Float bowl gasket?

Sorry if this is a stupid question guys... its driving me nuts.

Mikuni BS34's with 130Mains, 3rd clip spot on needle, and 37.5Pilots, stock air box, straight pipe.

I've been having a stumbling and "air leak" mimicking problem.

The bike soesnt idle very good, and worse at 1/8 throttle. When I try to pull away slowly, the power is inconsistant and stammery. Once under way its OK, but not super powerful. I've chopped the plugs at an 1/8th, and Cyl. 1,3,4 are nice and tan at the top with a light carbon ring on the metal. The 2 plug is white and no carbon - so I'm starting there.

I just took apart the #2 carb to boil it, and I've found a complete seperation crack in the float bowl gasket.

Is this the magic air leak I was looking for, for so long?!?

If gasket gap not a big deal, then what next. Spark is good, compression 130 cold. Air screws 1 1/2 outish.

Thanks guys...

Always remember that a float bowl can not be sealed, it must have a vent open to the atmosphere so gas can flow in and fill it. A leak in the bowl gasket can't leak vacuum because there is no vacuum there, just normal air pressure.

The carb holders (rubber intake manifolds) are notorious for having leaks. You can also have leaks around the throttle shaft and various places.

The straight pipes are likely what is making it run lean.

bountyhunter wrote:

The carb holders (rubber intake manifolds) are notorious for having leaks. You can also have leaks around the throttle shaft and various places.

The straight pipes are likely what is making it run lean.

There is only the #2 Cyl. running lean. I replaced the manifold, and gooped it first too. I've also hosed down the entire intake system with wd40, and cant find anything.



These are CV carbs, are you sure about no vac. in bowl?

Thanx for reply.

Tek9, Bountyhunter is correct. regardless of configuration, the difference in pressure between float chamber and vacumn at drawing port/s is/are the fundimantals of jetting, fuel heights & jet diametres are simply metering devices. since #2 is obviously lean, examine. Look for an additonal spigot leading to intake throat, many vm's used a port between carb slide and intake spigot to privide vacumne for petcock. bikes w/AIR systems used multiple vacumn sourses.(usually at carb tops) float chambers are vented to atmospere. sometimes w/ extras. close examination with a basic understanding often cures a plethra of probs. Oh Yeah. a holed/leaking diaphram or escaped return spring could keep slide from rising, and give lean plug reading. luck g

Other possibilities --
Floatbowl fuel level too low;
Pilot fuel jet clogged;
Pilot fuel passage clogged;
Pilot air jet damaged or missing;
Vac tube to petcock cracked, torn or ill-fitted (if from #2 carb).

Good Luck!

gane wrote:

Oh Yeah. a holed/leaking diaphram could keep slide from rising, and give lean plug reading. luck g

I got bit by that one. The holes or cracks in the diaphragm can be very hard to find. I screwed around with jets and needles for a long time until I realized the problem.

Patton wrote:

Other possibilities --
Floatbowl fuel level too low;
Pilot fuel jet clogged;
Pilot fuel passage clogged;
Pilot air jet damaged or missing;
Vac tube to petcock cracked, torn or ill-fitted (if from #2 carb).

Good Luck!

Well, fuel level - OK.
Pilot jet - Clean, same as rest.
Pilot fuel passage - Boiled carb, and blew thru them all.
Air jet - passes air at same rate as others.
Vac tube - plugged off, running on prime.

bountyhunter wrote:

You can also have leaks around the throttle shaft and various places.

hmm... maybe throttle shaft...

Gane wrote:

Oh Yeah. a holed/leaking diaphram or escaped return spring could keep slide from rising, and give lean plug reading.

Would this be causing my symptoms at idle - 1/8th? The bike seems to pull ok from 1/4 on...

TeK9iNe wrote:

Oh Yeah. a holed/leaking diaphram or escaped return spring could keep slide from rising, and give lean plug reading.

Would this be causing my symptoms at idle - 1/8th? The bike seems to pull ok from 1/4 on...

That's exactly where a tiny hole in a diaphragm gets you. At light throttle openings the vacuum differential is small and the piston rise is small.... but if you have a tiny leak, it doesn't rise enough. At higher vacuum difference (where the piston rises a lot) the tiny leak doesn't have much effect.

bountyhunter wrote:

At light throttle openings the vacuum differential is small and the piston rise is small.... but if you have a tiny leak, it doesn't rise enough. At higher vacuum difference (where the piston rises a lot) the tiny leak doesn't have much effect.

Ah... that makes COMPLETE sense!

Iv'e got it open again, so I'm super checking for this.

Update soon...

Cheers, thanx.

Those little slits in the rubber are hard to find (I missed them). They can hide in the egdes where the "curtain" folds are. I didn't know I had slits until I cleaned it and held it up to a light. Turned out I had THREE HOLES in each one.....:blink:

OK, so this whole thing is really weirding me out.

There are no detectable holes in the diaphram.
I've had the carbs off, and tested the vacuum slides, with a vacuum, of all things. All the slides rise to exactly the same height at the same throttle position.

There is still a considerable amount of air in the ratio.
The cylinder fires, but the plug stays almost completely white, but for a few splashes of gray on the insulator tip, and the metal base ring never changes condition. The plug is firing, and the pipe gets finger sizzling hot.

Where is the darn air coming from?!? :S

There are no air leaks in the carb itself, if I cover the end with my hand, open the throttle, then suck on the other end, I can't get any air at all.

this is the only thing keeping me from riding this thing.

Does anyone have any more suggestions???

Try spraying around #2 carb with propane gas (unlit of course, duh) while the engine's running. Gives broader coverage than spraycan method with WD40 or carb cleaner.

Good Luck!