Mikuni VM29 problems.

dutchz wrote:

...Crappy idle circuit should not be resulting in 25mpg gas mileage, so something else may be going on. Smaller or clogged air jets will cause massive rich midrange and it won't idle low. They are the small brass screws on the bottom looking into the bell mouth. Make sure you use a snug fitting screw driver for removal(I ground one down just for this purpose) they are soft brass and strip very easily.

BS30-97 air jet

Air filter(s) dirty/clogged or over-oiled?

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Good Fortune!

Would perform clear tube test to verify correct fuel level inside float bowl.

Don't rely on measured float height while carb is sitting there dry and upside down on the work bench.



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Clear tube test on Mikuni smoothbore carb.



Good Fortune!

The parts are right:

- needle jets (stock is O-6)
- needle (stock is 5DL31)
- air jet (stock is 0.9)

I have three carbs apart, but before I take the forth apart I'm going to do a fuel level test using a clear tube fuel level I made, just to see. I probable should have done that before any thing.

Okay, I need to know how bad would it be if the fuel level is at the top of the bowl gasket not below it? The fuel on the forth carb is at the top of the gasket line, and I just measured all the float heights and they are in the same place as the forth carb. So does that mean fuel is running freely into the intake ports? I had these carbs cleaned and bench synced, I would have thought the floats would have been tested.

that can cause problems, nice job on catching it. thats why I like to do my own work then you know,assume nothing, check everything while your in there just to make sure. I know it gets frustrating but your getting close now.

TexasKZ900 wrote:

Okay, I need to know how bad would it be if the fuel level is at the top of the bowl gasket not below it? The fuel on the forth carb is at the top of the gasket line, and I just measured all the float heights and they are in the same place as the forth carb. So does that mean fuel is running freely into the intake ports?...

Float heights are untrustworthy indication of fuel level, as exact same float heights in ostensibly similar carbs may produce different fuel levels among the carbs.

Would perform clear tube test on each carb. And don't attribute much concern and worry about the measured float height.

Too high fuel level may result in over-rich air/fuel mixture, evidenced by symptoms including poor fuel mileage and sooty fuel-fouled spark plugs.

Responding to an earlier question, imperfect choke plungers might contribute to the over-richness, but are imo not as likely a suspect as the fuel level or dirty pilot circuit.

Don't see where a reply was given to the earlier question as to whether pods were being used. Likely either pods or air box, but may influence diagnosis as to likelihood of whether fuel has risen far enough inside the carb to enter the carb throat.

Meanwhile, would sniff-test the crankcase oil and examine its physical quality to help determine whether there has been any fuel intrusion into the crankcase. Having no carb overflow circuits, smoothbores are more susceptible to such fuel intrusion, akin to carbs having a clogged overflow circuit.

Good Fortune!

Something that can throw a curve at you with old VM29's is sucking are past the slides. I've run into it several times over the past few years.

It shows up very noticeably when you got to synch the carbs. With the tops removed to gain access to the adjusters, you might experience the idle suddenly gets unstable.

Warm the bike up good before attaching the gauges. Keep the tops of the carbs on. Then, after the bike is warmed up, attach your carb guages and take notice of the readings on all 4 cylinders.

Turn the bike off and remove the carb tops. Start it back up and see if the idle has changed / become unstable / lumpy. If it did, the gauage readings are probably also different.

If that happens, the best bet is to set the tops back on the carburetors with no screws holding them. Perform your synch process, alternating the caps on/off per adjusted carb.

When you're happy with the synch, put all 4 tops on and check your guages.

These carbs are not getting any younger and the odds in finding sloppy / leaky slides is increasing as the years go by.

If they leak air by the slides with the tops removed, you can rest assured they leak excessive air around the sides with the tops on. That can screw up the fuel signal and account for a rack of carbs that refuses to play fair no matter how hard you work at it

Okay, I put the carbs back together and did a fuel level test on them and they were all at or just above the gasket line, I'll fix that after a good cleaning. Thanks for all the support and info.

I have some more questions:

1 - Should I remove the jet block and gasket for cleaning?

2 - Can the carb bodies be put into the carb cleaner dip can?

3 - Two of the slide lever linkages are loose and easy to move and two of them are kind of stiff, does that make a difference?

Thanks,
Dirk

I believe all the carbs should move and act the same linkage wise. remove all removable parts before cleaning, clean them then put them back. the carb bodies should be ok in your dip tank they call it carb cleaner for a reason, but don't put rubber or plastic parts in their just to be safe. see if some WD40 or even some carb cleaner will free up your slides without removing any parts?

TexasKZ900 wrote:

...Two of the slide lever linkages are loose and easy to move and two of them are kind of stiff, does that make a difference?....

Would assure that the small screw (#13) into bottom of jet block isn't over-tightened and distorting alignment with throttle slide.

Good Fortune!

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