DocAS

Again I need some advice from the experts out there. My car has rough idle when starting cold, just after 30sec the idle is fine. When it is getting warm idle stays fine. When it is really hot it has occasionally rough idle again, not as rough as when starting cold but rougher then in warming phase.

So far I changed vacuum hoses (1 was broken), checked carb idle settings, and checked for leaks with a starter spray.
I did not check timing but I don’t expect that to be the problem since it is running good when warm.

Any help is appreciated!

Thanks a lot!

oldcutlass

Could be the choke or carb is dripping fuel upon turning it off hot, it may need a tuneup, plugs may be fouling on startup due to flooding, etc...

Arrowstorm

My car does this also. It has a new Edelbrock 1406 carb, but I disconnected the choke because I could never get it adjusted right.

Actually, it idles rough until it reaches about 160°.

DocAS

Thanks so far!

My car has rough idle only for about 30sec after cold start (8 out of 10 times) then it idles fine. When hot it has rough idle only sometimes. So it will not get better with temp!

Carb is a Edelbrock performer 600 cfm with e choke.

I will ask to my mechanic whether he can make a tune up on the carb, although it is just 3 years old and idle mixture seems to be good!

Any more suggestions?

Thanks

CowsFlyTogether

I've experienced the same exact issue. My theory is that the fuel pump is slowly losing pressure over time so when you try the start the engine cold, there's a rough idle period in order for the fuel pump to build up pressure and also fill the bowls. Just a theory though, correct me if this is wrong

Koda

Cows, I don't think that theory flies. The car does not care if the fuel bowl is full full or partially full, and the float and needle regulate that via allowing flow in from the fuel pump. Think of it as I am drinking a drink through a straw and you are pouring more into the glass. I still get all I want provided you keep up, and you pour at wide open throttle fuel demands, so unless I am drinking at WOT as well, you'll always be ahead.

Granted, evaporation means empty fuel bowl, so the car won't start until the pump gets it there from the tank, but once it starts, it's good.

I have one car that idles a little rough when I start it cold when the outside air is cold. In summer, it idles right. Chokes can be like that.

I think DocAS's issue is choke and fast idle related for the cold starts. For the hot rough idle, I would like to know if the idle becomes rough as the engine gets hot, or is smooth until you shut it off and restart it hot. Choke and fast idle tuning for the first one, and I have an idea for the second one, but would like question answered first.

oldcutlass

If it's still runs rough after it warms, it could be choke related (sticking). Perhaps recheck your idle a/f mixture screws. Look down into the carb and see if its dripping fuel when it's idling.

CowsFlyTogether

Ah, makes perfect sense. Thanks for the clarification!

Koda

Yeah, that could well be. My thinking is I want to know if it just gets rough, which would be what your situation does, or if it's fine until shut down and restarted, which would be more of a heat soak vapor lockish thing, I think.

DocAS

Now that I read your answers I am aware that fast idle is not as fast as it used to be...so I will check choke and fast idle for the rough idle at cold start and recheck again a/f mixture at idle.

For warm idle:

Car idles fine when warming up, just after running for a while it gets rough, but not always. Your questions really made me thinking and I remember the car was always parked and restarted when I recognized the rough warm idle. But it is not limited to the time closely after restarting, it will appear also after a while.
It is also not limited to idle. When in P or N I it is also rough when raising rpm.
I do not recognize anything when on D and driving, but of course there are a lot other vibration around throughout driving.

Thanks

DocAS

Car runs fine now after:

- finally changing plug wires
- connecting choke to ign 12v under the dash, instead to the coil as the PO did
- re adjusting the idle a/f mixture

Thanks to everybody

oldcutlass

I bet the choke on the coil was the main culprit. Loading down the circuit.

White_Knuckles

Wow, talk about finding the closest 12V source! Yeah, I'm not liking paralleling the coil's primary with an electric choke either. Can you say voltage drop when the choke is energized?

Thanks PO!

DocAS

I did not measure, but I am not recognizing lights getting dimmed or stuff like that.

I will measure voltage next time I am in the garage and come back to you, but I am not expecting any severe voltage drop!

White_Knuckles

Sorry Doc, I wan't clear with that statment.

The voltage drop I mentioned was when coil power 12V was connected to your choke. When the choke is energized (creating heat) it pulls considerable current. As the choke was tied in with the coil, the coil may experience loss off power (voltage drop) where less than optimal spark may be the result? Let's say the coil had 11.5V when the choke is on vs. 14V when off?

Getting that choke tap off the coil was a good thing. Well done!

DocAS

Sorry knuckles, I missunderstood!

I did not measure voltage drop before I relocated the choke. I would have relocated it anyways for the reason you said. Since the car ran rough on idle I decided to do it along with the other things that have been necessary.

Thanks
Andi