fuel jets and possibly the needlevalves, since everything else was caked/untouched especially in the top floor of the carbs.

I tell you this because I do believe the needles in every carb are stock, but what throws me off is that they're only marked as "Y403S", which gives me absolutely no reference to anything...
Any ideas on that part?

d) you an me both! I know my writing can be a little rambling, but honestly I do try and work this out logically in the real world. Have checked and tested many other things I haven't specially mentioned, but never had this much trouble with a carb rack ever...

 

Looks like 33s to me - they have the funky air vent at the top

 

The Y-403S (actuyally, I think it's Y-403Z according to my info) are reproduction needles that correspond to the stock Y-14 needles, we've found that the repro needles are very high quality/precision, so that isn't (shouldn't be) the issue.

The brass vac ports next to the pilot mixture screws.........do you leave those un-capped or use a rubber cap over them (like the rubber caps used on the intake manifold brass vac nipples)? If I recall correctly, you can have running issues if you leave those un-capped, while on other models you have to leave those un-capped and attach a long vac hose to each of them (about 16" long or so) that drape over the carbs and then hang down and are open to atmosphere at their far end. I believe they are part of some type of emissions control system on XJ700 models (here in the USA) but there is absolutely no usable/explanatory documentation about them in the factory manuals.

In North America....the only place that the HSC33-equipped XJ700 models were sold --- the carbs were jetted really, really lean from the factory (in order to meet stricter and stricter emissions requirements) and was partially mitigated by using a very skinny ("rich") main jet needle....but that "richness" only occurs at above-part-throttle conditions. They also used a different emulsion tube, a different vacuum piston unit, and a different vac piston spring (the long slinky one) than their little brother HSC32 models (and all those items will physically interchange between HSC32 and HSC33 carbs, but they are different....and by the way, don't ask me what the differences are or how to verify the differences, because besides the vac piston return springs, I can find no visible or measurable differences between the HSC32 and HSC33 versions, even though there must be some).

My guess is that Yanaha/Hitachi was having great difficulties around this era (1983-up) with meeting both emissions requirements and providing performance, which created a lot of interesting mix of jetting and other internal components within the carbs. I know that a lot of people (including myself) have experienced a lot of tuning difficulties with the HSC33 series carbs.....much like what you are experiencing. I think the solution that I came up with was to plug up / block-off those brass nipples by the mix screw ports, and only then could you get the engine to operate properly. But I think others have reported the reverse siutation, and if I recall correctly, one method works on the 1985 air-cooled XJ700 models but doesn't work on the 1986 models --- and although the carbs are "basically" the same externally, there may be internal air flow passage differences --- even though there's no internal jetting differences between 1985 and 1986 model carbs, that could play a factor.

And for extra fun, California model carbs had an additional vac port up there (only on the #2 carb body) which ran to a "charcoal cannister" emissions device, and which caught fuel vapors out of the tank and re-circulated those vapors back into the #2 carb (so that carb was always going to run a little richer than the others, especially on very hot or very low outside-air-pressure days, as those conditions will cause more fuel-in-tank evaporation----and of course also depended on the proper operation / condition of the YICS system, which "shares" fuel mixture conditions amongst all of the carbs).

Holy Mother of God, this was the pinnacle of carb design tweaking madness.........

And if that's not enough, on both years there are some differences between California and non-California carbs, since (at that time) California had much stricter emissions regulations than the other 49 US states.

The FUEL LEVEL specs were also dramatically changed, from 3mm +/- 1mm on HSC32's to 1mm +/- 1mm on the 33's, raising the fuel level in the bowls significantly (and higher fuel level = less vacuum suction needed to deliver an equivalent amount of fuel thru the jetting).

If you held a gun to my head and made me guess, I would theorize that those 4 brass ports atop the carbs are part of some air regulation system that affects the rate of rise and fall of the vacuum pistons, which in turn lifts the main jet needle......and recall, the Y-14 (Y-20 in the USA) needles are among the "richest" needles (once off-idle) of any of the HSC-series needles, so if the vac pistons (and main jet needles) are rising too much (even at idle) that would cause a rich situation (up to about 2000-rpms, as only the pilot circuit is active at those speeds, it's providing 95% of the fuel supply, and thus not influenced by the main jet needle size or position). HESC32 series carbs had 2 rather large-bore "air compensator" jets in the rear of the carb throat (facing the airbox) which were eliminated on the HSC33 carbs....there are just wide-open holes on the 33-series in those locations.

Perhaps those ports were also designed to restrict vacuum suction within the bowls, thus reducing the fuel draw at lower rpm's, and lowering emissions.......in that era, about the only way to win the emission game was to really lean out the engines at idle/off-idle speeds, since it's at those speeds where the EPA emissions testing was done.....hence, if you could lean out the engine dramatically at those engine speeds, then you passed the test, and if the engine got "dirty" at higher rpm's, who cares? So we'll design things to be ultra-lean at idle/off-idle and just richen the crap out of it once the main fuel circuit comes into play to regain performance and operational stability.

I mean, that's what I would have done......

Again, no documentation from Yamaha on the "air compensator" jets purpose, but they vent directly into the bowls, and I've always assumed that they just provide atmospheric air pressure to the bowls so a vacuum lock doesn't develop in there. On the HSC33's, those restriction jets disappear and all of a sudden those brass nipples appear on the top-side of the carbs (as well as the overflow ports between adjacent carb bodies). Something subtle-yet-major was going on for all of these changes to occur, and I'm not enough of a fluid dynamics / carb theoretician to understand exactly what they were from a "system-wide" perspective.

Try plugging off those big carb nipplea top-side and see if that helps. Or, if they are already plugged, remove the plugs and open them to atmosphere.

 

Yup, and all the jetting etc. matches on info about 33's + the missing jets at the throat makes me believe they're 100% 33's

That said, despite having a massive headache st the moment I ripped the rack off again since the throttle axle started sticking for some reason, probably because I mistakenly left the petcock open and flooded the carbs...

Well took the bowls open again and actually compared the "original" pilot jets I have drilled out to 45# that were causing massive richness and my aftermarket 41-43# jets and the orifices are visibly different sizes. I mean I can't see a difference between the undrilled after market ones, but compared to the drilled 45# there's a massive difference. As a test I drilled one to supposedly a #30 and even that is visibly larger than the 41 and 42 aftermarket ones.
All this leads me to believe that:
1. Drilling jets is BS (sherlock moment)
2. The aftermarket jet sizes are all over the place.

So tomorrow I'm going to get some larger sizes of these cheapo jets and try something I can eyeball between these empirically determined lean and rich jets. Seems like it's a crap shoot to get the sizes right with these kinda jets...

Anyways, if that doesn't work my thinking now is that I was way closer on the mixture with the drilled jets and may just have the floats too high up. That's the next step after trying some "middleman" aftermarket jets.

@chacal thanks for that very thorough input again.

The needles are in fact stamped Y403S, I have quadruple checked and it's even visible on that previous photo. No idea hot that differs from the Z needle...?

I have actually blocked those extra nipples with rubber caps and since I got the enrichment plungers corrected and seal up, it makes no difference if they're capped or not. With the leaky plungers capping them clearly richened up everything and vice versa. Cyl 1. was the worst and real easy to test on the fly...

The nipples actually are just a straight channel into the enrichment barrel holding the actual plunger and spring. I have determined their only use is/was to provide extra air to the enrichment circuit by engine vacuum only. I can't see any other way of operation for these since they don't have a way of connecting into anything else internally

 

Well, maybe, but I can't visualize why you would need to provide any air to the enrichment (I'm assuming you mean the "choke" / starter circuit) since that circuit has a pretty precise, fine-control mechanism already built into it (the rotary thumb knob on the LH handlebar control switch). And that circuit is a rather "coarse" design anyway, and since it's only used (in theory and practice) for just a very insignificant amount of time (compared to all of the other air and fuel circuits and components in the carbs), going to all that trouble in carb re-design for just that typically un-used circuit seems like "a lot of sound and fury signifying nothing" (that's a bit of Shakespeare for those keeping score at home........).

And, if I recall correctly, emissions testing was conducted (at that time) on an engine that was already warmed up, meaning the choke system wouldn't be contributing to any emissions at all, as a warmed engine doesn't require choke/enrichment usage.

But like I said, I may be all wrong about all of this........I'm just trying to think my way out of the box of emptiness (information-wise) that Yamaha left for all of us.

 

Yes---especially if one used once of the aftermarket "performance" kit that supplies different sized fuel jets, recommends drilling the vac piston ports, cutting the vac piston return spring, etc. Their jets may be engraved as (for example) a #43 pilot, but that "#43" size may not have any relation to an OEM Hitachi "#43" size (oem sizes are millimeter measurements of the fuel passage hole on Hitachi fuel jets...a #43 jet is 0.43mm diameter, a #36 jet is 0.36mm diameter, a #122 main fuel jet is 1.22mm diameter, etc.). Note that this applies to Hitachi fuel jets only; Hitachi air jets use a completely different "formula" for air jet "sizes".

And Mikuni fuel jet sizes bearing no relation to any physical size, so for example, a #35 Mikuni fuel jet is not 0.35mm diameter......unlike the much more logical (to me) size marking criteria used for Hitachi fuel jets (even though most people would agree that Mikuni carbs are much more of a "performance-designed" type carb than similar Hitachi carbs).

Hitachi fuel jets are "sized" in a much more user-friendly manner, while Mikuni fuel jets are "sized" according to engineering principles: