RE: '70 Rallye 350

Hi everyone,

Before I list everything that I've done to try to get the idle less rough, let me describe the situation as specifically as possible.

*Abnormally-rough warm idle to where the entire car shakes (rather violently) in gear, less rough out of gear, and smoothes-out beautifully when idle is slightly raised. Outside of the idle issue, the throttle response is phenomenal, and it runs like a top. It just acts like a milkshake machine at idle.*




I've done the following:

1. Replaced points, condenser, plugs, wires, cap, rotor, air/fuel filters, set initial timing @ 7, & set dwell @ 32.
2. Carb. rebuilt.
3. Intake gasket replaced.
4. All vacuum hoses replaced.

I thought that maybe a previous owner may've put a lumpier cam in, but I have a pro-gas 468 Olds with a 542/560 hydraulic flat-tappet with a 286/292 duration on a 112 lobe, and it didn't shake as violently as this Rallye does. Furthermore, a radical cam wouldn't necessarily make a car violently shake as the cam characteristics eminate from the exhaust, not the engine. Since the exhaust is bolted to the car with rubbber hangers, and the engoine secured more-directly with motor mounts, wouldn't the car would be much less-susceptible to the effects of a hefty cam, and more responsive to an engine malfunction/misfire, perhaps?

I'm going to do a compression test next, but my only other thought is whether a leaky exhaust system would cause this rough idle? Almost half the exhaust is escaping before the muffler, and the mufflers themselves have holes in them. I tried the dollar-bill trick at the tailpipe, but I don't think enough exhaust is coming out to determine if I have a burnt valve; if that would be a contributor to such a violent idle.

Also, maybe it's fickle crack in the intake or near a vacuum source?

I look forward to whatever any of you can suggest; as this car is an 86k survivor, and a blast to drive, I just can't keep a topless can of pop in the car...

-Christian

 

Did your engine run like this before the tuneup? It could be numerous things but I would check the idle mixture screws and check for vacuum leaks first. Got a vacuum gauge? If so, what's it pulling?

 

split motor mounts?

 

DoubleV: Yes. Actually, it ran rougher before I did these things. I've already adjusted the mixture screws as well as identified all vacuum leaks (the biggest ones were around the intake, shich I replaced). I'll throw the vacuum gauge back on it tomorrow as post what I'm pulling.

boese1978: I did not check that. But that does raise a question; I thought that split motor mounts would only cause the motor itself to abnormally shake, and not the entire car...

 

I'm going to suggest setting your dwell at 30, bump your timing to 10 maybe more if it will take it. Then reset your A/F and idle to it's liking.