Check the plugs. Also check the ignition system. Keep in mind, many times ignition problems diagnose the same as carb problems. Also check the float level. Do a spark check, if it`s yellow it`s weak, if it`s blue it`s normal.
The last time we ran into a issue like this the plugs were bad.

 

i put new accel shorty plugs in the motor for the first startup and havnt changed them since. i pulled a few out about a week ago and some were black some were a grayish color so i figured missfire. now when i pulled them out yesterday they were all prity much black, but didnt smell of fuel. all the gray in them were gone. i have been running the car about 3 weeks now.

 

Accel = Junk. Say no more. The best place for almost any accel product is the trash. You have checked the firing order? as it`s easy to get some wires crossed. A few things elsewise that can cause it to backfire through the exhaust are too high float level, go with smaller jets, with black plugs indicates it`s rich. You may also want to check the butterflies and make sure there set correctly. Your squirter size is likely too large since it`s been drilled. After going with leaner jets, reset the idle mixture with a vacuum gauge, set it to it`s highest reading at idle.

 

ok here it goes....
i have a freshly rebuilt 454 that i put in my 81 firebird.
bored .030 flat top pistons so around 8.5:1 comp. with 049 heads, heads are pirty much stock with only a spring upgrade to fit my edelbrock performer RPM cam.
i have the timing set at about 34 deg. at about 2000, seemed to be were she liked it best. initial is about 18. now it runs awesome under 2000 but anything after that it backfires alot and sometimes will backfire through the carb. and will only sometimes backfire at around 1500, and this is only when i am just putting around. it runs very smooth when i am getting on it but when i let off is when it starts to do it. i was having some bog issues and was told to drill out my shooters a little bigger, but i think that i may have drilled them too big now. it doesnt have the response like it used to. i drilled then tested and continued process and was fine but now that i drilled it again out a little more this started happening. but it always had the exhaust backfire above 2500 RPM. my mind is boggled........

Put a vacuum gage on it and see what the needle looks like- sounds a little like either retarded timing (especially w/the backfiring through the pipes on deceleration) or the valves are too tight on it.

A compression or leakdown test will also tell you what the valve and ring seal is like.

 

i can only get about 15 pounds of vacuum. right now its actually at about 12 with an idle of about 800. when you say the "valves are too tight." you mean i have the rockers tightened down too much? as you can see from all my other posts i am fairly new to engine building as this was really my first one by myself. i am running 66 primary jets and 70 secondary. will a smaller jet cure my goof of drilling the shooters too big? and what kind of plugs do you recommend??

 

i can only get about 15 pounds of vacuum. right now its actually at about 12 with an idle of about 800. when you say the "valves are too tight." you mean i have the rockers tightened down too much?

Yes, tightened down too much. The gage would have a fast stutter to it if that were the case- not a nice, even, steady needle. The cam you have will cause some movement of the needle, but an open valve will certainly cause a bigger fluctuation.

The type of gage you use should be a "diagnostic" gage- w/undamped needle- that will show any small fluctuation, not a damped "MPG"- type gage made for watching for mileage.

Another thing that comes to mind is a broken valve spring. This is easily checked for, at least. Be sure you look very closely- a broken valve will sometimes realign itself w/the broken ends abutting, and can be hard to see.

will a smaller jet cure my goof of drilling the shooters too big?

No, you need a new squirter.

An AC plug in the 44 heat range would be a place to start.

 

yeah the vacuum is not very smooth. it fluctuates 2-4 pounds very fast. well tonight i will be pulling off the valve covers and setting the lash again. you know of a place were i can find those shooter clusters?? i have looked everywhere and cant find anyone who sells them.

 

yeah the vacuum is not very smooth. it fluctuates 2-4 pounds very fast. well tonight i will be pulling off the valve covers and setting the lash again. you know of a place were i can find those shooter clusters?? i have looked everywhere and cant find anyone who sells them.

I might be able to help w/the shooter- what is the carb make and model number? I'm thinking Edelbrock, judging by the "cluster" reference.

Before you relash the valves, look for a broken spring. Some guys have an easier time jiggling the pushrod up and down instead of twisting it to feel when the slack has all been taken up.

Good luck.

 

its an old holly 4010/4011. used to be a marine carb i was told. i have tried the method if pulling on the rod. i have hydraulic lifters so with the twisting method it was kinda hard for me to tell. was told to do it again when the engine was running, and haven't done that yet.

 

ok so i have come to the conclusion that i am a retard... i had my valves WAY too tight! i started losing the #1 exhaust valve and i gained about 10 pounds of vacuum. :thumbup: i am really not sure what i was thinking, i do remember tho when i was first trying to start the car it kept shooting flames out the carb and wouldnt run, so i tightend the valves and i think i went a little overboard when i did that. :spank: havent done the other side yet had to take a break because she was getting a little warm and i kept burning myself, and i wanted to post this to let yall know. i am hoping this is the cause of all the backfiring..i really really do...

 

well took it for a test drive, and holy crap she drives soo much better. and will actually want to idle going down the road with no backfire. but she still feels a little slugish...and i think that happened when i drilled the shooters...she doesnt have the get-up-and-go like she used to.