Carbs rebuilt - odd behavior

Hi All,

I'm going to appologize in advance for the long post but I have a lot of questions and want to provide some info up front. I have checked other carb posts and these have given me some ideas but I'd still like to get your input; btw: Thanks for all the carb rebuilding guidelines and tips, they really helped.

I just finished a carb rebuild on a recently purchased 1982 XJ650RJ.

I got the bike started today but noted several oddities and would like to get your collective input on directions to go with troubleshooting.

History of the bike: excellent maintenance records up to 1992 when I believe it sat for a while and wasn't ridden. A couple people before me went thru the carbs but couldn't get the bike running right. VERY clean bike purchased at a good price because it wasn't running. My personal experience: much background with automotive carburation (Holley & Quadrajet) including circle track racing but I've never worked on bike carbs or CV type carbs at all. OK on to the bike...

Took it slow and got everything as meticulously clean as possible all passages clear and no grit or dirt left of any kind. I found alot wrong while going thru the carbs so my hopes were bouyed that I would be able to get the bike right again. Previous rebuilders had swapped the pilot and main jets on one carb, another carb had the main jet simply loose in the bowl, lots of dirt and debris in the hard to reach corners, carbs way out of sync (visually the butterflys were set dramatically different in each carb).

After carefull re-assembly and bench testing (slide clunk test, static sync by butterfly clearance, pilot screws 2.5 turns out etc.) I set the float levels while the carbs were in a bench fixture (a hugely great idea that saved a lot of time and trouble - thank you guys). put them back on the bike and fired it up today.

Here's where the oddities start;

The bike fired up immediately and my initial joy turned to some concern when I realized i had forgotten to set the choke (start enrichment) lever before starting. Outside temp was around 50F. I'm thinking the bike should not have started so readily from stone cold with no 'choke'.

I then slowly moved the 'choke' lever and as soon as the slack came out of the cable and the enrichment valves barely started to move, the bike wanted to die. I left the enrichment circuit closed and just let the bike warm up a bit.

After warming a bit I blipped the throttle and the bike would hesitate briefly on the first tug and onced reved would drop back to about 2000 where it would sit for about 3 or 4 seconds before again returning to idle (just over 1000 rpm)

I shut it off and let the bike sit for a while and returned about an hour later started the bike and it went immediately to 4000rpm w/ throttle closed. I didn't do anything for a moment to see if it would settle to idle on its own. Just when I though it wasn't going to drop back to idle (after about 15 to 20 seconds) and started to reach for the idle adjustment... on it's own it did drop down to it's previous idle speed.

In order to reach all of the tuning points, I had rigged up a (clean) lawn mower fuel tank with ball shutoff valve and then a fuel filter to supply the gas. After those two short sessions (couldn't have been more than 10mins each, mostly idling) the bike had almost sucked this tank dry!

Ran out of time to continue so tuning/troubleshooting will continue next weekend. I have a colortune but have not used it yet. I think I've got bigger problems than mixture fine tuning right now. I will also be building a sync tool this week (thanks Rick).

Here's where the questions start;

When I set the float level I was looking for 3mm +/- 1 from the carb body right at the back of the carb near where the bowl drain is. I know the carbs are tilted forward when mounted on the bike...should the float level be set with the clear tube held at the SIDE of the carb (halfway between the front and the rear) instead of at the back?

What are the symptoms of too high a float level?...too low? Just how sensitive is this carb to float level? Is it better to err on the low side rather than having the bowl too full?

A co-worker and former bike racer/mechanic mentioned that the enrichment valves on the Mikunis hes worked with were sealed by rubber seats and was surprised when I showed him internal pics of the Hitachis where the brass enrichment valve appears to seal to the alum carb body with no rubber seal (albeit with spring and linkage pressure)...am I missing some parts? do these actually seal with only brass valve to alum body, no rubber o-rings or seals? Do these usually seal very well? am I relying on just the pressure of those springs to prevent any unwanted fuel from flowing thru this circuit? Could this be a source of extra undesired fuel even if all the enrichment valves are clean and (appear to be) seated?

The bike acts like it is getting too much fuel (cold start - no 'choke', dies at the slightest movement of the enrichment valves, consuming too much fuel from tank), but I'm at a loss from where.

The only thing I didn't disassemble (which i now regret) were the slides/needles. Could someone before me have raised the needles and this has caused these symptoms. Do these needles have multiple grooves? Does the position of the needle even come into play at idle and low revs (say below 3000)?

Some of the symptoms could also be vacuum leak related (idle not settling immediately). Though once the bike is idling it does so fairly steadly (not much fluctuation). I bought shaft seals but did not install them yet because of warnings about carb alignment and factory loctite'd fastners and because i wanted to see if I could get the bike going without the extra surgery, I WILL be replacing all the vacuum port plugs this week.

Regarding carb sync...there are two tiny (fuel?) ports on the roof of the throttle bore and these ports will be 'uncovered' to varying degrees as the butterflys open and close. I'm not too clear on what these tiny ports do in this carb (they seem similar to the 'transfer slots' on Auto carbs that help with transition from idle to part throttle). I set all the carbs statically so that all the blades were at the same position...('drag on slip of paper' tip) and set so that as the throttle was opened by hand, each blade would uncover their repective ports at the same time. If I now hook up a vacuum sync tool and it requires that I change these relative blade positions by more than just a bit in order to achieve vacuum sync, doesn't that mean that these ports will now be exposed differently from cylinder to cylinder? Does vacuum at each manifold take precedence over how much of these ports are exposed? Sorry for all the questions, I guess I just want to know what's going on in there.

Thanks for any insight and suggestions you can supply. I really want to get this Seca running fine and put on some miles this summer. Thanks again.

Bob M.

 

1. Did you pull the Air Jets up-under the (maybe covered or not) diaphragm?

When you find the Butterflys so "Out-of-whack" by PO's and Experimenters ... you NEED to check the adjustments of the "Big Needles"

Pull the Diaphragm assys and Masking Tape them on to the edge of a Level ... right above the bubble. Get the bubble centered-up right and rig the Level to stay put. (Vice?)

Use a deadly-accurate measuring device to get all four needles synced-up. Mark the spot down inside the piston for an eyeball reference if you need to raise or lower them as you get further into tuning.

No O-rings in enrichment circuit. Thats Hi-toddy for ya!

Come out to 3 or 3-1/8 on the Pilot Screws. Most Colortuners (Me too) found -- "Don't get any better than this" performance in that area.

Lay a light smear of synthetic grease in the diaphragm assy mounting channel to act as an adhesive holding the rubber down for mounting and a sealant against a possible leak.

If you pulled those topside air jets and followed the book stickin' 'em back-in, they might be in backward ... causing - HEADACHE - until they're right .

You got to be close.
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The ports you mentioned are the bypass ports. These work with the pilot jet and choke to deliver fuel at idle. You were correct in setting the butterflies evenly. At a closed position the back port should be just visible when viewing from the intake side or same side as the throttle linkage. Trust the vacuum meter. If one cylinder is pulling more vacuum than the others then that butterfly needs to cover the port more to even things out. This is where valve clearances are important. If they are not right then you will compensating for improper valve clearances instead of tuning the carbs. A weak vacuum will pull less fuel through the ports and vise versa.
I would check my valve clearances if the bike has more than 25K miles on it and then colortune. The colortune should tell you something. Follow this with the vacuum sync and see how that improves things.
On another note, I had trouble understanding the carbs at first due to my experience with Holley's and Quadrajet's. Main thing to remember is that these carbs have no internal fuel pump. The vacuum running through the venturi is the only way to get fuel through the carbs.

 

Thanks Rick and Blue for the quick replies

Rick -

I did remove the air jets and was very carefull reinstalling due to warnings about the Haynes manual. larger jet (195) towards engine side, smaller jet (50) in the center, my carbs did have the covers and they were put back in place.

I will pull and check the length of the needles. Is there a 'factory' dimension? or groove number for retainer clip?

Will turn out the pilots to 3 turns and seal the diaphrams

BlueMaxim -

I will check valve clearances too thanks for the reminder on their possible effect on vacuum

 

I'm sure there's a default setting or original stock set-up; but the Haynes book deals with it as a "We don't go there" ... "See the dealer" ... "Special Tuning Equipment Needed" ...

I look for any obvious alterations or mods. Sometime the needle gets "Clipped-off" ... other times the Outside Diameter of one (or all) will be messed with ... it makes a big difference when you DO know what to do RIGHT to fine tune.

Hopefully, nothing is altered preventing you from dialing it in right.

 

Hi Sliver

I had a similar problem with hi idle/slow to settle after I had rebuilt my carbs ('82 XJ650RJ) recently. The problem I had was the throttle cable slowing down the return springs on the carbs. I took it off, ran some oil down the inside of it, slid back and forth a few times then refitted.

The way I found this out was, removed the throttle cable then lifted the throttle linkage and let go, it snapped shut, so pointed straight to the cable.

Just a thought, hope it helps.
Lea

 

As far as interchange of parts goes, is the power valve (emulsion tube) the same between all the hitachi carbs used in these models (650 & 750's)?