rejet after cam change?

[Selland74]

I am getting ready to install a stg 1 hotcam and i was wondering if it would need rejetted afterwards. I have a big gun full system, k&n, and the carb has been rejetted.

[krt400ex]

no need to rejet at all. a cam does not change the air to fuel ratio. it just holds the velves open longer(or shorter) to change the power characteristics of a motor....and give u more power. there is more to it than just that, but that is basically how it works. no need to change jets

[400eXr1d3rZ]

If it holds the valves open longer or shorter, it's changing air flow. Rejet.

[Kaleigh]

but the fuel/air mixture is still the same.. its just letting more in and out.. my jets are the same.. i got 160main in my 400ex.. still the same as before I put my cam in.. i raised the jet to 165 and I was burning rich..

[krt400ex]

Originally posted by Kaleigh
but the fuel/air mixture is still the same.. its just letting more in and out.. my jets are the same.. i got 160main in my 400ex.. still the same as before I put my cam in.. i raised the jet to 165 and I was burning rich..


exactly

[GPracer2500]

woops...double post

[GPracer2500]

There are times when a rejet IS needed after installing a new cam. In many cases, if a rejet is needed it has to do with the new cam specs creating different pressure signals (vs. the old cam) within the carb's venturi. The size of a jet isn't the only factor that determines how much fuel a jet will deliver. The differential air pressure that pushes fuel through a jet has a lot of influence on how much fuel any given sized jet will flow (this is what main air jets and pilot air jets are all about--in case anyone was wondering). Think of it this way: how much soda can you suck through a straw in 2 seconds? Well, it depends on how hard you are sucking. Even if the size of the straw doesn't change, changing the vaccum signal to the straw can change how much liquid flows through it. Same thing happens in a carb. Changing cams can change the velocity of the air and the "character" of the air pulses moving through the carb as the valves open and close.

A similar thing can happen with high compression pistons--they can change the nature of the vaccum signals within the carb.

It's not so much that the engine needs more or less fuel than it needed before, it's that it's not getting the same amount of fuel it was getting before--even though the jet sizes didn't change.

In most cases the jetting differences between cams are going to be rather small though. I'd go so far as to say that most folks don't need to worry too much about rejetting with a typical cam install on a 400EX if their jetting was correct to begin with. If your jetting is off than a new cam could push you so far off that now a rejet is really needed.

[parts eeter]

Use a 42 pilot and 158 main or it will run hot and start crappy .Might not even start at all on a cold day unless u rejet.

[Honda4lyphe]

what id do is have some extra spark plugs and test it from there