Why start with the main jet?

[mmattockx]

So we have all seen the Factory Pro CV tuning guide that starts by sorting the main jet and then works down. They are certainly a credible source with lots of experience and I am not questioning that it works, but the question is why do they do it that way? It is backwards to the way we tuned 2 strokes for decades and makes more work for yourself because the lower circuits affect the ones above them but not the other way around. As well, if your bike won't idle and run OK in lower ranges because jetting is messed up it is very difficult to take it for a ride to check the main jet out.

When I jet a bike (with CV carbs, never done a flat slide or VM style from scratch) I sort the pilots first, then the needle, then fine tune the main jet. I never have to go back and reset anything because of changes I make on the following circuits that way and the rideability is never an issue for road testing.

Can anyone enlighten me as to why FP recommends doing the MJ first?


Thanks,
Mark

[tkent02]

ON CV carbs, the main jet does control all the ones below them.

[TxGSrider]

I believe factory pro is assuming you're reasonably close on your pilot circuit to begin with. They are providing tuning tips based on the idea that you're using their kit to begin with and they get you pretty close to start with.

[posplayr]

Can anyone enlighten me as to why FP recommends doing the MJ first?


Thanks,
Mark

Go pull out a factory manual that shows fuel flow diagrams for the CV carbs. You will find that flow through the needle/emulsion and the pilot flows through the main jet first. I( don't recall about idle.