another jet question

'76 900 cc motor is running well, idles good (has a little miss addressed in other forum for electrics) and pulls hard at full throttle. Main jets are now 125's, needle is in 4th slot or second from bottom, motor is stock (as far as I know), only modification is 4 into one exhaust. Problem is that there is still a flat spot from 1/4 to 3/4 throttle. With 125 jets shouldn't this be gone? suggestions please.

thanks

It would be instructive to know what type carburetors are being asked about. You main jets don't have much affect from 1/4 to 3/4 throttle. That would be needle jets/jet needles. Knowing the actual type of carburetors would suggest what needle jets and jet needles are being used. Also, if you are saying that the flat spot is from the entire rpm range from 1/4 throttle to full throttle, I would clean/lube my mechanical ignition advance before messing with the carburetors and adjust valve clearances.

Post edited by: wiredgeorge, at: 2006/03/23 08:32

Carbs are stock VM28's, mechanical advance was cleaned a month ago when I installed the Dyna S, valve clearance were adjusted at the same time, but the motor had set for a while so maybe the clearances have changed.

Thanks

Post edited by: CTSZ1, at: 2006/03/23 08:40

Redo valve clearances before making jet changes. Perhaps they are the reason for some of your problems.... I hate to keep asking but stock VM28s? Stock for what bike model? Kaw used stock VM28s from 1973 to 1980 and they came with main jets ranging from #125s down to #102.5s.

Still learning what all information I need to include. Okay, VM28's off of a 1974 Z1, when I got the bike they had 112.5 jets. Motor was worn out so I replaced it with the current '76 900 motor. Used the original '74 Z1 carbs. When I rebuilt the carbs I put on 117.5's because the bike has 4 to 1 header. Mixture was lean( missing, backfires, white/clean plugs) at all ranges so I moved up to 120's and then 125's. Now plugs are light tan instead of clean white, but throttle response is flat in the mid range. Needle is in the 4th slot or second from bottom.

Hmmm flat spot with Z1A carbs... mains should be about #120s with pod filters and 4 into 1. You are too big on the mains. Other than that, the stock pilot jet is a #20 and plenty large and the jet needle (5J9) runs to the rich side in a large needle jet. I would back down the main jet. Please address your comment regarding the flat spot from 1/4 to 3/4 throttle. Is this one long flat spot or in a specific RPM range?

Flat spot is from about 2000 to 5000 rpms approximately. Wide open it pulls hard in any rpm range. If at steady throttle it runs flat in the above range. It surges and has poor throttle response, except when I open it up wide.

By the way pilot jet is a 20 but the needle is a Y45. That's the needle that came with the rebuild kit. Old needle is a 5J9. I am also still running the stock air box.

Thanks