Jetting!!

[Turk]

Backfiring during decelaration!!

My 427 Side oiler
Low Riser 2X4 Manifold
Low riser heads
(2) 4160 570cfm Holleys
9.6:1 compression
MSD 6AL box

is in need of further attention.

Carbs came back from Holley totally rebuilt, I don't even know what jets are in it. They restored them to original, and I am asssuming they installed whatever belonged on them. It was a complete restoration by their Custom Shop.

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 to 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

[chopper]

Turk, I don't know if this is going to be of any help to you, but when I went through this, I ended up with primary jets at 60. Original jets were 64s, I believe. However, if you're backfiring on deceleration, it's more likely a problem with leaks in the exhaust system than jetting problems with the carb. The ERA cars use the "slip fit" (or "slip leak", which is more accurate) pipes and they can be a bugger to get sealed. I have Stainless Specialties sidemounts, and both SS and ERA had little useful advice during the trials when I was trying to eliminate the backfire which appeared when I installed the SS pipes. I tried different sealants and such and finally had the pipes removed and individually sized with a little pipe expander gizmo which I found at a flea market. We got almost all of the backfire problem resolved, although I still get an occasional "pop" out of the right exhaust.

I am running 600cfm 4160s with vacuum secondaries, MSD 7AL box with billet distributor, 10:1 compression and have both of my carbs set up with identical primary jets. The secondaries, as I'm sure you know, are not adjustable since that function is taken care of in the secondary metering plate. IMHO, if the plugs are showing the correct color, you're probably running close to the correct jet size. I suggest that you very carefully check for exhaust leaks while the engine is idling to see if you have some leakage around either the header/head mating or the header/sidemount joint (more likely the latter).

[Turk]

Guys,
Thanks a bunch. There is a concensus that my problems are leak related. I tried not using the Walter's exhaust sealer or any other silicone between the primary and the exhaust pipes. I guess I will have to now.

After reading the comments from some of you, I should have known. Two weeks ago it was the left side popping during decelleration, now it is the right side.

It couldn't and shouldn't be the carburation since they don't feed right and left bank seperately.

Thanks.

TURK