Back in May I dropped a used Goodwrench SBC 350 in my chevelle cousin, a 67 Olds Cutlass. Ran well after I worked the bugs out it until last weekend. Then started backfiring thru the exhaust when cold, next day backfired thru exhaust when goosed, next day wouldnt start. I think the Q-jet is to blame. But before I cough up money for a new or rebuild what I have I did the follwing:
1. Accelerator pump squirts fuel
2. Fuel pump pumps fuel
3. Choke doesn't stick
4. Compression is okay I think (150-155) all around.
5. All plugs cleaned and gapped again. (Accel shorties at .040 because of Dynomax headers)
6. Fire to all plugs.
7. Timing still on the money.
8. 12volts in and out to HEI in both start and run.
Thats about the limits of my diagnostic capabilities, Toolwise and smarts wise.
Any other suggestions before I either tow it to the shop or drop the bucks for a Q-jet job, a new Q-jet, or stick a couple of pounds of C4 under it? Thanks.

Sounds like a timimg chain to me.I think Good Wrench used nylon chains. Your timing is gonna look fine from the crankshafts point of view.

Greg,
I doubt that it is the timing chain unless the motor has a "baziliion" miles on it.
Bad,
I hope you don't mind being corrected but timing will change if the timing chain jumps, the timing chain drives the cam, which drives the distributor.
Greg,
One possibility, is water in your fuel. It will cause the problem you are having but usually will clear up. Can you tell for sure that the fuel is the real thing ? Try squirting about a ten second shot of carburetor cleaner down the carb. If the motor starts and runs for a second or two, you have a fuel problem. This will be a hard one to troubleshoot "long distance"
Hope this helps,


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Bill Koustenis
Advanced Automotive Machine
Waldorf Md

1971 Heavy Chevy - original owner
Team Chevelle #100

Gotta be timing.

All valves are seating just fine? It will idle well enough but upon ANY throttle goosage she back-fires. what about a gradual increase? does it back fire at a certain RPM?


Advance the hell out of it, around 16 BTDC. Then see how she responds. Maybe mechanicle/vacuum advance are gone and instant throttle response is crappy do to this?? rich/lean shouldn't cause this.

you Q-jet may at it's worse may cause a very rich idle mixture (or leakage) but if the Accelerator squirts, you are fine.

I had a 750 Q-jet on my 350, it was way to much carb.

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used to be "72chevelle350TH350" the fever got me too
BB414, TH400 (is OD and 12s to much to ask?), 3.73 posi,
13.634sec @ 102.5 MPH

Picture of me roasting the tires (http://www.auburnextremeracing.org/drivers/mike/)
Video of me staging (smoke of course) (http://www.notabusinessracing.org/videos/mike_chevelle_burnout.avi)

yeah, yeah just tryin to get my # of posts up. http://www.chevelles.com/forum/redface.gif

[This message has been edited by Gene Chas (edited 09-07-2000).]

Greg, send your Q up here for a rebuild with 1lb of C-4 and we'll call it even. http://www.chevelles.com/forum/wink.gif

Dont think the carb would do this unless it began to leak bad. When you did the compresion test, I assume the plugs did not look black and sooty?

If it isn;t the gas issue that Bill raises, then, Bubba, you may have a bad exhaust valve. But the comrpession test would have indicated this.

http://www.chevelles.com/forum/confused.gif

OK, my standard cure for funky problem that come on all of a sudden - WIRES. At least take a VOM and test continuity in all your wires. Hopefully you find your dud.



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Gene #62gold/ACES
67 SS396/(427L88) (http://www.chevelles.com/showroom/chaas00.jpg)

While checking the plug wires take a look at the cap and rotor also. I had a distributer cap crack once and it sounded like a rod bearing went out from the crossfiring.

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Philip #42
Anybody going slower than me is an idiot, and
anyone going faster than me is a maniac.

Under Construction Car & Page (http://www.citlink.net/~bowtie)

You didn't mention if the thing actually had spark! Does it? I am assuming it does, but just asking for clarity.

You mention that you have an HEI distributor. I have had the modules cause your symptom. Had one in a camaro that backfired so bad it exploded the mufflers. Most of the time the modules will just die and you get no spark. Ocassionaly they will go bad but still run.

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Steve Strasemeier (70SS 396, Fathom Blue/White Stripes)

I had REAL bad gas through my 68 once...wouldn't start...would have to keep the RPMs up if it would start to keep it alive...backfired like crazy. How long has this puppy sat?


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Joe Y.
68 malibu (http://www.mindway.com/~drone)
Bloomington/Normal, IL

Thnks for all of your posts. I dont think it is the timing chain because I have compression all around.
Bill K: The ten second shot of carb cleaner and later starter fluid didn't get so much of a hiccup out of it, so now I'm leaning more to ignition problems as stated by Ignition Man and 64elcamino. I checked the spark on all wires using an inductive timing light and things looked OK. Only pulled two plugs and checked in actual fire was at them.
But the plugs and 8mm Accel wires are less than 3 months old. Or did I make an assumption? By the way I hooked the HEI uup thru a 30 amp relay as suggested by CJLandry and put a new relay on after the problem started, but it didnt solve anything. I have again 12 volts going into the relay and 12 going out in start and run. So once again I be thinking maybe its a weak spark from the HEI. Please keep on feeding me info. With your help I bet I can get it running. Thanks, again.