Hi. I have a 72 cutlass 350 with a Edelbrock 600cfm and Edelbrock performer air intake. I'm not sure how to explain this but the engine idles smooth for about three or so seconds then has a little shimmy ( a quick little shake ) and back to smooth for three or so seconds and shimmy and so on. Any ideas? I feel I have my idle a bit high but if I bring it down the shimmy increases. I don't have a tach. I changed the motor mounts a while back am gonna check if the nuts are loose but I really don't think that's it. The carb is cleaned and tuned as well as I can from watching videos. Could it be a vacuum leak from the intake?

 

Are the fuel line rubber connections from the tank to pump in good shape? Could be sucking air or a bad fuel pump?

 

I merged your threads.

You mention you don't have a tach. How did you set the carburetor when you bought it? To properly tune one of our cars you at least need a tachometer, vacuum gauge and timing light.

 

It could be a lot of things, perhaps post what your tune up settings are and what distributor your running. Engine rocking is usually caused by misfires.

 

Agreed likely misfires. Does it pop out of the exhaust?

Basic things first as everyone here stated in this order:

1. Check all fuel & vacuum lines, carb base gasket, and PCV for proper function.
2. As Eric alludes verify timing and air/fuel mixtures are within spec.
3. Inspect plugs, wires, cap, rotor, point set, and condenser. Open hood at night to look for ignition arcing.
4. Disconnect everything from the carb...vac advance, PCV, brake booster, etc..eliminate all of the ancillary parts from the equation. Then see if it still does it? Might need to adjust idle up due to reduced timing with vac adv disconnected.
5. Hook a vacuum gauge to a direct vac source off the carb base or intake and tell us what it looks like.
6. Do the propane test to find carb base gasket and or intake gasket leaks.

Basic tools needed
1. Backset timing light
2. Vacuum/pressure gauge
3. Tach & Dwell meter.
4. Berszomatic propane torch.
5. Fire extinguisher and familiarity on its use.

 

Wellll the carb was tuned using online instructions from Edelbrock online. Will buy a tac for sure. As far as timing I have it have timing light and have it advanced as far as I can get it with the car sounding good. No backfires. I will look into this propane test, I put the intake on and have been second guessing my work. I guess a better description of what the car is doing is it feels a pulse. A pulse of super smooth and a pulse of a little shake.

 

My car does pretty much the same thing. I suspected a vacuum leak but I have covered all the bases and don’t find one. I found evidence of oil in #7 and now I have another intake gasket set on the way. Just did it in the fall and it didn’t go as smooth as I hoped. Disappointed but not surprised.

 

When the car is idling, peer down the throat of the carb and look for fuel dripping out of the venturi

 

Good point. This will cause erratic idle problems, more so when it's hot. So will a warped carb.

Today's fuel combined with heat soak can force gas to burbble out the vent or other passages.

The secret is to reduce the float level 1/32 to 2/32". Then take corrective steps to reduce heat soak.

Both of these are more technically advanced troubleshooting corrective solutions.

First, eliminate all the easy stuff I mentioned in post #5