Help getting my Carb right

[cobb_ben]

I have a holley 750 4150 DP(I believe that's what size it is...) The plugs were back and fouled out. It had 65 jets in the mains. I changed them to 63s and put the 65s in the back they were 74s. That made it run a whole lot worse. Then I put the 74s back in the secondaies and that made it run back like it was. I left the 63s in the primaries.

What it does to run bad, is that I will backfire(in the carb) when you get on it, unless you go real slow to put the gas down. It will run the POP... then it will get on down with it.

When I had the 63s in the primary and the 65s in the secondaries it would pop worse and not have near the pickup that it did with the 74s. I would pop a lot.

I was thinking that is was too rich because of the plugs being fouled out. And I thought that the POP was too rich and burning of the excess fuel. But maybe not???(puzzled)

I think maybe i need to make the power valve bigger, I don't know... And I though that it was a double pumper so that would make it not be lean...

THE ENGINE-- 400SBC aftermarket 4 bolt mains on 2 bolt block, 350 crank and 6in rods 10.5 to 1 conpression(I believe) It has Dart Pro 1 aluminum head with 220 or 230cc intake and 2.055 and 1.60 valves. It had retro full roller Comp cams with 530s lift(don't rember the specs exactlly. Full roller rockers. The intake is an aluminum air gap manifold. Then it has MSD 6AL box with MSD dizzy. It breaths thru 1 3/4 hooker pro comp headers, thru dual 3in glass packs with 2.5in "H" pipe. Then dumps at the trailing arms. Also electric fan and only water pump and alt off the crank.

ANY AND ALL HELP AND OR SUGGESTIONS WOULD BE GREATLY APPRICATED!!!

THANKS GUYS SO MUCH!

-Ben

[Pont406]

What is your vacuum at idle, initial and total timing?

[69rut]

If the engine runs "better" of different when you change the secondary jets, you have a problem with that circuit. The secondary throttle blades could be open. They should be closed. Check the carb linkage for something bent.

The 65 jets in the primaries is a good start for a motor like yours, but the problem with the backfire is the timing. Adjust that until you get the desired results. Some people will say get bigger shooters, or change the cam profile on the accelarater pump. This may also help, but set the timing first. Also hook up the vacuum advance to the "ported" location on the carb. This is located on the metering block, not the carb base.

The fouled plugs comes from the secondaries being open. The backfire is a too lean situation.

A bigger power valve will not help this problem. They are designed to add fuel during "wide open throttle" (WOT) conditions.

Another thing to check is the float level on both fuel bowls. There are also idel mixture screws in the metering blocks. They should be opened two turns. This is a good starting point. You will need a vacuum gauge to set them, but again fix the timing first. Then adjust the screws slowly, 1/4 turn at a time, and both side equally until you have max vacuum at idle speed.

[70GMCer]

back firing might have blown the power valve also.

[cobb_ben]

Thanks for the help. I have to work this weekend. But monday I will work on it. Thanks

-Ben

[cobb_ben]

No chance to do any adjustments yet... But I figgured out why I was smelling gas sometimes.. LOL You know those plastic clear see thru sight plugs... They suck!!! Both of them broke off and fuel was sloshing out... Leave the standard brass screws in place...