jetting help

[cardinalhvac]

we have a national mod 90 with 28mm koso carb. when bike is idling it surges up and down but takes 10 seconds or more to speed up or drop back off. there are no air leaks. is there something else that can cause this? i think i have a 35 pilot jet in it. she runs great off idle but is just pissed at idle? any help would be appreciated

[bulldogfallon]

The pilot sounds lean...

What may be happening is that your needle may be rich and your pilot is lean.

Try going to a 42 or 45 pilot and lean the needle 1 spot at a time.

We seem to end up with the needle 2nd from the top on our 90s, but that is with a 24mm carb

[WOLFPACK]

Are you running a high idle? If so, then what Gary has suggested makes sense as the pilot transitions to the needle.

What does your plug look like?

Any adjustment you make should be made only one step at a time. Jetting is not a spec, but a process. Gary has given you a likely end result. But for optimum performance you must go through a jetting procedure. If you just start changing things and it doesn't run right you'll never know what the problem is nor will you know if you have optimum performance.

Go up one size on the pilot and see if there is an improvement. If improved, continue to go up until it is either perfect or starts to give more problems. If it gets better then worse back off to the point that it was best. If still not perfect, then go to the needle. Same procedure one clip at a time. Then the main, same procedure.

Now watch your plug the see that it is not running to rich or lean. Adjust the main accordingly. Then continue to read your plug throughout the season. Humidity, temperature , altitude and compression effect jetting.

I hope this helps!

Shane