Assembling carbs

question for you guys, i’m assembling the carbs and i was wondering
A) how much light do you generally have shining through the butterfly when closed? I’ve got them all screwed on but some of them have a decent amount of line shining through
B) what are you tricks for holding the butterfly in place and getting the screws started while only having 2 hands haha i got them but damn there was some swearing.

 

forget the light, use the holes on the top of the carb throat. make the plates cover the holes the same, about half way and your good. now the idle knob should open/close them evenly.
if you hold the carbs throat up you should be able to balance the plates on the shaft, then a magnetic screwdriver should help

 

Halfway cover the holes?

 

This method beats using wires, paper clips or business card strips and is spot on.
Make sure plates cover the holes the same amount on each carb.
Halfwayish is about the "sweet spot" your looking.

 

The idle circuit hole. You will be adjusting the throttle plates by using the synch screws that are on the throttle linkages. Set #3 first (using the idle speed adjuster knob), then match the rest to it. (note that this is a different order than would be used for a vacuum synch).
Like so (the rearmost hole in the pic).

 

You can bench sync doing it this way in under a minute

 

I use the idle speed screw to:

set #3 at the back edge of the hole
Snap the linkage a few times
Adjust as needed

then set the other three to the back of the hole
Snap the linkage a few times
Adjust as needed

The back the idle speed screw out til #3 hole is half covered
Snap linkage a few times, adjust as needed
Fine adjust the remaing three to half
Snap the linkage a few times
Adjust if needed

Still only takes a minute or two to do it this way

 

hey guys,

I figured i would just keep this thread going rather than create a new one. carbs are back on, compeltely rebuilt. valves are in spec. i fired the bike up for the first time this afternoon. idled a little high tile it warmed up but then i had it idling without choke, turned it off and hooked up my vac sync tool to it. fired it up and BAM, rev'd straight to 5-6k RPM and sat there. just flying. messed around with idle screw, idle mixture screws, sync screws, nothing really had much effect on it. finally after about 30min of start/stop as i didn't wanna just leave it running at 6k i removed sync tool, put caps back on and figured i would start it again and see what it did. straight to 5-6k again. anybody have any ideas? intake boots looked good, EVERYTHING has been replaced in carbs, even throttle shaft screws. anybody have any ideas? i've sync'd quite a few sets of carbs in the past but this has got me stumped.

 

check to be sure you hooked the throttle cable to the correct location, not to the choke location.
make sure the throttle cable is not binding and has a little slack in it.
make sure the linkage is not hanging up on the cylinder head

make sure your vacuum hoses are tight and caps for port have clamps on them.

recheck carb to manifold connections for fully inserted to boots and that clamps are on correctly and tight.
do the same for airbox boots.

when you synced did you follow manual or start with carbs 3 and 4, then 1 and 2, then 2 to 3.

 

Verify that you put the airjets in correctly under the diaphragm.... large number toward the butterfly, small number toward the airbox

 

One item if you replaced butterfly seals , putting the butterfly plate can be slightly off ( don't ask me I know about this ) you may have to pull carbs and adjust position of plate ,recheck bench synch I like using 2 business cards cut in two, slide under plates adjust till there is a slight drag pulling on card this is usually very close .

 

never got as far as syncing it due to the rpm skyrocketing. cable is good. vac hoses were good, happened when connected to sync tool as well.

hmm interesting. it seemed to idle fairly normalish when warming up though.

yeah when i bench sync'd it it seemed good. if i have to pull them again i will make sure to check though

 

you can take a look at the sync setting with just the starter cranking a few seconds. not real close but you'll see a problem. might save you pulling the carbs again

 

Hi Guys,
I have same question as cg82 and imHo, the answer is still not clear. First start with the butterfly removed and peek up top, note 3 tiny holes( not 2)in vicinity of where top of butterfly contacts housing. #1 of these 3, ( maybe idle?) is 1/4" toward the Engine side and is not impacted by the butterfly position, as clearly shown in the pic in above thread. Cg82 and I, are asking about the other 2 holes, #2 & #3. These are 3 millimeters apart,and can seemingly be covered or contacted by the top of butterfly. So "our" question is
...when re-attaching the butterfly with my 2 NEW SSteel screws from Chacal(hoorah), should the top of the butterfly ( prior to synching) cover #2,#3? half of #2? all of#2 but not #3? blah blah
When "I" set the screws in the butterfly, the butterfly covered #2, and #3 not visible cause its behind or on other side of butterfly . My guess is, thats the pre-sync position. Look again at the pic above, you only see the #1 hole(irrelevant to the question)I will also add what do #2,#3 do? The butterfly almost seems to cover #2 hole exactly....leaving #3 to do some other function, since it's never closed or covered.
Thanks again for your help!

 

The rearmost hole (idle circuit) gets halfway covered by the throttle plate.
The forward hole in the photo is the enrichment curcuit outlet. There is no third hole on the HSC 32 carb that the throttle plate interacts with (and no #3 hole behind the throttle plate).

Inside your Carbs

 

The third hole (furthest from throttle plate) is regulated by the enrichment screw. It is something that should be checked when rebuilding the carbs to verify that a soft seat on the enrichment screws that the tip of the screw is visible and fully blocking the hole. Using the soft seat method damaged / corroded threads in the carb body can give a false positive that the enrichment screw has completely blocked the passage.

 

Oh...that third hole.
Mr. Dummy (me) checked his memory by picking up the wrong spare carb (a consequence of having more than one model of motorcycle in the garage).

The back two are both for the idle circuit, and having two helps to make a smooth transition from idle to main.

 

We are on the exact same track and I did soft seat the screws and of course they came through the small hole I would say a millimeter or two but that begs the second question if they're soft seated I'm assuming that the screw tip does protrude through the one or two millimeters and when I back it off two turns I'm assuming I'm not completely blocking off the passage. mine is an 81 xj750 should I be going more like 2 and 3/4 to 3 or should 2 and 1/2 be a good start? I've Been Around The Forum for a little while now but it's funny on YouTube it seems like everybody seems to think that if you wind that screw down that you're going to somehow break the tip and in my opinion the only way you could break the tip is if you had a small piece of sand stuck in the passage?
the tip is actually going to go through the hole and that whole screw is just going to hard seat itself and not go any further po sing little danger of breaking the tip unless you had something blocking the passage in my opinion.
As a sidebar, I would say the measuring of the fuel levels in the bowls is really difficult with fuel hose because it always wants to Kink as you're trying to make these hairpin turns with the fuel line so I'm going to come up with some contraption that will actually get that fuel line close to the two number two and number three bowls as those are almost impossible to get the tube depend up against , unless you guys have some particular kind of tubing that you used it a super flexible , the silicone tubing doesn't work that just disintegrates with the gasoline.
I am going to now bench sync with a guitar wire.

 

If the idle screw is tightened it will damage the seat that's cut into the carb, and that will alter the fuel metering (the metal gets distorted). You can also end up with the threads stripping out. The needle tip can crack and break off, but usually that's caused by dropping it on the floor (though ovetightening could cause a stress crack to form).

Don't tighten the needle. Soft seat only.

 

I'm aware, and was the "other poster" who made the smoothness comment (and also corrected my brainfart).
No need to send me pics. We have plenty that show what the insides are like,
Inside your Carbs

 

Ok. Well I actually got nervous when i saw the tip poke thru the carb, I was like," oh crap", I went beyond soft seating. But after reading I now see that was ok. I have to change the o-rings, the only thing that came out was the mixture screw and spring, so i will dig around for the washerand o-ring, maybe they get stuck in the seat over time.

 

The o-rings do get stuck easily. Take a sewing needle, heat it with a lighter, and bend the tip into a hook. You now have an o-ring retrieval tool.
Chacal has the correct o-rings in stock, and at a pretty good price if you can't find correct new ones locally.

 

What's the consensus for backing out mixture screws on stock XJ750 Seca... 2.5, 2.75 or 3 turns

 

2.75 is likely a better choice on any of the XJs but the standard is 2.5.

 

Guys, I put the carbs back on after the bench sync, (paperclip method( kudos to author)), and got the butterflies to "half" cover the front hole. I still cannot get over the nearly impossible task of re-installing the carbs. On my XJ750, it took me over an hour. First I confirm the air side boots are pulled out and sit in their boot groove, slide the carb in, so it's lined up(2 minutes), then the fun begins. I tilt the carbs downward, so upper lip of carb can catch inside upper lip of air boot (the boots get hung up on all 3 carb air intakes( r,l top). You need a non-sharp tool that has a 90 deg turn and long enough to release the boot edges from where they hang up(marv mystry on all boot mouths and carb lips(exh,int).
Lots of pulling and pushing,wiggling to finally get carb into air boot, then unhook edge of carb rack which hangs up on the engine side boots, push down and 'back', line up, then shove forward into engine side boots. Screw the braces down.

I tried the first time, pushing the air boots into airbox, slid carbs in and the carbs went onto the engine side much easier....but it was impossible to "pull" the air boots back out and into their boot groove. I wanted to avoid damaging these boots with prying tools, saw my folly and removed the carbs.

 

install carbs to motor first.
you can loosen the bolts on air box to get some play
then start with #3 airbox boot heat it in very hot water this will soften it enough to install in box and on carbs. next boot#2.

 

make one out of 1/8 music wire
grind a cheapo philips screwdriver so the end is round, push the air boots around with it
i push the boots into the airbox

 

I just use my fingers, but then my airbox boots are pretty soft still.

 

Yep, I did this to remove the carbs. But reversing the process doesn't work.

 

It works for me, and a lot of others. You just need practice.
I've gotten to where I can pull the airbox boots back out and into place with just my fingers.

 

It might also be an issue that my airbox boots are 30+ yrs old. If I bought new ones, the rubber shouls be more flexible no? Do you add any lubricant to boots?

 

Mine are the same age as yours. I don't lubriate the airbox boots. I just get them warm (unless it's summer).
New ones will be more flexible, for a while.

 

Roger that.
On other note, I bought the sync tool, will pull the two bolts and clean chamber with wire brush n Hoppes. I was reading if engine gets too hot, the o-rings can melt and the tool can get stuck in the chamber. Doesnt the sync have to be done with a warmed up engine though, with rpms at around 2500?

 

Helpful tip. Don't use the YICS blocking tool. Its not needed in order to get a good running synch.

 

Well let me say this I did buy a yics tool but mine is the rod with the O-rings and you attach a long screw to it to slide it in and there's probably a half inch of play once she said it in there and then supposedly you put the second nut on close it up and then run around forever with this yics tool just in there and you never take it out and in the event that you ever do you know you change the the O-rings then of course there's a tool that shows on the xjbikes website which has that rubber stopper looks like a cork and stops it is specific position is the other guy right in saying you can put the yics tool in or should I just clean the port out like I did already and close it back up

 

So you are saying don't bother with this $28 tool that I bought and just clean the port out put the nuts back on and go ahead with the sync.
I'm sure this is going to beg the question as to why we need to have a sink tool or why Yamaha Engineers think we need to have a sink tool to properly do a vacuum sink is that so should we all be agreeing in this format that's BS

 

The one reason I could see needing a sync tool would be in the event that you were only syncing carbs in pairs and wanted to isolate the other pair

 

it is the risk vs reward issue. we have had a few members break the port when removing bolts that have not been of the bike for decades.
tuning with the sync tool is not going to give you much in the way of performance. syncing with out the tool is syncing how your bike runs real world.

 

It's also a matter of perfection vs. good enough that you can't tell the difference.
Unlike Yamaha, we don't have to make sure the bike passes emissions testing (even Ye-Olde Corner Motorcycle Shop has to adhere to federal regulations regarding emissions, though that isn't enforced). Carb synchronization has the potential to cause a machine to fail even the loose emissions standards of the 80's.

The synch to get from using the tool is (to me) indistinguishable from the synch you get without it. Though there is a strong case for the potential need to use the tool on an engine that is experiencing a realtively broad difference in wear across any two cylinders.

I would not reccomend the blanking tool, since that is meant to permanantly disable the YICS system. The result being a tradeoff: slightly less effort when setting the carbs up, but a loss in fuel economy. It can be used to synch with, but is less handy for that purpose.

If you wih to synch with either tool you can, but the extra time and effort is not worthwhile IMO (several of us here share that opinion, but the collective has no definitive position and there are dozens of conversations about that dating to the start of the forum).

As a side-note: When I tore my engine down to swap cases I found that the YICS ports were very lightly covered in carbon, with no restrictions (it looked like what you'd see after putting smoke on a bullet mould to keep the sides from sticking). The engine had around 20,000 miles on it at the time. I did clean the ports since the head was off, and everything easily accessible, but I'd wager that cleaning would not be warranted that soon (or even regularly) unless the engine was burning oil (blowby getting from the breather into the airbox, and then getting sucked back into the YICS ports would cause more carbon to be deposited).

 

I only see three holes in that Barrel which is tough to see anyway one at each end very close to where the threads end for the bolts, and a hole in the middle. And if we don't use the tool at all basically all you're trying to do is make sure that the pressure on the vacuum gauges are all reading 20 or 21.

Got it!

 

My bolts came off with that familiar short crack, then uncrewd fine. I cleaned it with my gun cleaning kit, which is perfect for this job. Hoppes, a smell that brings back many warm memories of my Dad teaching me how to take care of my 16Gauge. We never bagged much but a few ducks, but he loved me enough to know, there is value teaching a man to fish and hunt, and I guess for us here, to Tune an engine!

 

I get no smoke from exhaust, and am happy for that, and with only 9600 miles, this bike just needs an owner that gives a hoot.

 

I think I'll bag the odd tool I bought and store in my extras box. I can't wait to crank it up, and sync it....I am assembling the mini gas supply tank...just need to finish the mini tank prep...

 

Money well spent. How are you planning to vent the new tank?

 

I keep refilling a 100ml syringe, without its top, connected to gas line. I got it idling at 1100 with choke off, but I was in over my head trying to sync it with a 4 bottle manometer. I couldnt remember how many turns I made, in or out,quarter, half... I am simplifying now, reduced my bottles to 2, and will do 2 at a time tomorrow, sync 2-3 first,then 1-2,then 3-4

 

nope
do 1/2, then 3/4, then 2/3

 

+1

Though since 3 is the master you can do 2 to 3, then 1 to 2 and 4 to 3, if the bench synch has 3 in the right baseline state.

 

i think if you do 2/3 first it gets messed up when you do 1/2, don't confuse me i can't walk and chew gum

 

SOS. Send help! FML! So i've taken these damn carbs off 3-4 times since I last checked in. To recap, when I fire it up after the bike warms up a little bit the RPM goes straight to 4500-5000. I replaced the intake gaskets as I thought those were suspect. I checked the float levels and had to adjust 2 a little. put them back on, and straight back up. I turned the pilot mixture screws all the way in just to see, still revved up. I backed the idle adjsutment screw all the way off, still revved up. I've replaced damn near every part on these carbs. I pulled the plugs and they were pretty damn black. This last winter i tore the engine apart and replaced the head gasket and got the valves in spec. WTF do I do here, I don't know what else to check. What else can make it do this. Does anybody know of somebody who may have a good set of carbs that I could borrow to bolt on and rule out the carbs? I need to find some way to rule something out, either the carbs, or the gaskets/air system/the engine.

Any suggestions?