jetting?

[carlygirl]

When changing your piston (stock to aftermarket) but not compression do you need to re-jet?

I would have to say no.

[d3ktrix]

Nope, even when you change compression a jetting change isn't ever really required as long as it was good before.

[400exrider707]

Originally posted by d3ktrix
Nope, even when you change compression a jetting change isn't ever really required as long as it was good before.

For most of us changing a piston whether changing the compression or not, you will not need a jetting change, but to say never, well thats a bold statement.

A jet flows a certain amount of fuel at a certain rate depending on the vacuum pulse sent to the carb by the piston. Putting in a fresh piston, especially one with higher compression often increases the strength of the vacuum pulse to the jet, which means it will pull more fuel through the same size jet. Now like I said most of the time its too small to notice a change, but I have seen some mildly modded 450's running 185+/- main jets, and I have seen the same bikes get Nmotion big bores and drop main jets down to about a 155-160. The vacuum pulse was a lot stronger.

[d3ktrix]

Originally posted by 400exrider707
For most of us changing a piston whether changing the compression or not, you will not need a jetting change, but to say never, well thats a bold statement.

A jet flows a certain amount of fuel at a certain rate depending on the vacuum pulse sent to the carb by the piston. Putting in a fresh piston, especially one with higher compression often increases the strength of the vacuum pulse to the jet, which means it will pull more fuel through the same size jet. Now like I said most of the time its too small to notice a change, but I have seen some mildly modded 450's running 185+/- main jets, and I have seen the same bikes get Nmotion big bores and drop main jets down to about a 155-160. The vacuum pulse was a lot stronger.

Ya agreed, when I answered the question in my head I was thinking of a small change from like 10:1 to 12:1 and used "isn't really ever" as an answer. But thats only a 2pt compression change. Not a good example, only a minor change.
When someone going from say 10:1 to 14:1 or a BB kit like your said is more likely to need a change.

Though for the OPs original question of keeping the compression the same I'd say its very unlikely to need a change. I won't say never tho :P