Confusing Fuel Leak and Intake Manifold O-Rings Size

[SlowOldGuy]

I rebuilt the carbs on my '85 over XMAS and decided to change the intake o-rings.  The old ones where smashed flat and appeared to be seeping a bit of fuel.  The rebuild was due to what appeared to be fuel residue dripping off of a few of the manifolds.  Just a random drip or two, nothing gushing.

I originally thought it was oil dripping down from the valve cover gasket, but it had a strong fuel smell.  When I pulled the carbs, they appeared to be coated with fuel.  I used the Marine Stabil which has a green dye in it (it's also supposed to be better for ethanoled fuels) and there was a green tint all over the carb throat and even above the diaphragms.  

A check of the fuel level showed it was approximately in spec below the float bowl gasket surface.  I wiped the fuel residue out of the throat and left the carb assembly hooked up to an aux fuel tank for a few days.  Sure enough, a fine coating of fuel formed once again in the throat of the carbs.  Since the fuel level is not that high, I'm having a hard time figuring out where this fuel residue is coming from.  It's like a fine coating of fuel, not a detectable leak.  The only conclusion I can come up with involves the tendancy of alcohol to "climb" (or wet) the wall of a container.  It's like the fuel is climbing up out of the needle jet.  

I replaced the needle seat o-rings and tweaked the floats down just a smidge and ran another test on them.  After a few days, the fuel residue reappeared, but it wasn't as bad.  Hopefully I've slowed it down, but it still concerns me.  I'm positive that it's not enough to fillup a cylinder, it's such a slow seep that it should evaporate.  However, it obviously accumulated enough in the intake to seep past the o-rings.

And this is another thing that has me kind of confused.  It's always run fine, starts right up, warms up quickly, idles fine and has excellent throttle response.  So, how can fuel leak past the intake o-rings and there not be a vacuum leak?  Seems like if fuel can seep past the o-ring, then air should  leak past even easier?   Unless this only occurs when the head is cold and it only take a little heat to seal back up.

Everything is back together and still running fine.  Still waiting for the weather to heat up a bit so I can clean it up and see if it still wants to seep any.

Anyway (and here's the payoff for anyone that's read this far and I'm sure Henry's either fallen asleep or gotten bored and wandered off by now), during this exercise I figured out that the manifold o-rings are a 40mm ID x 2mm cross section and I bought 50 of them from McMaster.  I have extras if anyone out there plans to replace them.  They are $1.50 each (as opposed to ~$4 each from Yamaha).

Thanks for playing,
DavidR.

[racerman_27410]

David,

I was experiencing a similar issue with my stock carbs.... seems enough fuel was creeping that i could start my FJ without even using the choke....the carb joints were wet with fuel from the bike just sitting and my fuel level was in spec also.......for a trial i got a fresh set of late model carbs from Andy, converted my FJ to a fuel pump and the issue seemed to be solved.   I have since rebuilt my stock gravity feed carbs with new emulsion tubes and new float needle valves.... have not had a chance to put them on and try it out.....maybe we can get to the bottom of the issue if my rebuild does not cure it.


Frank

[SlowOldGuy]

Frank,

Sounds like the same thing.  Creeping is probably the best description for what I'm seeing.  

I don't think it's a particularly bad or dangerous situation since it's just a micro-thin coating of fuel.  It's more annoying that anything.  And the engine runs fine.

It seems to coat everthing; throat, diaphragm and slide, slide cover, etc.  Almost like the fuel is evaporating and condensing all over the internal surfaces of the carb.  But no external overflow leaks (except the few drips I've noticed seeping past the intake o-rings).  I wonder if it's the alcohol in the fuel causing it?  

I've had more carb problems in the past 2 years than I've had in the past 20+ years.  It's got to be fuel composition related.  Theres no way fuel can "leak" to the top of the diaphragms.

If I can get the time, I'll pull the carbs on my '93 and see if they are experiencing the same thing.  Or, maybe I'll pull them off the FZ.  It's currently got at least one idle jet clogged, stupid crap fuel!

DavidR.

[racerman_27410]

i would be curious to see if the other carb sets were experiencing the similar symptoms.

i never did pull the late model carbs off to see if the joints were indeed still wet but the bike would no longer start up without the choke so i figured it was good enough until i could get all new parts for my stockers.

Calvin put some stabil in his FJ carbs and it made the biggest green mess i have ever seen.... he did leave them for a little over a year (while he was on the blood thinners) but everything inside his carbs was blocked solid with the green crud..... i think given the option now i believe i would have to advise draining the fuel system completely for any kind of long term storage.

they got the gold mine.... we got the shaft (gas)

Frank

[andyb]

Maybe we can convince mister R to put a post up in the reference section that details all the o-ring sizes used in the carbs and intake assembly?  I had them all at one point but lost them, and this is another good size to add to the list...

[SlowOldGuy]

But you already get a list of the o-ring sizes, they are included on the contents list of my kit.  :-)

Putting them in the files section is too much work for my tiny brain.  Feel free to copy and paste:

Needle seat o-rings.  Stock is 7mm ID by 1.5mm X-section, but I use a slightly larger 7.1mm ID by 1.6mm X-section

Fuel transfer tubes and overflow fittings:  6mm ID by 2mm X-sec

Idle Mixture Screws: 3mm ID by 1mm X-sec

Petcock seat: 5mm ID by 2mm X-sec

Intake Manifolds: 40mm ID by 2mm X-sec

All are available from McMaster-Carr

DavidR.

[LA Mike]

I rebuilt the carbs on my '85 over XMAS and decided to change the intake o-rings.  The old ones where smashed flat and appeared to be seeping a bit of fuel.

And this is another thing that has me kind of confused.  It's always run fine, starts right up, warms up quickly, idles fine and has excellent throttle response.  So, how can fuel leak past the intake o-rings and there not be a vacuum leak?  Seems like if fuel can seep past the o-ring, then air should  leak past even easier?  

Thanks for playing,
DavidR.

David & Frank,
Question:  if you have a vacuum leak past these O rings will it cause your RPM's to climb and then not you able to adjust them down. My brothers '93 rpm's climbs w/o the choke and we can't dial them down. I did not see a leaking fuel as you have describe

Mike

[SlowOldGuy]

Hey Mike,
That's a definite possibility.  A rise in speed is caused either by an air leak, sticking throttle linkage, not enough slack in the throttle cable, or a binding cable.  Have you tried spraying carb cleaner or WD-40 while it's running and see if that affects the idle speed?

Send me your address and I'll send you a set of the o-rings.  Couldn't hurt.

DavidR.