VM29 Jetting Trouble

I'm about to pull my hair out so I hope someone can help. I've got VM29's on my 78 KZ. It's a 1075 with mild cams and a custom 4into1 with a Supertrap. I've got the carbs jetted with 120 mains, 17.5 idle jets and needles set at the 3rd slot. Idle air screws out 1 full turn. I'm running K&N filters as well. I put new intake boots on and synced them up.

Now the problem. This thing won't idle to save it's life. I've tried adjusting the idle/air screw a 1/4 turn at a time but no good. I thought about going to a larger idle jet but the thing is burning really rich at idle. The RPM's hang when you give it throttle as well. I removed the carbs, checked the vent tubes, idle and main jets and slides but everything seemed to be in working order. The slides move freely and the cables aren't binding. What am I missing here crew? Wireman, I think I might just send these damn things to you if I can't figure them out!

Thanks in advance.

i had the same problem with my 29's. they worked great with the throttle open but midrange and idle sucked.
i too had 120 mains with the needle on the third clip. maybe this is an inherent flaw with these carbs?

Man I hope not! I know I can make these things work! Other people have done it right?

chetriply wrote:

Man I hope not! I know I can make these things work! Other people have done it right?

My 29's purrr.(sorry...not trying to rub it in,lol)
YES...yes You can get them to run good/great.
Some people have had problems with the 29's,and some have had great success.
Here's a few things to look into...

The vent lines...where do You have them connected,and routed?When I first installed MY smoothies,I had the vent lines wrong,and it ran like crap.
Can You post a pic of Yours,and show the vent lines?
Also...You had the slides out,I'm assuming?Did You happen to notice the small gasket that sits on top of the "smoothbore" insert?...

I'll get You a pic of what I'm talking about,if need be...

Here is some info from Z1Enterprises website --


Mikuni 29mm smoothbore carbs

Once power-hungry riders realized the potential of the Z1 engine, a large array of performance parts appeared on the scene. One of the most important of these was the Mikuni 29mm smoothbore carbs. The stock carbs did not adapt well to the increased fuel demands of high-lift cams and high-compression pistons, nor the back pressure changes of the barely flow restricted 4-1 exhaust systems. The 29mm smoothbore carb was by-and-large a cure for many tuning problems. The top of the carb is identical to the 26mm (and later KZ1000 28mm) carbs. The float bowl went back to using the 17mm drain plug directly under the main jet. Great for racers as it allowed for quick changes of the main jet. None of the VM29's have an overflow pipe fitted in the float bowl.
lNote: some very early sets of 29mm smoothbore carbs have a float bowl with a small drain screw angled to the side.

Below is a reference of jet settings for the 29mm carb on Z1 / KZ engines. This is a good starting point for a bike with 4-1 exhaust, after market cams (up to 0.410" lift) and high compression pistons (approx 10.5:1). Each bike setup is different, so actual settings will vary.

29mm smoothbores Z1 / KZ900 / KZ1000
Main Jet 120
Air Jet 0.9
Needle Jet O-6
Jet Needle 5DL31-3
Throttle valve (slide) 1.5mm cutaway
Pilot Jet 17.5 or 20.0 (or 25 in the later versions)
Air Screw setting 1 1/2 turns out

Havin a rough day,there Patton?:laugh:

Couldn't figure out how to post a GIF file of the exploded view, so here's one way to see it --

Go to Google.
Do an image search for Mikuni VM29.
Should see a thumbnail of exploded view.
Click on the thumbnail.
Click to enlarge the thumbnail.
Should see full screen size of exploded view.

Part #87 is the jet block.
Part #88 is the gasket that fits underneath the jet block.
Part #13 is the small screw that threads from underneath through the gasket and into the jet block.

If the screw is removed and then jet block removed to access and replace the gasket, when re-assembling note the small screw is not centered into bottom of the jet block and serves to keep the jet block properly oriented in line with the bore of the carb. DO NOT TORQUE the small screw -- just turn it barely snug. Otherwise the jet block will slightly twist (as in Leaning Tower of Pizza) and prevent proper movement of the throttle valve Part #86 up and down over the jet block.

Believe the jet block gaskets are available from Z1E (but maybe not shown on their website).

It's true -- the 29's really will purrr.

chetriply wrote:

... burning really rich at idle....

Some possible reasons:
Pilot air jet partly clogged.
Pilot jet partly clogged.
Float bowl fuel level too high.
Dirty or obstructed air filter.
Starter plunger stuck open.

Hoping for you a successful fix.

I have tuned a couple of sets of 29s... It would really help if you described what it is actually doing at idle. Does the idle hunt up and down (change rpms)? Does it bog? Does the idle hold for a bit then race?

First, look at your throttle cables. There should be visible slack. Next, replace the orings on the mixture screws. Your jetting is about right for your set up. The mixture screws should be turned out 1/4 turn for #17.5 pilot jets.

I pay absolutely no attention to the jetting guidelines posted for the 29s as the carbs will not work when jetted and adjusted as described. The last 29s sold were sold with #25 pilot jets and it is impossible to make 29s with 1.5 slide cut outs work with that large a pilot jet... just too much gas.

Do NOT take off the blocks to examine those gaskets already mentioned. Those gaskets are no longer available and you WILL tear yours almost every time. When torn, the bike WILL NOT idle. The gasket material is super thin and making new ones that allow the block to seat flush are almost impossible.

Last, if you real irritated with the idle, try taking some JB Weld and plugging up those little holes in the tip of the jet needles. The holes allow air to be drawn even those the screws are mostly shut and are there to add "tuneability". Who the heck buys racing carbs and wants to tune the pilot circuit? You mostly just want it to idle smoothly without a lot of fussing. You can try tuning the idle with regular VM style air mix screws IF you stretch the springs under the screws so that the air screws have tension on them from the spring so they don't back out. They will be on the last couple threads since the thread on them sits farther out than on the VM29 style airscrew.

i recently "aquired" a set of early (1973) vm28's and they had the tips of the idle pilot jets soldered shut. i never knew why they had done that until now. they must have been drag carbs or something similar.