The 327/350 in my 68 is running quite rough and I'm running out of ideas. Its a 57kmi original and has never been apart. When I got it I found everything to be original except for an early 70s holley spreadbore in place of the factory Qjet, and some aftermarket electronic ignition conversion.

When I first got the car home this summer, I found that it would always start but ran like crap. Once it warmed it would idle better but not great, and throttle response was horrible. I picked up a good Qjet core and sent it out to a friend of mine who is a Rochester guru, and who holds several national drag racing records running a 70 Cutlass in a stock class, with a Qjet. It came back with the correct original metering rods and factory L79 jets. Thinking I'd just pop this on and have all my problems remedied, I was disspointed. It ran about the same.

This weekend I changed plugs and wires, and checked the timing. Everything checked out OK. Next I removed and plugged all vacuum lines thinking that a leak somewhere in the system might be causing the engine to run lean. No improvement. I can get the engine to idle at around 950 RPM but the exhaust note is more like a prostocker than a stock 327. The motor does not respond to idle mixture adjustments at all. Throttle response is still horrible. Flicking the butterflys makes the engine almost stall before recovering.

What am I missing?

 

What is your compression in PSI. You may have some broken rings or some valves not adjusted correctly,

 

You have a bad vacuum leak. Since its been there since you got the car, I would suggest it could be a bad intake gasket. Use a spray bottle on stream and send a stream around the edge of the intake manifold. If the leak is external, you will see the water sucked in and the engine nmote will change. Also use it around the base of the Q-Jet. I do assume you used the correct gasket for the Q-Jet. A wrong gasket can cause a big leak. If there is no change around the perimeter of the manifold, or the base of the carb, it may be internal. An internal leak should be accompanied with smoke as sit will suck in oil to the cylinders, and burn it.

Good Luck on finding the problem.

 

Would not hurt to try a set of points to see if it is the black box ignition.

Compression check and a plug check can tell you a lot about the condition of the engine.

-Mark.

 

I agree with most of the above suggestions. Vacuum leak would be my first guess but if not, a compression test should be the next thing you try.

Good luck.

 

First thing I would do is a compression test. This sounds like a timing chain to me.

BigBlockk

Later.....

 

I've heard of people using a small propane tank with an extended burner
to run raw propane around the intake while the engine is running.

If there is an intake leak, the engine will run smooth when it sucks in the propane and burns it.

NOW, don't do that before someone who knows chimes in!!
Anyone else heard or better even done that?.....with success

 

I also have the L79 in my 68 and just had my motor completely rebuilt to factory specs as part of ongoing restoration.
I went from the wrong Holley carb to the correct Rochester and was warned that if the gaskets (metal and fiber) were not installed in the correct sequence that it would create leaks and terrible running problems.
Guess mine are on correct as it runs like a swiss watch.
You might want to make sure you have the correct gaskets, in the correct sequence and position.