My engine was idling smoothly until a couple weeks ago when I took my first extended highway trip in a while (about 50 miles). Ever since that trip my idle is rough nearly every time I come to a stop light. It runs smooth while driving, but as I come to a stop it idles smoothly for only a second or two, then stumbles pretty regularly. When I accelerate again, it smooths out, although if I accelerate slowly it continues to stumble until maybe 1500 rpm. This is only when warm; when cold the idle is smooth. However...

Also new since the same highway trip: after cold starts I hear a regular tapping noise that sounds like a valve out of adjustment and is present mainly when under load such as going uphill; that noise goes away as the engine warms up. When cold the idle is smooth. Oh, and one other note: I have some pinging (I think--it sounds like someone shaking an empty spray paint can under the hood) at high speed or hard acceleration. The pinging got better when I retarded the timing to factory spec (0 degrees for 1974) but didn't completely go away. (Side note: when I bought the car a few years ago, the timing was set to 8 degrees retarded; it runs better at 0.)

The engine is original with 85K miles, and idle rpm is 750 in Park. The car sat for many years in a garage before I had it. Compression test a few months ago was OK but leakdown test showed pretty bad leakage on most cylinders; however it was running smoothly until recently.

What to check first?

Vacuum leak. Intake gasket, carb gasket, carb throttle shaft, etc...

There's something the matter here, for sure; you should not get pinging with base timing set to 0° or even to 5° BTDC. Tune-up parts and technique suggestions in this thread. Sounds like you're due (or overdue) for a valve adjustment. Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Could also be a faulty PCV valve (that counts as a vacuum leak), and you will want to see this.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Your oil might not be flowing very well up to the valves either. I would also try a quart of Marvel Mystery Oil to help free things up.

_________________
Chuck Rivers, Webmaster

Exhaust gasket leaking, blowing HOT exhaust onto an intake pipe?? Look for exhaust stains on the intakes using a mirror and good flashlight. My 2 cents.

_________________
"Louise", a 1976 Dart Custom project, (now sadly reverted to being just an "organ donor" to our other project Darts.)

Sounds like it might be valve recession from sustained higher temps on the hwy. That makes the valve lash tight, which causes rough idle. Try a valve lash adjustment and see if it changes. Once the valve start receding, they can continue doing that faster with time. You might try cooling down the engine and/or using lead substitute, although I'm not sure the latter really will help.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Last time I had the car in the shop the mechanic said the carb gasket was leaking vacuum. He tightened it at the time but I will definitely check for leaks around the carb. I do hear hissing from under the hood but I don't notice it being louder than it always was.
I did adjust the valves a few thousand miles ago. Also, the carb is pretty new (it was installed right before I bought the car and I believe it was NOS because it came with very old-looking Holley packaging and is a nice shiny gold color). I've got the NGK plugs, new PCV valve, new crankcase breather, and new 195 degree thermostat. Plug wires had resistance within specs last time I checked. I'll keep in mind your post about cleaning out the idle circuit (is it the same for the 1945 as the 1920?). Regarding the ping, I've considered that maybe my timing mark is off, but I haven't checked that yet. The ping was always there before the rough idle started.
I sort of suspected (without having any expert knowledge to back it up) that maybe a valve was sticking, but only at low rpm and only while warm--like maybe a valve stem gets tighter when hot and doesn't quite get enough oil at idle. Is that what you're getting at?
I like checking things that are easy to check.
I can see why you would suggest that (now that I've looked up valve recession) but it seems unlikely in my case. I was only going about 65 mph max (~2500 rpm with 2.71 gear ratio and 205/70R14 tires if my math is right), and it was a cool day, maybe 50 degrees F. If that's what it is I will fix it when I rebuild the engine.

I would suspect that if a valve is sticking, it would be worse when cold. Anyway, the MMO will remove any gum or varnish on the valve stem or guide. I have seen a motor with a rough idle get cleaned up 20 seconds after putting in the MMO.

_________________
Chuck Rivers, Webmaster

I would still do valve lash again to see if that is it. I had a car that would need it every 1-2000 miles and I finally pulled it apart and some exh valves had recessed 0.2-0.3"!! It started happening faster as time went on.

Lou

_________________
Home of Slant6-powered fun machines since 1988

Probably not relevant to this discussion, but I once made the mistake on my race car of doubling up on the cheap Fel-Pro manifold-to-head gaskets in an attempt to prevent exhaust leaks from a warped manifold. Used copper-cote spray between the gaskets and on both sides. It prevented any external blowouts (for one race at least), but we discovered after blowing our last head gasket that the exhaust had actually leaked between the two manifold gaskets from the #3 exhaust port directly into the #2 intake port. The head gasket had blown on cylinder #2.

We ended up "fixing" the warped manifold by throwing it in the trash and switching to Dutra Duals, but I also only use the Victor Reinz manifold gaskets from now on. I've never had luck with Fel-Pro manifold gaskets, even on a straight manifold.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Fel-Pro manifold gaskets suck. Victor-Reinz are slightly better, but It is very much worth your while to get the good gaskets (Remflex) for the manifolds-to-head and intake-to-exhaust junctions, unless you like doing the job more than once in a row.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Did Fel-Pro gaskets go downhill? I have never had a problem with them.

_________________
Chuck Rivers, Webmaster

Fel-Pro and all the other Federal-Mogul brands (including Champion, Moog, and Wagner) have been on a downhill trajectory for years, as engineers have been replaced almost wholesale by MBAs who think of it all as generic "product" and outsource it to China.

In the case of manifold gaskets for Slant-6 engines, there is no comparison between the junk you get in the Fel-Pro kit and what you get in the Remflex set.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have similar symptoms at times , My motor likes cold as in the first start up or winter
since I have stick it's easy to work around & compensate a bad mood if it shows up
For now I am not ready to go advance repairs. what I found helps is 5 w 30 synthetic oil in winter
10 w 30 summer (less resistance) more important I take it easy . My D100 would cruse at 75 mph 4th gear / low RPM's all day . But it will react negative so I just drive mellow , the pay off is 10 fold , I just slug around town .
Being the D100 is a plus 30 year old pickup it's customary to drive it old man slow any way
Your symptoms as mine (at times) are not just from one Aha cure . My truck has 86,000 in 32 years
low use year after year has some draw backs also .

_________________
Rediscovering what has been forgotten by many
the slant 6

And the winner is...valve adjustment. At least I think so. It's running better now that I've adjusted the valves, although I think more adjustment may be needed.

The first symptom I noticed, when I originally posted this in the fall, was rough idle that would smooth out at higher RPM. But as the weather got colder, it got worse and wouldn't smooth out even at higher RPM. It seemed there was a magical combination of ambient temperature and engine temperature where it would run OK, but that combination was elusive.

Two weeks ago I finally had decent weather to do the valve adjustment, and I found each valve needed about a quarter turn looser to have the same easy-slide fit that I set last time. The engine is running much better now, although depending on the weather it can be either perfectly smooth or god-awful rough (it runs better in the morning after sitting in the 50-degree garage overnight than in the afternoon after sitting outside in the 35-degree weather all day). I'll try another adjustment, maybe to 0.011" and 0.021" just to see what happens. This time I'll also be extra careful to make sure everything is good and warm before I start.

This begs the question why did the lash get so tight? Aside from Lou's suggestion of valve recession, one possibility is: I did an upper-cylinder treatment with SeaFoam (spray it in through the spark plug holes, let it sit for a few days, go out for an Italian tuneup). The goal was to free any rings that might have been stuck, but possibly a side effect was to clean off some buildup that was preventing the valves from rising to their full height. If that's true, then the clean valves would sit tighter against the rocker arms? I'm sure the experts here can tell me if that's plausible or not.