304 tuning help..

OK. 74' 304, Edelbrock intake, truck avenger carb, HEI, gapped at .050 and fenderwell headers.
Why is it back firing out of the exhaust badly. Timed at 7 degree below the TDC line (is that correct, or should it be above the line?)

I replaced the intake gasket, but did a poor job on the RTV, so I may have a leak...at the header flange as well. I hear about carb spray while it's running to hear for vac leaks?????is that correct or I am I waiting for a fire to happen?

On a better note...it looks good now

 

Spec says 5-6 Degrees Before Top Dead Center - mine runs great at 10.

 

Backfiring out the exhaust is typically either running to rich, or a exhaust leak.

 

points or electronic ign? dont know much about the intermediate jeeps but if it does have points dwell can cause a backfire

 

Backfiring under load and a tad above idle. I am sure there is an exhaust leak, and make an intake leak since I wasn;t too happy with my RTV job when I replaced it last week.

I just dropped the truck avenger on, so it may be a combo of running very rich and exhaust leak????

 

This is kind of what I am thinking to. It feel like it has a miss and with the timing light showing a miss, I may need to do a compression check and then verify my 12v signal for the HEI isn't fluctuating down.

 

could be any number of things. sticking or physically damaged valve, bad lifter, dist off by a tooth, bad advance, exhaust leak...

check compression in all 8 cylinders to either rule out or further incriminate the valve train, then check the plugs and make sure all 8 are firing.

does the backfiring stop or change under varied RPM and/or throttle position?

 

Check the timing, should not bounce at all! must stay steady, 304 i think had lose timing chains from the factory. if you get the timing set under load and then lose it under free spin. It would tell me the chain.

 

I say check the distributor. I had the same problem on my 304, fixed all the leaks and still backfired....I swapped out the prestolite w/ HEI and never had the problem again.

 

Usually a backfire is a result of an electrical issue or a valve problem. However are you sure it is backfire? A backfire sounds like a 30/30 blast whereas alot of times people confuse missing or loud popping as a backfire. Missing and popping can be any number of things including ignition, fuel delivery, vacuum, exhaust, etc.

If you have a timing mark that is floating around on you that is certainly a timing chain, gear or cam issue. I would start with a compression test and work my way up from there.

 

cool/ I was unaware there was a difference between popping and back firing. I haven;t had time this week to work on it, but It sounds like a combination of everything...exhaust, timing, carb too rich.

I replaced 2 bent pushrods when I had the intake off, so since the jeep sat for 15 years before I got it...I have a feeling the PO really over revved the motor. I didn;t see an odd wear on the rockers or bridge supports that would lead me to think there was a valve problem....but there could have been a stuck lifter etc that went unnoticed.

I'll swap the header gaskets first and see what happens and then do a compression check and post back with the results. I have a feeling the motor (only 62k miles) led a hard life and the new HEI and truck avenger just made them more noticeable.

If the top end needs to be rebuilt I may as well just drop in a JY 360 since it'll probably be cheaper in the long run.

 

I may have missed something. so just clarifying. did it run correctly after you installed the HEI or did it start doing this emediately after you assembled? is the Dist installed correctly? and is it tightened down properly?

 

A bad coil can give this symptom, too. Cheap and easy to replace.

 

It ran very well with the old points distributer and new coil, but I replaced to an HEI, installed a truck avenger and resealed the intake.
I have an exhaust leak that i'll knock out tomorrow and i'll try another 12v source to make sure its constant...but otherwise...it's a leaky intake or something buggered in the top end.
Intake was resealed due to 2 bent pushrods from the PO.

 

what size truck avenger? Maybe too much carb.

 

its the 670 version. I wanted to keep the same carb if i swapped out to a 360...and the 304 has an intake and headers..so it should be oK.
My buddy who builds race motors called tonight and recommended I put a set of .035 gapped autolite plugs, lean out my mixture a bit, clean up the plugs (all crossed up after messing with it) and I should be good to go. Didn;t sound like an intake or exhaust leak since it was at idle to him. So I'll see what happens tomorrow. Hopefully it'll fix the miss and pop.

 

Your buddy may be right but the 670 is too much carb for a stock 304...

 

sorry guys but I really see something not right here. So after you put the HEI and Carb on then emediatly it started back firing. is this correct. if so its the dist. you have it out a notch. your timing is all off. easy fix. just check this for me please.
pull #1 plug. rotate engine untill #1 piston is to the top. this is hard to see but you can put something in the plug hole to feel with that won't scar the piston. when its at the top pull the dist cap. look at were the rotor is pointing and mark it on the block or someplace! put the cap back on. that marked spot for the rotor is #1 plug wire. start the plug wires from there. I am sure you no the firing order if not we can tell you. I really think you are just out a notch!