Irregular Rough Running

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Prints Valiant

Prints Valiant

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My 1973 198ci Slant 6 has an occasional rough idle. So much so that it stops running. However if shut off for an hour or so, it runs fine -- almost as smooth as new. The fuel system is OK (carb rebuilt recently), as are the emission controls. Has new vacuum hoses, spark plugs and wires. Most of the time it idles and runs well within a minute or two of starting. Sometimes it can't be kept running. Suggestions?
 
Check first that it may be a grounding problem. Make sure your engine is grounded to the body really well, (Firewall, frame rail etc.) & make sure the I.C.U. box is grounded well too. (Run a separate ground wire for that, it's a good Idea anyway) Car may lose ground when it's warmed up, slight expansion of the metal, paint, a bit of rust, could cause the problem. Just my 2 cents.
 
Do you have a factory service manual? If not, go download one for free at mymopar.com. It has all of the correct tuneup specs. That way, you can run through everything and first verify it's all in tune and eliminate that. Next on my list would be the classic vacuum leak. They can cause all sorts of issues. But verify the tuneup first.
 
Sometimes hard to keep running when cold- your choke and fast idle cam need adjustment.
 
I wonder if the distributor cap may be cracked or corroded or the coil flaking out when warm? Ive also seen the pickup in the distributor go goofy when warm.
 
This can be so many things, it is hard to where to start.....
If the engine stops suddenly, with no change in the engine sound, I would be looking at electrical. Such as loss of electrical power, loose connections etc.
If the engine sound changes, & then dies, I would be looking at fuel. Maybe dirt/water in the carb or a jet blocked.
 
Prints Valiant said:
My 1973 198ci Slant 6 has an occasional rough idle. So much so that it stops running. However if shut off for an hour or so, it runs fine -- almost as smooth as new. The fuel system is OK (carb rebuilt recently), as are the emission controls. Has new vacuum hoses, spark plugs and wires. Most of the time it idles and runs well within a minute or two of starting. Sometimes it can't be kept running. Suggestions?
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Sounds like the ecu or the pickup is getting hot and failing. Then starts cold fine. Typical.
 
When did the problem start? Looks like a few new parts. Were these replaced trying to fix the problem, or did it start after replacing them?
 
SIX225 said:
When did the problem start? Looks like a few new parts. Were these replaced trying to fix the problem, or did it start after replacing them?
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This mechanical problem, among many others, was present when the car was purchased. After two extensive rounds of replacements (including gas filters), gas tank cleaning, and a carburetor rebuild, this running issue remains. Now work through the group's suggestions!
 
SIX225 said:
When did the problem start? Looks like a few new parts. Were these replaced trying to fix the problem, or did it start after replacing them?
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I hope you're not the type who doesn't replace anything until the car won't run. Sounds like so far everything done was normal maintenance parts. Though these days new sure doesn't necessarily mean "good".
 
Check your valve adjustment. If they're too tight, you can get a rough idle when the engine warms up. Beyond that, and making sure you don't have any funky (cracked/spongy/holed/etc) vacuum hoses, you have the correct PCV valve, etc, tune-up parts and technique suggestions in this post, and carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download.
 
I would look at the base of the carburetor when it starts running rough and see if it is freezing up, you make need to get some heat directed at the base of the carburetor.
 
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