main jet wrong size?

[Guest]

Are there symptoms of having an incorrect size main jet? I took the bike out on the highway last night and got on it. As I was rapidly accelerating and in 2nd gear it would almost seem like the clutch was slipping (maybe that's the problem?) and the engine would rev to red line, but it would almost seem like power was lost to the wheel. It would kind of hiccup like this a few times until I shifted and then again at red line it would do the same thing. Any thoughts? I haven't done plug chops yet to check fuel mixture.

[mmattockx]

Lean will surge and have no power, rich will bog but without surging (it will just fall on its face when you whack the throttle open). Your description sounds more like clutch slip than jetting issues. You will feel the jetting well before redline because full throttle acceleration will have you fully on the main jet by 3500-4000rpm at the latest (assuming CV carbs).

A better way to sort jetting is to hit the choke when you are on the carb circuit you want to check. I would go out on a back road where there is no traffic so you can change speeds and fart around without worrying about getting run over. Get into 3rd or 4th gear so the engine can pull hard for a while without revving off the tach. Slow down until the revs are around 2500rpm, then pull full throttle. Let it rev up to 4000rpm to be sure it is on the main and pulling solid, then hit the choke. If it cleans out and pulls harder you are lean. If it bogs slightly you are pretty close. If it bogs and falls on its face completely you are rich.

FWIW, this test works for all carb circuits, but you need to put some tape on your throttle housing and mark the throttle opening in 1/8 increments so you can accurately know which circuit you are testing.


Mark

[jdion81]

I'm thinking its lean, but when trying to diagnose with the choke, there was no change.

[mmattockx]

I'm thinking its lean, but when trying to diagnose with the choke, there was no change.

That is likely a touch lean then. When rich enough you will get a slight bog on the choke. If you have the jets and changing them is not an issue I would try one step up and see how that works.


Mark

[LAB3]

Lean will surge and have no power, rich will bog but without surging (it will just fall on its face when you whack the throttle open). Your description sounds more like clutch slip than jetting issues. You will feel the jetting well before redline because full throttle acceleration will have you fully on the main jet by 3500-4000rpm at the latest (assuming CV carbs).

A better way to sort jetting is to hit the choke when you are on the carb circuit you want to check. I would go out on a back road where there is no traffic so you can change speeds and fart around without worrying about getting run over. Get into 3rd or 4th gear so the engine can pull hard for a while without revving off the tach. Slow down until the revs are around 2500rpm, then pull full throttle. Let it rev up to 4000rpm to be sure it is on the main and pulling solid, then hit the choke. If it cleans out and pulls harder you are lean. If it bogs slightly you are pretty close. If it bogs and falls on its face completely you are rich.

FWIW, this test works for all carb circuits, but you need to put some tape on your throttle housing and mark the throttle opening in 1/8 increments so you can accurately know which circuit you are testing.


Mark

That's a nice method, I'll keep that stashed away.

[KEITH KRAUSE]

The choke (starter) circuit, is designed to operate with the throttle closed. Opening the throttle by-passes the choke circuit. So the above testing method incorporating the choke is useless.

[KEITH KRAUSE]

Are there symptoms of having an incorrect size main jet? I took the bike out on the highway last night and got on it. As I was rapidly accelerating and in 2nd gear it would almost seem like the clutch was slipping (maybe that's the problem?) and the engine would rev to red line, but it would almost seem like power was lost to the wheel. It would kind of hiccup like this a few times until I shifted and then again at red line it would do the same thing. Any thoughts? I haven't done plug chops yet to check fuel mixture.

Sounds to me like the clutch is slipping. I don't know what you mean by "hiccup" but if it "accelerates rapidly" and "revs to red line", then both of those suggest the main jets are fine. A lack of fuel would decrease acceleration and the motor may not be able to red line. Have you serviced the clutch at all?
As for main jet condition, just inspect the mains and verify they're the correct size.