New VM28SC on KZ1000A2 - how to dial it in?

I just installed freshly rebuilt VM28 SC Carbs (no accelerator pump, 2 screws on the carb top) along with brand new intake boots. The carbs were bench synced are jetted for PODs and 4 into one.

The bike fired right up and idles fairly well after adjusting the idle screw, however it keeps back firing when idling.

No fuel issues (clean tank) and there are no vacuum leaks.

There are no hesitations revving up the bike, but coming down from high rpm's it back fires on the way down.

Any pointers where to start would be greatly appreciated.

Were those carbs on that bike previously or did you buy and install them on there?

back firing when idling --- would assure correct ignition timing, with alignment of the F mark at idle rpm.





coming down from high rpm's it back fires on the way down --- would suspect too lean pilot circuits.
Try turning in the pilot air screw on each carb clockwise, say 1/2 turn, to enrichen the pilot circuits. This reduces the air allowed into each pilot circuit.

Would also assure all valve clearances are within specs. A too tight intake valve clearance may allow combustion within the cylinder to escape through the intake port into the carb.

Good Fortune!

Were the carbs tuned to run on this particular bike before or is it a new install?

Thank you all for your feedback! The carbs have been rebuilt and bench synced. Jetting was for 4 into one with Pods. The carbs were not specifically tuned/set-up to my bike (78 KZ1000A). Even though the carbs are originally from a Z1 they should be pretty close to a 78 KZ1000.

I took the timing cover off and to my surprise I found that the advance had been changed to a Z1 Advancer (3 digits). The 78 KZ1000A should have a different advancer (one with 4 digits).

Since I dug into the timing I went ahead and installed a new Dyna ignition.

I checked the timing with a timing light prior to changing the points out for the Dyna, and low and behold the timing was way off!

Since I did not have an Advancer for a 78 to 80 KZ1000 I used the one presently installed, but changing to the correct advancer will improve things further.

After the timing was adjusted I tinkered with the pilot air-screws and attempted to sync the carbs. Boy, these VM28's are not liking to stay in sync...

So far I managed to tune the bike to a state where it is not backfiring. It idles pretty well, no back-firing. The only thing I could not accomplish is to get rid of the "popping" when coming down from full throttle..

My next step I think will be to change out the old Z1 advancer for the correct one.

The "popping" from coming off full throttle probably means that it runs a bit too rich under full throttle... but I guess that is better than too lean at full throttle.

Do you think there is a way to dial in the VM28's to get rid of the "popping" when coming off full throttle?

Thanks again for all our feedback. Your advice has been great!

If you mean by backfiring popping through your carbs at idle then it is running to lean, go up one size on the pilot jets, your VM28 probably have #15 pilots go to a 17.5. Popping through the exhaust at closed throttle could be either to rich or to lean. However if they are popping through the carbs also then I would say your idle circuit is to lean.
It is virtually impossible to set up carbs that were never on a particular bike for pods and a header by "bench" tuning them. You have to do that on the bike and that is what you doing now.

Also, bear in mind that the VM28SCs are not the easiest to tune, with their external linkage. Take your time and definitely invest in a carb adjustment screwdriver which will make your life a tad bit easier.

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baldy110 wrote: If you mean by backfiring popping through your carbs at idle then it is running to lean, go up one size on the pilot jets, your VM28 probably have #15 pilots go to a 17.5. Popping through the exhaust at closed throttle could be either to rich or to lean. However if they are popping through the carbs also then I would say your idle circuit is to lean.
It is virtually impossible to set up carbs that were never on a particular bike for pods and a header by "bench" tuning them. You have to do that on the bike and that is what you doing now.

I thought pilots didn't come into play until the bike was off idle? I'm a noob at this BTW.

Pilot jets are part of your idle circuit.

I run the same carbs on my bike from time to time. I also have vm 28mm (3 bolt caps) which I pull off from time to time to try and find the right needles. Your 28's sound a little lean at idle. The poping is a lean symptom when backing off the throttle. The air screw should be 1 1/2 out. The pilots should be 17.5 or 120 pending on your altitude. If fuel level is right, that should eliminate the poping. The advancer will only bring your timing up higher at idle. If you need more on the top end, open up the stops a bit. What is critical is make sure both weights hit the stops simotaneousley. (CRITICAL) or your timing will bounce all over the place. Bend one or the other untill this is acheived. Happy tire schorching.

These carbs are bad about the needle seat wearing. Were they replaced? Roy