TOPIC:

Hi!
I was wondering if there's a wau I can figure out what size main jets should be in my 1979 KZ1000ST without playing hit-or-miss swapping jets back and forth?
It has a Kerker 4 into 1 exhaust system, Wiesco flat top pistons that put it at 10.2 compression, and individual K&N pod filters on the carbs. It starts and runs good, but when I let off of the throttle and downshift it starts popping quite a bit. Not so bad you can feel it, but you can hear it clearly out of the pipe. It currently has 140 main jets in it, I'm thinking it may need 130s. Anybody out there with a similar setup?
Thanks in advance!

I am not sure if the ST had VM28 w/pump or VM26 w/pump. In any case, it likely used #102.5 main jets. I would certainly not use any mains larger than #112.5 else you are overjetting relative to other internals of the carburetors. The problem with the backfiring on decel is not related to the size of the main jets. It happens when the throttle is pretty much shut and thus in the idle circuit. With your set up, you should have #17.5 pilot jets but you may also have a small air leak that causes the backfire... cracked vacuum caps, bad o-ring on pilot screw(s), intake manifolds cracked or not sealing good, carbs not seated... that sort of thing.

I solved my decelleration pop by reclipping the jet needle to a richer position. Like George points out it's almost always a lean indication.

savedrider wrote:

I solved my decelleration pop by reclipping the jet needle to a richer position. Like George points out it's almost always a lean indication.

If I raise the needle is it going to affect my performance off of idle?

Popping on Deceleration is known as a Lean Misfire, caused by lack of fuel to ignite in the chambers and then passed to the exhaust system where by some chance encounters something to ignite the very lean mix and thus pops. I wouldn't think that going from a 140 to a 130 main would cure it as WG has stated and I think that savedrider has the solution by re-clipping your jet needle to provide a richer setting thus providing a good mixture for ignition. Your performance off-idle will change somewhat but I'd bet it would be for the better, but not knowing what carbs you have, I could be wrong.

hat0791 wrote:

savedrider wrote:

I solved my decelleration pop by reclipping the jet needle to a richer position. Like George points out it's almost always a lean indication.

If I raise the needle is it going to affect my performance off of idle?

I'd just try it and see. I had mine clipped in just about every position before I settled for the position that gave the best performance. In my case it was a richer position.

Well, I'll give it a shot then tonight after work.
Ill post my results in the morning!

I was sitting at a stoplight and revved the engine slightly and noticed smoke drifting by me. It's running really rich off of idle so I don't want to raise the needle just yet. I've got a set of 130 main jets, and a 17.5 pilot ordered that I'm gonna try.