How can I have a smooth-running car?

May 1, 2011 | By Richard Prince


I have a 1966 Pontiac Catalina convertible with the high-compression 2-bbl. 389 cid engine and have never been able to get a smooth idle. The engine stumbles slightly and the vacuum is 17 inches at 600 rpm with the transmission in neutral. If I put the automatic transmission in gear the stumble goes away but the idle is very rough.

The correct carburetor has been rebuilt twice. The plugs, plug wires, points, condenser, rotor, and distributor cap are all new. The ignition timing is set to 6 BTDC and the dwell angle is 30 degrees. The engine was completely rebuilt about 30,000 miles ago. I have replaced all of the vacuum lines and verified correct connections. A mechanic used propane to check for a vacuum leak around the intake manifold gaskets.

I drove this car on a 3500-mile vacation trip this past spring and on the first half of the trip I had a problem with oil blowing out of the breather.

I recently checked the compression and five cylinders were at 185-195 psi, the #4 cylinder and #6 cylinder were at 170 psi and the #1 cylinder was at 150 psi. I left the compression gauge on cylinder #1 for a few minutes and it did not lose any pressure.

I would really appreciate any advice you can provide.


The rough idle and engine stumble that you’re experiencing may be related to the

somewhat low compression you’ve measured in cylinder #1. A general rule of thumb holds that a healthy engine should not have any more than a 20 psi difference in compression between the highest and lowest cylinders and you’ve got a 45 psi difference.

The lower reading in cylinder #1 can be due to any number of different causes, including poor valve sealing, poor ring sealing, a breach in the head gasket, or a damaged piston. A slightly bent connecting rod will also yield abnormally low compression.

Oil coming out of the breather indicates excessively high crankcase pressure and this is often the result of compression blow-by.

If cylinder pressure gets past the rings and into the crankcase during the compression stroke for one or more cylinders the excess pressure that builds up in the crankcase will find the easiest way out, taking the oil mist normally found in the crankcase out with it. It is likely that you have a cylinder sealing problem but before taking the engine apart you’d be wise to look at other possible sources of the problem.

You’ve eliminated most of the ignition system as a cause of the stumble and roughness by replacing all of the normal service parts.

You don’t mention anything about the distributor and excessive wear there could be causing your trouble. You also don’t mention the ignition coil. A weak or defective coil will usually cause the engine to stumble and run rough.

In spite of the fact that the carburetor has been rebuilt twice it could still be a problem. I’ve seen a lot of carburetors that were “rebuilt” by cleaning the main parts and installing all the parts in a rebuild kit, with no change in poor performance because underlying problems, such as mis-matched metering blocks, incorrect metering rods, a warped body, modified jets, etc. were not fixed.

Have you looked at fuel pressure and volume? Excessively high or low pressure can cause the engine to run poorly. A vacuum gauge can tell you a lot and though you’ve used one you don’t give me enough information to draw concrete conclusions.

Whether the 17-inches Hg you’ve measured is good depends on the cam profile and other specifics of your car’s engine. Was the needle steady at that reading or did it fluctuate? If it did fluctuate was the fluctuation regular or irregular? How, if at all, did it change with changes in engine speed? Have you tried changing the ignition timing? How do you know

that where you’ve set it, at 6-degrees BTDC, is best for your engine? A stock 1966 389 cid Pontiac calls for 6-degrees BTDC but are you sure that your engine is completely stock? It’s quite common for a non-stock cam to go in and this will change the timing specs.

Have you had the engine “scoped”? It may be worth investing in having a diagnosis done by a professional with an oscilloscope before you start taking the engine apart.