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Rebuild of Chevy 350

2K views 7 replies 7 participants last post by  zukidave 
#1 ·
I have an 85 Chevy with a rebult 350. For the last couple of months my truck has been backfiring through the carburator when it's still cool. It's not knockin at all so I know it's not a lifter. I'm thinking it's the exaust lobe on my cam. Any suggestions on what it might be? Also it's only a two bolt main so I was wondering if it's even worth rebuilding or should I start with a 4 bolt main core?
 
#2 ·
From what I have read on alot of forums, 2 bolt mains are stronger than 4 bolt mains. You just have to have the other two bolts splayed (sp?) in or somethin' like that. I wish I could elaborate more, hopefully someone can.
 
#3 ·
When was the last time you checked your timing?

I had a rebuilt 350 in my burb that was really hard on cams, but it never caused me any backfiring issues. Motor just gradually lost power and the valves started making a lot of noise.

After having mine rebuilt once and swapping it out to the 455 Olds, I don't think I'd bother with it ever again. Next time I'm just going to throw in a crate motor. After all the little odds and ends that needed replacing, the quirky tuning, and never knowing how good of a job the builder really did, I think the crate is the way to go.
 
#4 ·
Most likely, the ignition timing is too advanced. Could also be that one of the intake valves is adjusted too tight or it is leaning out. An exhaust valve will not cause backfiring through the carb.
 
#5 ·
you might check to see if your head gaskets are ok. My 88 had some backfiring and turns out that the head gasket was blown between 4 and 6. while the engine is running, pull the plugs from the cap and if the motor doesn't bog down when you pull them, then you know that your not getting any fire from those cylinders. pull one at a time until you go all the way around.
 
#7 · (Edited)
The 2 bolt main chevy block is good well past 500 hp the with proper parts. Its not like it can't take any power. Have it balanced use good pistons, rings, rods/rod bolts, etc. If you splay the main caps (adds two extra bolts to each main at a ___??? angle that ties into the main webbing of the block) of couse the two bolt block will take more power, however, the 4 bolt block can take some abuse. It is capable of producing more hp than the 2 bolt when prepped. In stock form the 2 bolt is weaker than the 4 bolt. Also, some sneaky rebuild shops like to charge more for work on 4 bolt blocks. (They assume some people don't know what the hell is going on, damn crooks.)

As for your backfire problems, You said it only does it when it is cool? I would do like 4x4Freak said, and check the timing first. What kind of carburetor do you run? When was the last time you cleaned it? You probably have already checked your spark plugs, if not, that would be something I would look into. Also if you decide to clean your carb, you might want to retune it. Edelbrocks (AFB style carbs) seem to need more attention than Holley's and quadrajets. As far as the rocker arms being too tight, I would make that a last resort, because if it was running fine when you first got it back together, how could it get even more tight? If anything, I would think in that siuation it would it would be that a rocker arm loosened up. Also an exhaust valve can cause a motor to backfire, specifically in the event that it is not opening causing all of the backpressure to exit the way it came in. Hope this helps.
 
#8 ·
If it run's good when warm I think I would look at the choke. Sound's like a lean miss fire.Bad cam, timing, lose rocker, They would effect the moter at all temp,s.As far as the two bolt four bolt thing the two bolt block has stronger main web's more meat. The main cap's are weaker that's why the hod rod guy's convert the 2 bolt block. GM say's the 2 bolt bolk is good to 500 hp or 7000 rpm's if built right.In 20 year's drag racing I never had a 2 bolt block let go in 10 sec car's
 
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