ok guys, i got an edelbrock 600cfm on my little 289 and have had it on there for some time. about 6 months ago it started backfiring out of the carb. so i replaced the points and adjusted the timing, still backfired. replaced plugs, wires, cap, rotor and coil. still backfires. i dug deeper, took heads off and checked for bad valves. none were bad, lapped valves and reassembled heads. reinstalled and adjusted the valves. STILL BACKFIRES!!! i am leaning to the carb now. was messing with it today, seems like it only does it when the secondaries open up. so what do you guys think? i am at my wits end. im goin to email edelbrock on monday and get their feedback as well. i have messed with the idle mixture screws and also replaced the jets and rods because it was running rich when it was running. oh the engine has about 30k miles on it and i also forgot to mention i already rebuilt the carb, totally. thanks for the help guys.

 

I think you were on the right track with timing. But, you need to check the valve timing as well. I know it's only 30k miles, but if you have an el'cheapo timing set, it could easily be the culprit.

 

you might want to look at the vacumn line from the intake, if thats clogged you get some backfiring through the carb port. speaking from exp.

 

Backfiring is generally related to timing. Double check the firing order of your plug wires, then check the timing again.

 

by checking valve timing, do you mean the timing chain itself? if so, i did then i had the heads off just to be sure. it was perfectly fine. also the plug wires are in the right order. that was one of the first things i checked. i havent however checked the vacuum hose. i only have the one that goes from the carb to the distributor. i think it is fine, but will check it out today.

 

make sure you dont have it retarded too far, makes large fireballs when you do.. Also check to be sure you dont have flat lobes on the cam. If the cam is headed south, the lift and duration will be much smaller on some of them. A cam that is way to small will pop through the carb and not rev over a certain RPM.. If the exhuast valve isnt opening all the way when the piston comes to TDC, the compressed gasses need to go somewhere. As soon as the intake opens they head out and you get a backfire through the carb..


Just things to check.

 

is there a way to check or a cam going south? i remember looking at the cam while i had the heads and intake off, looked good. i didnt take it out however.

 

Ya know, backfiire is the wrong word to use. If it truly back fired, the fire would go inside the engine and not out the carb or exhaust. Yeah, we all know what we are talking about, but maybe the term should be front fire, or out fire or something. Ok, I'll crawl back in my cave now.

 

remove one valve cover at a time, start the engine and see if some rocker arms aremt moving as much. Put some cardboard or another barrier on the bottom side so you dont get oil everyehere.

It could also be very lean, when the secondaires open if its not getting enough fuel it could pop. For a similar effect but on a much larger scale, a nitrous backfire is the same thing but much more violent and destrcutive.

 

hmmm, will try running it with the valve cover off, im good at making messes, lol. i believe the squirters for the secondaries are working correctly, but will check again.