Check My Thinking..

[noahrob]

OK, here is the truck...1973 Blazer with a 383, Stage II Quadrajet, mild cam and a 4 speed, mechanical fule pump and HEI.

The issue is that my truck backfires when I have the RPM's above 3000rpm and then suddenly let my foot off the accelarator. Backfire, meaning a huge fireball shoots out of the exhasust (according to witnesses). This generally does not happen in 1st and 2nd gear, only 3rd...

1) I checked the timing first, and it is fine, running at 9 degrees, which I think should be fine...

2) Looked for a vacuum leak, but all is well there...

3) Checked the plugs and they were caked with carbon...

Conclusions:
a) Just getting too much fuel into the cyclinders and when the carbon heats up, blame there are detonations even after my foot comes off the accelator.

b) Timing chain is stretching so that when I let off th eaccelarator from under a load, the slack doesn't close the valve all the way and blame there is detonation..


I am open to any suggestions, as my brain hurts.

 

[FOR MUD]

Check your exh system for leaks , when air can get in it ignights the unburnned gas and makes it backfire.

 

[dontoe]

Running rich IMO!

 

[noahrob]

Leaks? in front of the mufflers?...I only say that becuase there is nothing after the mufflers

 

[FOR MUD]

Yes the closer to the engine the more likely it will happen.don't forget the manifold could be cracked or the gasket is leaking.

 

[Chevy305]

If you are getting detonation, I'd check your ignition system again. Check your vacuum advance and coil/ignition module. Also when was your last tuneup?

 

[noahrob]

I'll check vacuum tomorrow and tune the carb again...Thanks for the feedback, sometimes I make myself a little crazy thinking about the same thing over and over again...

 

[4X4HIGH]

Check your exh system for leaks , when air can get in it ignights the unburnned gas and makes it backfire.

BINGO, we have a winner. This is what i would HIGHLY conclude as your problem.

 

[Hogback Fabrication]

I would vote for running very rich under certain conditions. Carbs are VERY hard to get to run perfect under every load and speed.

 

[noahrob]

OK, I tightened every bolt in the exhaust system, and to "FOR MUD"'s point, the header bolts were lose. Also, I did a vacuum test and the port on the carb connected to the vacuum advance is dead at idle and then runs up to about 15 (don't know the units) as you hit the gas. The intake manifold is running about 15 at idle, the when you mash the gas it dips for a second and the shoots up to 25+. I am not clear as to what I should have expected, but this seems logical to me...

I do not know how to check the vacuum advance module on the distributor. Woudl anybody be willing to give me some instructions, please

 

[4X4HIGH]

CAhecking the vacuum advance unit is easy. Take the hose off the carb and with the engine running suck on the hose and the engine timing will change if the vacuum cannister is good. You'll know easy enough just by listening because the engine will change sounds and the RPM should go up.