327 idles great, backfires with throttle

[1986c10]

I dont remember seeing anything abnormal about the power valve, is there something specific I could inspect? It would be nice to fix it.

I ended up buying a cheap basic holley 600 carb to toss on it so I can drive it, should be here tomorrow.

Haha, I was heavily debating putting another turbo LS in this thing but I decided not to. Atleast for now.

[Captainfab]

The main concern with the power valve is making sure it is of the correct vacuum rating for your engine and that it is not blown. If the carb is not equipped with a power valve blow out protection valve, once the engine backfires it will more than likely rupture the power valve diaphram. That will compound your problems.

[1986c10]

I know it had a 6.5", it might need to be stepped up a little as my vacuum was pretty high. I'll tear it apart, it backfired a few times so I'll verify its not blown. Thanks for the help.

I slapped the new holley on and was able to drive the thing to the gas station! First time it has been driven on the road in probably +25 years. I noticed that the trans seemed to be stuck in high gear. I crawl under the truck, its got a 4l60e with no electronics hooked up.... The guy I bought it from had an LS in it and couldn't get it to fire so he put this engine in. Probably left this trans in because it was easier.

[Captainfab]

A 4L60E will NOT function without a controller unless it has been converted to full manual

[LostMy65]

I'm trying to track this down on a 350.
I found this in another thread:
.
"If it backfire here's a generic rule of thumb:
If it backfires through the carb/TB it's the timing
If it backfires through the exhaust there's a valve sticking
Simple enough right?"

[geezer#99]

Close!
Backfire from carb is lean and timing retarded.
Backfire from exhaust is rich and too much timing.

What exactly is your 350 doing?

[cadillac_al]

They also backfire nicely when the spark plug wires are crossed; especially 5 & 7.

[LostMy65]

My 66 backfires through the carb.
My 65 runs perfect.
I've swapped the carbs, I've swapped the distributors. I've double checked the routing of the wires. 65 continues to run great, the 66 still backfires under load. I'm starting over. I pulled the front clip off the 66 and I'm going to double check that I'm not off a tooth at the distributor from TDC. I'm going to pull the timing cover and make sure the chain hasn't skipped a tooth. I'm going replace the wires and plugs but make sure the wires get routed correctly. Basically walk through everything again.

[LostMy65]

I forgot to mention, with the front clip off, I'm going to run a fuel line straight from a clean can to the fuel pump. I'm going to run a wire straight from the battery to the coil.
Basically, a full process of elimination.

[geezer#99]

Originally Posted by LostMy65

My 66 backfires through the carb.
My 65 runs perfect.
I've swapped the carbs, I've swapped the distributors. I've double checked the routing of the wires. 65 continues to run great, the 66 still backfires under load. I'm starting over. I pulled the front clip off the 66 and I'm going to double check that I'm not off a tooth at the distributor from TDC. I'm going to pull the timing cover and make sure the chain hasn't skipped a tooth. I'm going replace the wires and plugs but make sure the wires get routed correctly. Basically walk through everything again.

Are the motors identical?
Same intake, exhaust, headers or stock manifolds.
Does it backfire when run 1 minute, 5 minutes or 30 minutes?
Does it matter what the ambient temperature is?

[LostMy65]

Originally Posted by geezer#99

Are the motors identical?
Same intake, exhaust, headers or stock manifolds.
Does it backfire when run 1 minute, 5 minutes or 30 minutes?
Does it matter what the ambient temperature is?

The motors are not identical. But they are both sbc. 350 & 283.
And the 350 ran great before. Nothing changed with the exhaust, or with the intake.
Beings I struggled with this for awhile, I'm just going to go back to square one. I'm going to pull the valve covers and make sure every rocker arm has full travel, and every cylinder has a similar compression. I may even do a leak down test while I'm at it. It may sound overboard, but if I do figure it out, I'll be reassured of the engine's overall health.
I do know it's a very low mileage engine.

[geezer#99]

When you check the rockers check the valve stems are all the same height.
Could be a burned valve.

[LostMy65]

Originally Posted by geezer#99

When you check the rockers check the valve stems are all the same height.
Could be a burned valve.

Great suggestion. I need to check that they all spin correctly too - as long as I have open.