Jetting a Race Engine

Jetting a Race Engine

Changing jets on a race carburetor sounds easy enough. For the most part it is one of the first things a racer learns and adjusts. Like any other part of an High Compression, High Horse power race engine a great deal of care must be taken before a Tuner just jumps in. The very best way a Tuner can determine the proper jet for a race engine is on the Dyno. Reading A/F Ratio numbers and reading BSFC numbers tell a tuner the fuel the engine requires.

At the track we have all pulled out a spark plug, looked at the color and jetted an engine from there. A Tuner that has experience can get very close making jet changes this way. If you have never “read” a spark plug be very careful tuning this way. It can be very difficult to understand what you are seeing. It is hard to tell the difference between carbon spotting and aluminum spotting on a plug.

A carburetor jet size is very simple. The higher number the more fuel a jet will flow, thus richening the engine. There are numerous jet manufactures and not all the jet number from one Mfg. to another flow the same. We highly recommend you use one manufacture and stick with them for all your jetting needs. Most quality jet manufactures offer a chart that will show flow numbers and drill size for a given jet. Jets with a different number on them may have the same Hole size drilled in them; one of them may have an angle cut into the jet that allows more fuel flow.

When changing jets it is the perfect time to double check other components of your fuel system. Check your fuel bowl gaskets, check to make sure your floats are not stuck, look for dirt in the bowl, etc. After changing your jets and putting your carburetor back together, turn on your fuel pump and check for fuel leaks before starting your engine. An Experienced Tuner will always double check fuel level after changing jets.

Never guess at a jet change, look back at Dyno sheets if need be. If you have never rear a spark plug to find proper jetting, find a spark plug representative at the race track and have them help you.