little tech question

[chucks1967]

Got a 66 with a 350 CID out of a 71. Im having problems with it back fireing through the carb. I haven't messed with it yet, but this is me thinking the problems out loud. 1. Timing is to advanced, 2. I have a floating out lobe on the cam, or 3. the carb is out of tune.. most likely not the cause.. new carb same problem. any other ideas.. Im fixing to tear into it this weekend... Im tired of replacing power valves!

[Martin64]

If it were me, I'd make sure the timing in correct, then go from there.

[Franks Fab Shop]

yea sounds like timing... and check see that the plug wires are in proper order too.

[Bears63]

+1 on the timing and checking plug wires.

[Ham-er]

Plug wires correct?(1 8 4 3 6 5 7 2 ). Carb not dumping extra gas into intake?

The Yes it MUST be timing! Ignition timing or valve timing though? Or possibly both? Old chevy V-8 timing chains do stretch a bit but also the factory installed cam gear may have "fiber" teeth. Supposedly to make the engine quieter? I have found many many cam gear teeth in the bottom of the oil pan on these things. The replacement cam gears have metal teeth NOT fiber.

How is your compression?

[markeb01]

One way to check the timing chain is to set up a timing light and run the engine up and down the rpm range smoothly and suddenly. It all is well, the pointer will move up and down the timing marks smoothly. If the chain (or gear) is shot, the reading will bounce all over when the chain waves from tension to slack. You might also check the end play in the distributor shaft. I've seen some of incredible clearances.

One other thought is the distributor may have been installed 180 degrees out. The engine will run, but will also backfire like crazy.

[Corts60]

Let's not also forget that it could very well just be a lean condition.

When you set your idle/mixture screws, try and do it with a vacuum gauge. After you ensure the timing is correct, of course. Does it pop both when it's cold and warm or just cold? Could also be an improperly set choke causing the lean condition.

[Eagar1]

You may have a bent, burnt, worn or stuck intake valve, but its most likely timing, it may even be a too tightly adjusted.

[Eagar1]

Oh and bad cap, or plug wires will do the same thing.

[Eagar1]

Ummm, last thing, bad or maladjusted points if you still have those.