Friday, August 21, 2009

Jetting a clone carb

Here is what I have seen on my dyno:

If you are still running the stock air box:
A .029" is slightly better than a stock .028" If you are allowed to run the valvecover vent tube to a catch can and plug the hole in the airbox, do it.

If you are running Box Stock rules, only the main jet can be changed:
.037" to .039" seems to be the ticket at low altitude. .038" being my "go-to" jet.
When the engine is cool, a .037" makes more power on the top end and can help the engine turn more RPM, but most times, it is at the expense of HP at or about 4800 RPM. (you won't see that on anything other than a water-brake dyno)
A .039" almost totally gets rid of the mid range dip in HP, but it costs you on top end. A .038" is a compromise and once the engine gets good and hot, seems to run better than the .037"
Side note- On the road courses where we are running them 30 minutes wide open, we also drill the low-speed jet. to .019"-.020"

If you are running BP or Superbox, The etube and both jets can be changed:
I like the 140 etube, .036 jet and .019" to .020" low speed jet.

These have been my observations on on my dyno with the Yellow engine and the New Greyhound HF engine. My on-track experience has only been with the yellow engine.