Idle speed variations? 74 Pontiac GP 400

[Oliverd642]

Hey guys I just have a quick question about the idle and what it involves on my vehicle.

The car has been running perfectly for 3 weeks (the time I’ve owned it) today however it started and idled really low, just about stalling. I wouldn’t be surprised if it wasn’t even turning 400rpm. It stalled a few times but managed to drive it home. Any slight touch of the gas pedal brought the engine to life and it ran perfectly! Letting go of the gas pedal it would go super low in RPM and sometimes stall. It’s got the original Rochester qjet carb, I have not touched it since I got it.

The weird part, I gave it full throttle on a nice open road and after that it idled perfectly. I’m wondering where or what I should be checking to make sure the problem doesn’t come back?

 

[melsg5]

has there been any dramatic change in outside air temperature today, much warmer?
Are you familiar with how the choke works on your car?
If it's the original carburetor it is a hot air choke in all probability.
On the passenger side of the carburetor is a round plastic disc secured with 3 small plates.
By loosening the screws that hold these plates you can vary the tension of the choke spring inside the housing.
On a 72 degree day, there should be just enough tension on a completely cold engine, that after stepping on the gas pedal once, the choke has sufficient tension to completely close.
If you can take some pictures of your carburetor there may be further suggestions.

 

[Oliverd642]

has there been any dramatic change in outside air temperature today, much warmer?
Are you familiar with how the choke works on your car?
If it's the original carburetor it is a hot air choke in all probability.
On the passenger side of the carburetor is a round plastic disc secured with 3 small plates.
By loosening the screws that hold these plates you can vary the tension of the choke spring inside the housing.
On a 72 degree day, there should be just enough tension on a completely cold engine, that after stepping on the gas pedal once, the choke has sufficient tension to completely close.
If you can take some pictures of your carburetor there may be further suggestions.

Hi! So sorry, I forgot to reply! The problem I described happened the one time and never returned! Now I have something else... it runs completely fine durning normal driving but if I “kickdown” the engine will start to buck and the exhaust will pop as the revs get high. Problem with the carb? I believe it’s the original carb that came with the car. 4bbl Quadrajet. Yes it has the hot air choke and it seems to be still working just about perfectly.

 

[melsg5]

Always start with the ignition since that is what causes most problems.
Check the cap and rotor for wear and look for any obvious indications of arching inside the distributor.
Do you know how many miles are on the plugs?
Do you have an ohmmeter to test for a bad plug wire?
Make sure none of the plug wires are touching anything hot such as the exhaust manifold or rubbing against anything.

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[Oliverd642]

Always start with the ignition since that is what causes most problems.
Check the cap and rotor for wear and look for any obvious indications of arching inside the distributor.
Do you know how many miles are on the plugs?
Do you have an ohmmeter to test for a bad plug wire?
Make sure none of the plug wires are touching anything hot such as the exhaust manifold or rubbing against anything.

Okey thanks for the info! The spark plugs look fairly new but I’ve no idea about the wires, cap and rotor. Something for me to check out this weekend. Maybe it’s a good idea for me to replace all of the ignition stuff as I don’t know how many miles are on them.

 

[melsg5]

You determined they look new by removing them and checking the gap?
You also should confirm they are the correct plugs.

 

[Oliverd642]

You determined they look new by removing them and checking the gap?
You also should confirm they are the correct plugs.

I did not check the gap, I don’t even know what it should be. I’m still learning. It didn’t do this before either, it just happened yesterday. It will drive completely normal until you stomp on it and the revs get high. I’ll check the things we discussed this weekend and report back!

 

[Oliverd642]

Hello! I just got finished with replacing the distributor cap, rotor and wires. Runs like a dream now! Even my speakers have stopped crackling! Problem has gone I ordered some new spark plugs too but they haven’t arrived yet.

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[melsg5]

Excellent to hear, sounds like that was overdue

 

[Oliverd642]

Excellent to hear, sounds like that was overdue

Yep!

 

[melsg5]

Those parts are in pretty bad shape.