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361/727... Rough idle only when warm.

jmbass98

jmbass98

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When cold. Car idles, shifts, and runs well.

When warm. Idles rough, engine shaking, shifts hard.

I don't understand what would cause that difference other than the choke. Which seems to be adjusted and working properly. Anyone have ideas other than that?

Edit:

Note that no amount of timing or idle / fuel mixture adjustment while car is warm fixes the issue...

Changing from manifold to ported vacuum changes nothing either.
 
Do you have a stock exhaust manifold with a heat control valve?
 
You may have an obstruction in one of the idle fuel restrictions that the choke "covers up".
Mike
 
Typical symptom of needing a valve job. Sort of funny that this has fallen from common knowledge but if you asked this question in ‘75 you’d get that answer. If the engine is generally original it’s probably a good diagnosis too.

With what you describe I’d expect 3-6 valves wouldn’t lap in properly with compound, mostly exhaust.
 
My guess is a vacuum leak. When the choke is on it runs rich and that covers up the vac. leak. When the engine is warmed up and running rough, close the choke butterfly slowly and see if it smoothes out. If it does, start spraying some brake clean (or a flammable aerosol) around the carb base and intake where it seals to the head. A power brake booster can cause a vacuum leak as well. You can squeeze off the hose with vice grips to diagnose or pull the hose and plug off the intake port.
 
toolmanmike said:
My guess is a vacuum leak. When the choke is on it runs rich and that covers up the vac. leak. When the engine is warmed up and running rough, close the choke butterfly slowly and see if it smoothes out. If it does, start spraying some brake clean (or a flammable aerosol) around the carb base and intake where it seals to the head. A power brake booster can cause a vacuum leak as well. You can squeeze off the hose with vice grips to diagnose or pull the hose and plug off the intake port.
Click to expand...
Seems like messing with the mixture when warm would have some effect but he said it didn’t. To your point though, spraying some carb cleaner around the common leak areas would help find this if present.
 
Please elaborate/explain what you mean by "shifts hard"?
Neutral -> drive
1st->2nd
2nd->3rd

I agree with the valve diagnosis. Could be sticking or incorrect lash adjustment.
Try pulling a plug wire at a time while it's running crappy. You'll need rubber gloves.
Use one hand or insulate the fender.
 
If it runs like crap naturally you press the gas pedal more which makes the transmission shift higher and harder. The only connection between the engine and transmission that would cause it to shift hard is the kickdown linkage.
 
Hmm... I do have a ticking / clicking noise in the engine bay, would that be related to valve issues?

Would that also only cause issues while the engine is warm...?

I was thinking about replacing the heads anyway. Maybe time to do so?
 
Slowly closing the choke does not smooth anything out. Will eventually just kill the motor, obviously.
 
Sorry for the questions. Thanks for helping. Just trying to make sure I don't spend a lot of money on parts for the wrong issues.

I sprayed around for more vacuum. Found nothing. Reseated the intake manifold and carburetor. Made sure the throttle blades were closed properly in carburetor. Replaced hosing. Readjusted mixture and idle. Switched from manifold to ported vacuum and retimed. Issue still persists ONLY WHEN WARM. This is frustrating...

Could this be electrical somehow?
 
As someone said in an earlier post the problem "may" be masked by the choke.
Put a vacuum gauge on it as suggested by 493 Mike. That will tell you a lot.
Do a compression test and as Nate S said it may be valves, but that test will check valves etc.
Even though the cam may be a hydraulic lifter it is possible to over adjust them which would show up more when it gets hot.
The problem could also be in the idle circuit of thee carburettor.
Can you borrow a known good carby?
Do you still have points? Ignition can cause some weird *** issues.
Essentially you need to go through a process of elimination until you find the source or sources of the problem. It may be 2 things so keep that in mind.
 
Noise is coming from timing chain... Seems like it's going bad. Misfires unless I advanced the timing hard. Proper timing seems to show less RPM drop and smoother idle, but still rough-ish, poor acceleration, misfires, etc... Time for a new chain, it seems.
 
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