My 1970 Olds Cutlass Supreme with 350 2bbl has had a rough idle problem for a while. The car has 202K on the original engine.

So after verifying the timing, fast idle (900), curb idle (700) and slow idle (575), I hooked up the gauges.

Compression: All cylinders read 120-130, except one which is 138. Seems OK for a high mileage engine.

Vacuum: fast idle - 18" steady needle. Curb idle - 16" slight flicker. Slow idle 12" lots of flickering.

Based on the vacuum numbers, someone suggested the car needs a valve job. Any thoughts or suggestions ? Also, what exactly causes the needle to flicker so much at slow idle ?

Ron

 

I'd look for vacuum leaks and worn vacuum lines. Some of your vacuum issue may be related to the state of your tune and possible bad settings. When was the last time it was tuned up. The other thing that comes to mind is the timing chain, by ll means with the miles on her it may be time. You compression seems consistent.

 

The chain was done about 25K miles ago. Tuneup was recent. Dwell and plug gaps were just verified.

I also checked for vacuum leaks by watching the vacuum gauge while spraying carb cleaner at the carb base and at the edges of the intake.

Ron

 

Agreed.

Compression fine.

Vacuum low.
Please review these vacuum gauge animations to give us more information about gauge needle behavior.

To check valves, you need to do a leakdown test (there are fancy tools to do the best job, but a rough and ready version may be possible using the hose from your compression tester, depending on its construction).

Please confirm dwell and timing settings, and also check timing chain (you can just feel it for slack with your finger if you remove the fuel pump, or you can put a 1⅛" socket on the end of the crank and move it back and forth while observing the distributor rotor - if the crank moves more than the tiniest bit before the rotor moves, you need a timing set.

- Eric

edit: Never mind.

 

I'm going to assume your chain is fine since it was changed 25k ago. Can you tell us what your dwell and timing settings are? Did you use a vacuum gauge or a tach when adjusting the carb for the highest vacuum or rpm? Are both the a/f mixture screws an even amount of turns out from all the way closed? If you look down into the carb while the engine is idling, is there a dribbling of fuel?

 

Dwell is 30 degrees. Timing is 10 degrees at 1100 rpm. Vacuum hose is off vacuum advance and plugged. No fuel dribble. I used a tach when setting the timing to verify 1100 rpm.

I looked at the animations, which are great BTW. The flickering mostly resembles #3, but instead of being slight, it's more pronounced +/- 2" and it's faster than the animation.

Ron

 

Have you adjusted the 2 a/f mixture screws on the carb and have you checked how many turns they are out? You may have some carbon build up on valves also. I'd add a good quality bottle of carb/injector cleaner to a tank of fuel. Follow the mix directions on whatever brand you use.