Well, I've tried most everything.

[SBC Maro]

This is killing me! I have a 355 that I just CAN NOT floor without fireball-out-the-carb backfires.

I've tried 2 distributors, 2 coils, new plugs and wires (Plugs gapped). 5-6 different initial timings with a mix of no vac advance, ported, and full. Tried 3 different carburetors (600, 650, 750 double pumper) and I just can't get this shaken out. This is starting to absolutely drive me nuts.


The car runs great it idle (This is a 355 with Dart heads and a very mild cam) with about 8-12 degrees of initial timing, on full vacuum advance. Yea, I disconnected the vacuum advance when setting initial

As soon as I peg the car though, with an exception of doing so at a very high RPM, the car will backfire out the carburetor, and shut down. If I lean into the throttle, and avoid pegging it, it works out, and picks up very well.

I thought maybe I was running lean, but with a 750 mechanical secondaries double pumper? I'm out of ideas here.

[SBC Maro]

I'll get out there and take a look, Give the neighbors a few more minutes of precious rest before I start banging around out there

[500caddy]

check for vacuum leaks around the intake.. if you have power brakes make sure the booster is not leaking vacuum it will cause a lean pop. the next thing is are you running a mech or hyd cam?? to much preload on a lifter will cause it to. just a base line put as much initial timing you can 20deg or more is fine total 38deg have all the timing in before 2500rpms. did you make sure the timing pointer was dead on??

[500caddy]

denny, read again i said for a base line tune..i have seen gauges show nothing and it was a vacuum leak. know thats out of the way. three carbs, two dizzy's. if the timing pointer is on then it sounds more like valve timing or to tight of a preload on the lifter only takes one!!! i would lock out the dizzy and set it to 38deg a forget about it and look for the real problem but thats just me..

[500caddy]

he already said the timing was tried at 8-12. two dizzy's doing the same thing ????????? not likely.. three carbs all doing the same thing again not likely. as for the vacuum gauge if you have no base line and the car idles fine tell me how your going to read it???? the car pops on heavy accel not sure how a gauge will show anything there...i'm not sure way you would run as much int timing as you can?? i run all may sbc with locked out dizzy's. is there something that i don't know about as to way you wouldn't run 20deg of int???

[SBC Maro]

Ah! I let this thread go, I haven't been able to get on in a while here.

I set my initial to 8 degrees, and we're still having the same problems here. I have really steady vacuum, about 17" at idle.

I'm starting to think it's a valvetrain problem. I want to go through and totally readjust all the valves and see if it helps at all. There is a tick on #5 intake, and I don't know if it's a lifter or a loose rocker arm. I was trying to hot adjust it, and gave it about a 1/4 turn tighter, and the motor seemed to lug a little, so I backed it off, not wanting to do any damage inadvenrtantly.

Both of these distributors were new, one brand new (the Accell Blueprint Points-style) and a newish HEI with an Accell SuperCap. The HEI was in use on the last motor, and worked like a dream.

Also, it should be noted that only the 750 is a double pumper, the first 600CFM was an edelbrock, and the 650 was a Holley, non double-pumper.

I'm leaning towards valve adjustment thought. Thanks alot for the help, You can probably imagine how frustrating it is to have a brand new motor, and and endless supply of problems. Sigh. Hopefully this thing will run right someday

[R Pope]

Could be a bad plug or wire,too. Even new ones can be NFG.

[C9x]

Secondary float level low?

[fordbeast]

sbc maro, im thinking timing with quick accelation backfire otherwise carb problems.

[Racecar100]

Plug wires touching each other will cause upper RPM back fire. It will cross fire from one wires to the other. My friend was working on a 37 ford with Buick Nail Head motor did the samething and I seperate all the wires from each other and it stop backfiring.

[SBC Maro]

Denny, I'll definitely give that a shot.
This morning (before I read this) I readjusted all the valves. They were rather loose, although not terribly so.

It does feel better, but it still likes to pop a little on acceleration, not nearly as bad however. As for the secondary float level, this carb has sightplugs, and the fuel is sitting right at the bottom of the sight window.

Pope: On the third set of plugs (Bosch, then Autolite AR133s, now Accell 576s) all gapped correctly, and switching to HEI required a new set of wires (Set from the old motor, then Accells for the Points, now Accells for HEI). Sigh.

Let's pray the you're valve adjustment technique clears this up, I'm getting ready to trade this thing for a 90's ford escort, hah. It would be nice to punch this thing from a dig and light them up

Thanks so much for all the help, even if I don't fix this glitch, I'm learning a great deal

-Rob

[500caddy]

can i ask what gap your using for the plugs?? i would only put 1/4 preload on the lifters. i never run more then 1/8 to 1/4 on any after market cam and lifters... but if you using anything bigger then a .035 spark plug gap it will do it ever time. you should be using a gasketed nonprojected gaped at .035.plug

[Tohmer]

What kind of break in did you do on the new motor. 15-20 minutes at 2500 rpm usually does it. If the break in was not done well then the cam may be rounding lobes and causing this issue. You can continually adjusy but the lobes will go away. Just a thought.

[erik erikson]

What kind of break in did you do on the new motor. 15-20 minutes at 2500 rpm usually does it. If the break in was not done well then the cam may be rounding lobes and causing this issue. You can continually adjusy but the lobes will go away. Just a thought.

If the cam was on it's way out the door it would pop out the intake and exhaust.

[SBC Maro]

It's fixed!

Welp, whaddaya know. It was right under my nose the entire time.

It turns out that instead of replacing everything, I should have used a little more intuition. Felt retarded, acted retarded, was retarded. Apparently I didn't index my balancer correctly. When I was reading 12* initial, I was getting more like 4 degrees ATDC. My friend's father is a mechanic, and we were just shooting the breeze about the Nova, and we decided to take a look. Cranked the timing up a bunch, drove around listening for knock, and did some minor carburetor adjustments. I punched it at a stop sign, and all hell broke loose.

This. Nova. MOVES! now that I can floor it, that is

Thanks for all your help guys, I learned a great deal about tuning while trying to track this problem down. I'm going to go grab some timing tape, get back out the dial indicator set, and degree the balancer correctly. I'm surprised that the car even ran so retarded. When the #1 cylinder was at absolute TDC, the balancer was reading about 16* BTDC, if I remember correctly.

Again, thanks for your time and suggestions, there is much of wisdom that you can only get from the combined experience of this many people.