Mikuni VM29 problems.

I looked back at some of your previous post and it seems that you've had this problem for a while but its been getting better? I think this is whats happening-for some reason(dirty jets/clogged passages/massive air leak/whatever) your idle circit is not pulling enough fuel to run, so you are dialing it up to keep it running around 1500RPM. this brings your slides into play and your needles a bit, so your not really running on the idle circit but rather the transition between idle and part throttle. when you blip the throttle your giving it more fuel then you take away the air by closing the throttle, temporarily richening it up so the RPM's drop. I know you think everything is spotless because you had them ultra sonicly cleaned but maybe they're not. I had my 68 triumph carbs soaked in a professional hot tank from friday till monday and the idle circit was still blocked. I think you need to go through the idle circit with a fine tooth comb.

what jets do you think someone may have "filed" on? do you mean the air jets inside the carb mouth, or the ones in the float bowl area? don't worry about the choke rubbers. also make sure your advancer is free to move against the springs. If someone tampered with your airjets that might be your problem. I would replace anything that may have been molested! on a side note: sometimes we do things to our bikes that seem like good ideas at the time, but later we're embarrased to own up to them. the fact that you say "someone may have filed" on your jets tells me that maybe you filed on them since you know a file was used! Its ok if you did we just need to know. that could be the missing piece of the puzzle that helps us crack this nut! if your airjets have been "bored" out they will let in more air, which would be the same as putting a smaller pilot jet in it, making you run lean no matter what adjusting you do.

not a direct answer but something i have had on my 29's..pilot circuit must be clean and flowing or the idle suffers...even pretty clean on my 29's can be cleaned and blown out to be crispy response and sweet idle without constant fiddling with the idle screw settings...when the idle circuits are working right ....the idle screw never needs re-setting for a long time...

so it goes...prob not your issue...but gave me fits...cleaned em twice... until i finally did one last circuit purge and fixed it...

why two cables...29's should snap closed with one new lubed and free sliding cable /un binding cable...never needed two cables on 29's....set one up right and it is way more responsive and snappy imo...

First off I want to thank everybody for taking the time to help.

I don't understand how to clean the pilot circuit and idle,
is there someplace to see how to do it, or should I have them ultrasonically cleaned again?

man i wish i could rember how i finally got to the littel circuit...patton has a picture that shows the teeny tiny screw that is hidden and gets missed....other than to say i had cleaned them twice before with great results,then one time later the next year "I" clogged the pilot circuit and they ran worse than before i cleaned em...i was sad to have to pull them off again and this time carefully with a jewelers flat head screwdriver and a bright mini flash light peeked in every tiny hole and passage,removing everything i could find and using a few cans "chem tool" carb spray and really well directed bursts of air,over and over....until a burst off air and spray cleaners had mist coming/shooting out all four carbs the same, in like holes and passages ...that is when i found a tiny screw that i had missed)i believe it was in the air cleaner side throats and in one of those tiny intake holes....

sorry to not explain it better...patton showed a pic then ..think it was him.. ...and that got me thinking it was worth one last take apart and really spraying and letting each passage soak then high comp air blasts.....put em back together and on the bike and it was like i had never ridden my bike before...big difference.....in fact i need to do em that good again.....oh and i put new plugs that last time too...just was unreal the difference in those 29's if that passage has any grime....imo

good luck

TexasKZ900 wrote:

First off I want to thank everybody for taking the time to help.

I don't understand how to clean the pilot circuit and idle,
is there someplace to see how to do it, or should I have them ultrasonically cleaned again?

Search for "Carb Cleaning 101" and you will find all the info you need.

1 thing to add to that, is to make sure that you use the little straw included with carb cleaner sprays, to squirt carb cleaner up through the pilot hole in the bottom of the carb while looking in the bore of the carb, to make sure that the passages are all squirting cleanly into the bore.
You MUST VISUALLY VERIFY the flow of fuel from the bowl into the carb bore by spraying cleaner through everything and watching in the bore where it goes. All the carbs should look similar in amount of spray.
Dont stick your eye right up to it, and use some safety glasses.

You said something about the colortune... do you own one? or did someone do it for you?

This should help:


Tune the mixture screws properly! X number of turns is BULLSHIT, and especially with pod filters. You should en up with them all at different positions via colortune confirmation. With pods, you need to check and change these settings once a month at least, but it gets very quick and easy! The only time you'l ever have a bike with screws close is complete stock, and even then, they can be off by more than an 1/8th turn. With pods, can be 1/2 turn or more!

Carb tuning/jetting isnt just guessing jets. Its a refined art of feel, sense, and knowledge that is quickly dying.

Good luck! - keep us posted. 1 change at a time, and be patient.

Okay, I just finished up a work project early so I'm going to start pulling the carbs apart this afternoon. I really appreciate the info. I have a set of magnifying goggles and a SureFire mini light ready to go. I'll take pictures of everything so everybody can see what I'm doing.

I bought the ColorTune with plans of using it at least once a year, and yes the mixture screws are not all the same but they are very close to 1/2 turn.

Thanks again.

When you have the carbs apart, let us know what size the following parts are:
- needle jets (stock is O-6)
- needle (stock is 5DL31)
- air jet (stock is 0.9)

With the jetting you are running 120/20 you should be close. 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.

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?

[Click on image to enlarge view.]



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.



Good Fortune!

Clear tube test on Mikuni smoothbore carb.



Good Fortune!