Carb Jetting!
 

I posted this elsewhere, hoping to find a quick fix.

Backfiring during decelaration!!
My engine with 2 four barrel Holleys sitting atop a low riser manifold on a 427 SO is not doing what they should be doing. I probably didn't do what I was supposed to do before I installed them.Sent the carbs to Holley Custom Shop and had them totally restored to original and just installed them.

I don't even know what jets are in it. I am assuming they installed whatever belonged on them.

The car seems to run fine, except during decelaration there is serious "pop, pop". they are almost like mini backfires. Sounds cool, yet I have been told by others who are more knowledgable than I that looking at the exhaust and the color of the pipes, my carbs are running too rich. Pulling 6 or so of the spark plugs shows that they are not too far off the desired color.

I am going to have someone rejet, the carbs, but not having had this done before, I am curious to see if anyone else is running a similar setup, and what would be good numbers for jets for starters.

Front carb, and rear carb...would they requier different jets?
I appreciate any help on this subject you could provide. If what i posted regarding the engine is not sufficient let me know what other pieces of the puzzle you require.

Thanks,

TURK

Turk,

Most of the time you get backfires when you deaccelerate is due to air leaks in the headers. Tighten all the header bolts when the engine is cool and see if that helps. If it doesn't consider replacing the gaskets and and retightening the headers. IF your timing is correct, most of the time these air leaks are the cause. When you take your foot off the gas, the increased vacumn draws air in through the leaks and the oxygen in the hot headers then fires the raw gas that was not burned in the cylinder. This problem is usually worse if you are running an overly rich mixture, but based upon your plug readings this appears not to be the case.

bkozlow

Ditto....leak in the exhaust for reasons stated above !

Thnaks for all the input in here as well as on two other threads on the subject. I think I am convinced the problem is leak related. i am NOT convinced I will be able to stop it. I am willing to give it a hard try this weekend.

Thanks again.

TURK

Turk, the pop is , like others have said, an exhaust leak. As far as jetting, you have 2 choices. Go to a rear wheel dyno with air fuel mixture reading capability and jet the carb accordingly. Second way, put your cobra in third gear at 2000rpm, floos it to the red line, keep it floored, and then shut it off, pull it over , pull a plug and read it. Then jet accordingly. When you do this, do not let off the throttle and then kill the motor. As soon as you shut the throttle blades at high rpm, the motor goes lean as there is no fuel. This tends to clen the plugs. good luck, scott.