Reasons for strange jetting

I am new to motorcycles, I have a lot of experience tuning webers on Porsche and VWs so I am used to tuning carbs with one barrel per cylinder. But I have been having a hard time getting my 79 KZ1000 ST tuned. It ran sweet cold but when warmed up it starts to run fat off idle with a bit of bog then it takes off fine. Honestly accept for the off idle flat spot when warm it pulls very nice. After lots and lots of reading I tested things like voltage and battery, new plugs ect. All good there. I put a new stater in and it seems to be working fine. I was also getting some fuel purge when sitting after a but the bike would piddile gas on the ground. So after much reading I decided I needed to check the float height/ needle valve function. Plus I wanted to get the carbs apart to see what the jetting was and that is when I ran into this.

Cyliders 1 and 4 135 main jet, 15 pilot
Cylinsers 2 and 3 130 main jet, 15 pilot

oddly all of the plugs appear to be equally rich, not surprised that they were rich just equal with the uneven jetting.

The bike has the VM28SS carbs with pods (I know, I know)
Kerker 4 into 1
B8ES plugs
not sure on valve adjustment yet.

Any suggestions on why the jets are missmatched and what do you suggest? Based on what I have read It seems I need 120 mains and 17 pilots? Is that right?

Thanks,

Double A

Attached is a picture of my nearly completed project

sorry meant 17.5 pilots

Not an expert by any means, but bigger jets on 1-4 is probably because the outer pods get more air than the inners. Find an air box and rejet would be the best bet. Thats about all I can offer.

Is the accelerator pump working? Are your air filters the cheap wire mesh kind?


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I don't know if 0.05mm main jet diameter difference on the main jets would make that much of a difference. You say they are equally rich so I doubt it. It sounds to me like you need to adjust your pilot screws. The pilot screw setting determines low RPM performance. My pilot screws are on the engine side, and my guess is yours are as well. The pilot screws may have covers over them. If they do, you will need to pull the covers by drilling a hole in them, screwing in a dry wall screw, and pull off the covers.

Start by screwing the pilot screws all the way in counting the turns. Don't turn them in tight, just slight touching the bottom. Record the number of turns for each pilot screw Mine were the same when I did that. If yours are not I would take the average of the turns measurement for each carb and then set them all to that average. Start the bike and let it warm up. but keep a fan on the engine so it doesn't over heat Then set your idle for 1000 RPM. You will need to have the tank off and run the gas from a bottle. Now turn each pilot screw out one turn and see if the idle RPM goes up or down. If it goes up, keep turning them out to find maximum RPM. A half turn to maximum is close enough. Occasionally rest the RPM back to 1000 RPM as you do this If the RPM goes down from your initial averaged setting, then you are going the wrong way so follow the same process but turning the screws in to find the maximum RPM. Once you find the max RPM again turn them all the way back in counting the turns on each pilot screw. They should be the same if you followed my instructions. If they are not not, average the number of turns and set them all to that. If they were different by a lot you may want repeat the process.. If they were the same to a half turn I would leave them at the averaged number. I would not try tuning each carb pilot screw separately.

If your carbs are leaking gas out the front ends your float valves are not fully closing. Fix that first. I would use vition tipped float needles.

Hope that helps.

Yes accelerator pump is working, I have not measured the delivery amount like in IDA carbs but the accelerator pump jet is in fact squirting fuel with each stroke of the throttle. Filters appear to be gauze with steel mesh. I cleaned them with my k&n filter cleaner and lightly oiled them.

I think I have the pilot screws "pretty close". before I knew there was different size main jets were inside I had the pilot screws at different setting than the inner ones with the smaller jets. it makes sense now that I know there were different sizes. I have them all adjusted to best idle. But there was about a 3/4 turn difference between the cylinders with the 130 and the ones with the 135 jets. Now it all makes perfect sense as to why, but when tuning I was thinking I had mis adjusted valves or low compression on certain cylinders. they seem to function in the same manner as idle mixture screws on Weber carburetors. I will say the cylinders with the larger jets when backing out to best idle "fell off" much more after best idle than the cylinders with the smaller main jets. Like it wanted to die as soon as you passed best idle rather than the typical lump and drop in idle you get. So it went from best idle to sputtering to near death in 1/16th of a turn.

I ordered (Z1E) up a set of 112.5 mains and 17.5 pilot jets. weiredgeorge recommended in a different thread 110 mains if the acc pump is operational but I went with the 112.5's for now because the plugs are not crazy rich and since its 80 degrees here already and soon to be one million degrees outside for the rest of summer I erred on the rich side.

I also ordered the clear tube tool to check full height. Adjusting the floats has made a tremndous difference. When I started #1 was 20mm #2 was 18mm #3 was 19mm and #4 was 26mm. I set them all to 26 and no more fuel puking out and the off idle bog is nearly gone.