Wednesday, January 13th, 2021
Reprinted with Permission from Race Car Books.
If the high speed bypass poppet is located after the barrel valve, it will shut whenever the throttle is lifted for an out of shape run.
That will return the engine to the rich AFR momentarily when the throttle is hammered back on. The enrichment is useful to decrease the chances of a momentary lean condition from throttling under load. A momentary lean condition can cause a backfire that is reduced by this method.
Some fuel system standard layouts provide this location. Enderle and Alkydigger is an example. Others locate the high speed ahead of the barrel valve. Rons and Hilborn are examples. This protection is not provided by that layout. For MFI setups with a barrel valve separate from the nozzle distribution block such as an old Crower, move the high speed bypass connection after the barrel valve, in the connection line or an added output in the distribution block.
For MFI setups with a barrel valve and distribution block that are a common assembly such as Hilborn, an added port in the distribution block location can be done to move the high speed bypass to this location.