Carburetor Dillemma..

[Jeremy D.]

Yesterday when I was playing with the timing on my LT-1, I ran into a bit of a problem. I was gradually turning up the idle screw on the carb to increase engine RPM and everything was great until I got to 2800-3000PM. The car started backfiring, although still running. I DID have the vacuum advance connected, on purpose. I turned the idle screw down, the backfire went away and the car ran fine. I just happened to peer doen the carb throat, and noticed that on the primary side, the passenger side booster had fuel gushing out of it, while the drivers side had apparently nothing coming out. "OK, blocked passage" I thought. I took the primary side float bowl and metering block off, soaked them in cleaner, blew out all the passeges/bleeds, put in brand new Holley jets, took out the power valve and it seemed okay, cleaned the accelerator pump nozzles, etc. I put it back together with brand new Holley gaskets, fired up the car, and it did EXACTLY the same thing. The car runs great up to about 2800-3000 RPM, then backfires like crazy. Fuel was pouring out of one booster, and not the other. I had shot carb cleaner through both boosters while the carb was apart:they're unrestricted.
The carb is a 4150 Holley 780CFM, list 4555, blah-blah-blah 2121, factory jetting(70) and power valve (8.5)The carb was rebuilt by Holley Custom Shop two years ago, and the gaskets are the reuseable Holley gaskets. WHAT THE HECK IS WRONG WITH MY CARBURETOR?!?!?!! I'm gonna pull it apart again today, but ANY insight is WAYY WELCOME!!!! Thanks Guys!!

[Jim T.]

Re: Carburetor Dillemma..

I am not a expert but have maintained my 70 and 68 with Holley carbs since 1973-1974. Why did you change the jets? My experience, the only reason to change the jets is to go smaller or larger not to just put new ones in. Did you vacuum test your power valve? I always replace the power valve in a tear down like this, one can be bought in a blister package. I would use a 6.5 Holley power valve. I have a ZZ crate engine in my 70 and use a 4.5 power valve since my transmission is the turbo 400 and at idle the engine vacuum in gear with the air on is lower than a car with manual and no air. My 68 327/350 L79 with Holley double pumper spread bore I use a 6.5 power valve. You did not mention replacing the needle and seat, this regulates the fuel level. Clem is an carb expert mayby he will see your post and offer some insight.

[Joe C.]

Re: Carburetor Dillemma..

Jeremy:

If you mean "accelerator pump nozzle" for "booster", then it is very possible that the ball check was left out during the rebuild.

Joe