Rough idle... only when warm.

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Jonathan Bass

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Been trying to get ideas on what people think this issue could be. I'm lost at this point.

When cold. Car idles, shifts from park smooth, and runs well.

When warm. Idles rough, engine shaking, shifts from park hard, performance loss.

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?

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.

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.

I read on a classic Mustang forum somewhere that someone was having a similar issue and replaced their PCV valve and that fixed it. I ordered a new one, so I'll test that when I can.

But otherwise. Does anybody have any ideas? I'm frustrated.
 
Sounds like you covered all things vacuum.
What year, make, engine?
Curious, If you simply place your hand over the carb when its warm, as to choke it, does the idle improve?
Joe
 
Details man! power brakes? Slant 6 or a hemi? Somewhere in between?
power brakes my have a booster leaking vacuum.
Had a dirty carb, would run not bad until it warmed up,choke would come off and it would get rough. Pulled carb and cleaned it. Solved!
Plugs burning clean?
What do you have for a carb?
 
361/727. Power brakes yes, but I disconnected the brakes to eliminate that issue. Pretty much brand new Edelbrock 1411, I got it about 3-4 months ago. Choking the carburetor does not improve idle.
 
The thing that confuses me most is. Why would the transmission shift from park harder at lower RPM when I'm having warm rough idle, than it would at a cold start with higher fast idle RPM?

Shifts smooth first start up of day. Shifts hard when warm.
 
The thing that confuses me most is. Why would the transmission shift from park harder at lower RPM when I'm having warm rough idle, than it would at a cold start with higher fast idle RPM?

Shifts smooth first start up of day. Shifts hard when warm.
Motor mount. Trans mount. Cable shifter? Cable routed too close to exhaust. Been there...
 
Jonathan,
After installing a brand new Edelbrock last year. I had an
Intermittent backfire and ran hot, thought it was timing or a secondary ignition problem. Tried everything with no luck.
A wise ole mechanic said the carb was no good, he "didnt care if it was new".
Had it rebuilt, never ran better, smoother, cooler and stronger. Guessing it had an internal Lean issue.
Who knew?
 
Is the fuel boiling in the bowls? Did you put a heat isolator between the engine and carburetor? Have you verified the float level and fuel pressure?
 
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Carterbrocks run hot. Use a phenolic/plastic/wood or that 1/4 fiber gasket between the intake an the carb. when choked it changes the fuel circuit. Kinda sounds like your float level may be very low. Choked creates alot more vacuum so it pulls more fuel, if your float level is low, your fuel pressure to the boosters is going to be lower and its gonna run lean. When i starts to run like crap, shut it down and pull the top of the carb off as fast as you can to see what the fuel level is.
 
I do not have a carb spacer... just a gasket and adapter plate. I can get a spacer and heat insulator gasket and see what that does.
 
If I give it VERY light throttle with foot on brake while in drive, the rough shaking idle will smooth out... would that be an indication of anything? Increasing idle speed does not achieve same affect.
 
Definitly get the insulated gasket even if this isn't your issue. You have to have one on an Eddy carb.
 
What ignition you have? Also does it get progressively worse as it warms up?
 
My brother had a similar problem on his Firebird . Turned out to be the fuel pump. When hot it couldnt keep up .
Worth checking fuel pressure cold and hot ....
 
Trying the spacer and gasket today.

I have a new MSD Blaster coil, new ballast resistor on firewall, new voltage regulator, and original distributor that it had when I bought the car.

Problem existed with previous coil, ballast, and regulator.

Will check fuel pressure.
 
Trying the spacer and gasket today.

I have a new MSD Blaster coil, new ballast resistor on firewall, new voltage regulator, and original distributor that it had when I bought the car.

Problem existed with previous coil, ballast, and regulator.

Will check fuel pressure.
Cold fluid flows slower than hot fluid. (soft shifts when cold) and trying to compensate with idle speed because of a rough running motor can cause harsh shifts, so for now just deal with the running quality and the shift engagement issue will probably go away.

Sounds to me like the carb is going lean when the choke comes off as it warms up.
As mentioned above once its warmed up and running crappy try using your hand to cover part of the top of the carb to restrict the air going in.
If it runs better as you restrict it tells you it’s lean.
It also works to point out a rich condition, as the restriction causes the motor to run worse.
 
OK what ignition? Points, electronic, what make etc. Can be a component on its way out.
 
MSD Blaster 2. Brand new. Same issue with last coil. New ballast, new voltage regulator. Only thing stock is distributor. Going to replace that soon. Upgrading to electronic.
 
OK if it is points prob a condenser on its way out and if it is electronic can be a pickup in it. You have to be more specific so we can help.
 
Could be a few things:
[1] Lifter preload [ bottomed out ]. Unlikely but possible. As the engine heats up & parts expand, bottomed out lifters have nowhere to go & stop the valves from seating.
[2] Carb. Remove it & disassemble. Unlike brand H & clones, you won't need gaskets if you are careful. Blow out passages with compressed air. In the primary clusters, you will see a thick & thin brass tube hanging out the bottom. The pinched lower end of the thin tube is the idle fuel feed. Poke a piece of wire in it to make sure it is not blocked. Be careful, the boosters have a L & R, not interchangeable. Re-assemble with 7/16" float setting. Set idle mixture screws 2.5-3 turns out from seated.
With the carb back on the car, loosen the tops [ 2 turns on the screws ] on the metering rod pistons. Swivel the tops slightly so you can see the piston but it remains captured. Tighten screws.
Warm up engine & adjust mixture screws [ in gear if auto ] for smoothest/highest idle speed. Look at the pistons; they should be down & not moving. If they are moving, you need softer step up springs. If they are moving, use a thin screw driver or wire to push the pistons down. If the engine note changes, it means the pri throttle blades are open too far at idle. This can be caused by a big cam, worn cam, PCV that is oscillating & not closing [ usually caused by low vacuum from big cam ].
 
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