Dr L-88

I have been told by several people that I should replace the power valve in my 750 DP Holley with a power valve plug, and when doing so, will have to re-jet the carb with smaller jets, probably 5 or 6 sizes smaller.

Carb is on a street-driven 427 ZL-1 with hyd roller cam, .570 lift, timing set at 36 degrees total.

Your thoughts?

Thanks,
Rex

mr.beachcomber

First of all, removal of the power valve will require increasing the size of the affected jets as the power valve is part of the fuel metering system and adds fuel as manifold vacuum drops.

If you have a late model double pumper, you should have blowout protection for the power valve from engine backfires. (I have used/raced Holleys w/o backfire protection for the past 30+ years and never had this problem, but YMMV.)

The only reason to change your power valve and/or jets should be correcting a lean or rich condition. For example, back in the late 60's street rodders used to buy a Holley 3310-1, change the front 10.5 and 8.5 back power valves to 6.5's and decrease the front and rear jets by two sizes to eliminate a rich condition on the street under acceleration and full throttle. If we were racing a SBC, we might swap the 10.5 front power valve for the back 8.5 and use a 6.5 power valve in the secondaries. BBC basically used the 3310-1 in its out-of-the-box configuration with a Chevy "open" plenum intake manifold. If used with a divided plenum manifold, individual jetting was required if the diverter tabs on the boosters weren't filed off.

By now, I've probably told you more than what you wanted to know about power valves and jetting Holleys; however, I just wanted to point out that out-of-the-box jetting/power valves should only be changed for specific fuel metering situations.

Have fun with your 427!