Having trouble with Mikuni VM29 Smoothbores

Alway's better to start fat on the jetting and take fuel away then
start lean and burn it up chasing the right jetting.

Whelp, I took the bike for a quick 5 mile ride this morning before work and took a peek at the plugs when I got home. Same deal 1 and 2 look lighter and 3 and 4 are sooty black. I'm about ready to throw in the towel on the VM29s or at least buy a set of used stock carbs to see if the problem persists when I switch carbs. I may switch the components over from 1 to 4 to see if the problem moves then. What else could it be? This problem has been with me since I got the bike and I've Bought new coils, updated the entire electrical system, rebuilt the entire top end (including using a new head and cam set up) Cleaned, bench synched and set float heights on the carbs multiple times. I feel like the problem has got to be in the carbs some where but I'm just stumped. If I buy some used carbs are kz650, kz650, gpz750 and others pretty interchangeable? If so what are some of the best kind to look for ?

Try adjusting the float level with the bike on the centerstand.
Steve

Compression test results (especially cylinders #3 and #4)?


Is choke circuit malfunctioning whereby enrichment circuit fails to fully close when supposed to be fully closed?

Is choke required to start cold engine?

Have carb float repairs failed (especially carbs #3 and #4)?

Are carbs properly bench-synced?


Using brand new B8ES plugs,is there healthy fat blue spark on plugs #3 and #4?

Could experiment by test running with hotter running B7ES plugs in cylinders #3 and #4.

Possibility of broken oil ring (perhaps when installed) could allow crankcase oil into the combustion chamber while the engine is running.
Is smoke visible from the exhaust:
• When cold engine first cranked?
• During initial riding pullaway after starting a cold engine?
• On acceleration with warm engine?
• On deceleration with warm engine?

Good Fortune!

Would also check the BS30-97 jets.



Good Fortune!

After I get mine going again I'm gonna try 22.5 pilots and 122.5 mains.

Yes Swest I set the float levels with the centerstand. I even put a wood block under the front wheel and checked the carbs with a little level.


Patton- I will try to get some compression numbers in the next couple days , after that I will be away for a few weeks. I would be surprised if those cylinders are off since this is a problem that existed before the top end rebuild I did where I replaced the rings and honed.
I'm going to look closer at the choke mechanism. I installed new orings in all the choke plungers but maybe something else is going on. I use the choke to start the cold engine but turn it off almost immediately
I have quadruple checked the float levels. They all sit at 4mm below the gasket surface
I bench synced the carbs using the wire method on the forum. They were a bit off before I did this but I believe are squared away now. I will double check next time the carbs are off.
The spark is healthy and blue in all cylinders. This issue has actually followed me after putting new coils and wires on the bike. I've switched back and forth between B7ES and B8ES plugs as well
'I have not noticed any smoke. I

I'm toying with the idea of just getting another rack of carbs while I'm away so they're waiting for me when I get back. If not only to see if the problem persists. How are the pumper carbs?

I'd compare the emulsion tubes and air jets. They may have been modified or replaced with the wrong ones. Who knows what a PO did before you got them. Used pumpers will have to be rebuilt, rubber parts etc.
Steve

When bench sync. VM29's the number two carb is the lead carb.
start with that one and adjust the rest to match it. When I did mine I backed the
idle speed screw all the way out and adjusted #2 till it just bottomed out then using the idle screw I raised
it till a small drill bit slid under it with just a little drag then adjusted the other three to match.

On my 33's #3 is the lead carb.
Steve

Thankyou, I'll be away for the next few weeks but hopefully I'll figure this all out when I get back