VM29 Jetting Trouble

Thanks to all.

Wireman, when the bike is sitting at idle and you give it throttle, it will hold at 2-3K rpm and then gradually decrease until the engine dies. You can blip it again and the same thing. Adjusting idle and throttle cable has little to no effect, neither does idle adjustment.

I installed new throttle and idle cables and adjusted them until there was visible slack. I installed new intake rubbers as I thought I had an air leak. I installed new Mikuni 17.5 pilots and 120 mains. I have not changed the o-rings on the mixture screws but I will. I also installed new K&N pods.

The thing doesn't have to run like Beemer to suit me, I just want it to idle a little and not hang when I blip the gas.

chetriply wrote:

... when bike is sitting at idle and you give it throttle, it will hold at 2-3K rpm and then gradually decrease until the engine dies. You can blip it again and the same thing. Adjusting idle and throttle cable has little to no effect, neither does idle adjustment...stalled new intake rubbers as I thought I had an air leak. I installed new Mikuni 17.5 pilots and 120 mains. I have not changed the o-rings on the mixture screws but I will. I also installed new K&N pods...The thing doesn't have to run like Beemer to suit me, I just want it to idle a little and not hang when I blip the gas.

Symptoms could be caused by intrusion of outside air into fuel mixture before reaching combustion chamber.

Suggest spray test with engine idling for air leak between carb and engine (at manifolds, clamps and vacuum caps). Avoid carb intake area. May use WD40, starting fluid, propane gas (unlit), brake cleaner, or carb cleaner.

This is not about leakage on the intake side of the carb from the air filter.

The pilot adjustment screw on each carb is a pilot air adjustment screw. Screwing it in clockwise reduces the air which enrichens the idle. Screwing it out counter-clockwise increases the air which leans the idle. On my Yoshi-headed Z1 with 29's, I began at 1 and 1/2 turns out and adjusted from there. Only 1/4 turn out is too rich for me, but realize applications differ.

If there's no change at all in idle speed when adjusting the pilot screws, probably something still wrong inside the carb.

When correct, should purrr and compare favorably to any Beemer idle. :lol:

Thanks Patton.

There is no air leak on the intake side. As I stated, I replaced the carb holders with new ones. This was my first thought. I checked it using propane. The idle/air screw is exactly that. It adjusts the amount of air/fuel at idle. If screwing the idle/air screw all the way out doesn't change the idle, the idle jet is too large.

That's why I'm pulling my hair out! I've tried all the simple solutions and tricks I know. I've been working on bike professionally for 9 years but this is my first set of 29's. I've never had this much trouble with a set of carbs. Usually, the problem presents itself and you find the solution.

Thanks to all for your help and sorry if I'm bitching too much. I may just throw some CV's on it and call it good;)

chetriply wrote:

... no air leak on the intake side...If screwing the idle/air screw all the way out doesn't change the idle, the idle jet is too large.

Believe the newly installed 17.5 pilot jets should be small enough.

No specific mention has been made of having cleaned the tiny orifices in the air jets. If obstructed, could prevent sufficient air from reaching the pilot jet regardless of pilot screw adjustment. Used wire to clean mine. Sorry about beating a dead horse regarding sufficient air supply, but the ultra-rich pilot circuit condition keeps me suspicious.

Also, have the float bowl service fuel levels been checked with the clear plastic tube method. If too high, can cause overly rich mixture. The upside-down bench measuring float level test is ball park at best.

With the air jets clean and clear, and air passages from air jet locations to pilot jets clean and clear, and service fuel levels correct, would begin with 1 and 1/2 turns out on the pilot and adjust from there.

Remotely possible that the choke plungers are not completely closing off the enrichment circuits.

The idle hanging might possibly be cured with correction of the above items. But could possibly result from slight interference (rubbing) between a throttle slide and jet block.

Hoping for you a successful fix.

Post edited by: Patton, at: 2007/05/15 00:22

Getting back to those gaskets...

There's a small hole in each gasket(if You in fact have them installed.I think Some have removed them?)
If that hole in the gasket doesn't line up with the hole in the,"Jet-Block",then You will have problems.

Again...the vent lines...how do You have them set up?

Are You getting any fuel seeping out of the air-filter side of them at all?Not the bowls,but right AT the filter side openings...

Not sure if it has been mentioned but did you seal the manifolds to the block? Also if you can't get it running, your more than welcome to send them to me.

Alright folks, it has to be something I did this last time I pulled them off. Before that, they at least ran. They wouldn't idle but the rpm didn't hang like it does now.

So, I reomved the new intakes and resealed them. Checked every freakin' orifice I could find (on the carbs) and installed another set of brand new 120 mains and 17.5 pilots. Also checked float levels and they are spot on. Now, I have to work on customer bikes (Harley-Davidson) for the remainder of the day.

Hope I have positive results to report. Thanks again to all.

Post edited by: Patton, at: 2007/05/15 11:27

Easy to miss noticing this tiny air jet part #12 when cleaning the carbs, but it's there and is does screw out for cleaning.

(while Sandy rofl at diagram note)




Post edited by: Patton, at: 2007/05/15 11:52

Post edited by: Patton, at: 2007/05/15 11:54

Thanks again Patton:)

:blush: Forgot to mention early on about first checking all valve clearances and setting to specs. And compression check too. Also ignition timing -- static test adequate if engine won't idle for timing light. Am presuming correct cam timing and ample spark (good coils, plug wires, plug caps, and plugs).

Undoubtedly above preliminaries already all done in this instance before undertaking any carb work, but mentioned here as information to any others who might be interested.

Hey ChetRip... its wiredgeorge... not wireman hehe, Even I got standards... Sounds like you have done what needs to be done... only thing I can think of is that a weak return spring is being used. Don't bother wiggle and jiggle... find another spring and try it. I had a set of 29s doing EXACTLY what yours are doing and it drove me nuts. Believe it or not, the spring on a VM24 LOOKS the same as a spring on a VM26/28/29 but IT AIN'T... slightly smaller gauge wire and if it is used on a 29, will not shut the slides with authority and they have to "idle down" for the idle to drop down to normal. I have seen 24 springs used on a couple 29 assemblies so can only guess that maybe MIKUNI stuck the wrong springs on these carbs in a couple cases.