The Stovebolt Site Forums The Shop Area Engine Shop Rough / missing idle problem.

Ever since I had the 216 engine in my '41 rebuilt and got it up and running a couple years ago, it has not idled properly.

When it is idling, the engine runs rough and emits a popping sound in the exhaust. When under load it seems to smooth out and run ok.

I have replaced the plugs & wires, distributor cap & rotor, points, condenser, coil, and carburetor. I have also messed with the timing and carburetor settings, all to no avail.

It seems to get slightly worse when it has been idling for a long period of time.

Here is an audio recording: http://dl.dropbox.com/u/65606249/cough.mp3

Does anyone have some ideas on how to fix this maddening problem?

What RPM is it set at idle?

Have you take a vaccum reading?

Do you see any evidence of exhaust leaks around the manifold?

I can't get to the audio recording from here but from your description, it sounds like the timing could possibly be slow.
Reving it up utilizes the vacuum advance to bring the timing up some.
Do you have a timing light? If not, you could do as I've done for years and slightly loosen the distributor where you can turn it by hand ever so slightly while it's running to advance the timing a little, Rev it up till you hit the sweet spot then lock her down again.
Jerry W.

The rpm at idle is about 500 according to an electrical meter.

The vacuum is about 20 inches of mercury, with a slight fluctuation.

No leaks can be seen or heard near the manifold - the audio recording was taken at the tail pipe.

Reving the engine does not take away the problem. It only seems to run smoothly when under load - of course then I'm in the cab instead of listening at the tail pipe, but it seems smooth.

I already have it advanced further than the factory settings, advancing more, even all the way to knocking, does no good. If anything, running it on the more advanced side causes the problem to be slightly worse.
When it is retarded, it makes a fairly constant, spitting sound from the tail pipe, and the truck loses most of it power when driven like that. I can not find a distributor position which gives a nice smooth idle without a popping sound in the tailpipe, though its a possibility I'm missing it if the region is extremely narrow.

tight valve
vacuum leak
fouled plug

I'm back to working on this truck again.

The truck still has this same problem of making popping sounds in the exhaust when idling at any speed. When a pop is heard, a small fluctuation in manifold vacuum frequently coincides. It fluctuates down by as much as 2inHg from 20inHg. It will also suck a piece of paper into the exhaust pipe when the pop occurs. Additionally, the engine will frequently shudder slightly when there is a pop.
When driving under load, the problem does not seem to occur and the engine seems to work fine with plenty of power.

Checked for burned valves by doing a compression test. All cylinders showed 120-123psi. Can valves be burnt if the compression checks fine?

Readjusted the valves.

Checked for vacuum leaks by removing the carburetor, blocking off the place were it was, removing the pushrods, and pressurizing the manifold with compressed air. Only air leakage found was some seepage past the valves and guides. I also checked for vacuum leaks with the engine running, spraying acetylene around the joints. No change in idle noticed.

If I retarded the ignition to the point that the idle slowed considerably, the popping seemed to mostly stop but the truck then couldn't accelerate.

This is really quite frustrating. If I somehow put the camshaft in with the dots on the timing gears not matched up, could it cause this problem? Is it worth replacing the entire ignition and fuel systems? Worth removing the head to check the valves?

I had the exact same problem. Drove me nuts. It would randomly pop under idle, higher RPM's would smooth out. Did everything you did.....finally dropped in a rebuilt distributor. The drive gear on the distributor had so much slop in it it was throwing the timing off and on. Autozone carried one for under $100 after the core charge. Might want to pull the dist. and see what you have....

Hey Sput. Trust me, I feel your pain. Have a look at the posts "235 running like Crap". That's my ordeal! A couple of learnings (for what they’re worth)...
• To check the distributor for wear – take the cap off, get it to the point where the points are open and wiggle the centre shaft. How much does it change the point gap? A little is OK, but not much. Given it runs well at higher RPM, I’d be less convinced this is the issue.
• You can check that your plugs are firing consistently by putting a timing light on each lead. If the light is flashing consistently, your plugs should be firing consistently. If you find it doesn’t flash consistently, I’d check leads and plugs.
• The sound you recorded, along with the tail pipe sucking up the rag lead me to lean more toward an exhaust valve leak. If you’re sure the valves are adjusted (go with too much clearance rather than not enough), you may need to think about the head.
• When I pulled the head off mine, I could see it was in serious need of seats and guides. (It was popping exactly like yours.)
Good luck!!
Glen

Thanks for the tips, guys. I'll act on them this weekend.