Dies at idle when AC turned on and in gear???

[sick472]

85 K20, 350, auto transmission, with a Holley carb....

The truck has gotten to where it may die at a stop if the AC is on (especially prior to warming all the way up). I can compensate for this by bumping the idle screw up, but then it kicks pretty hard when put from park into forward or reverse. So, I have set the idle speed to eliminate the kicking, but cannot run the AC without the threat of it dying at a stop.

Normally, the warmed up truck will start fine without touching the pedal, but if it dies at a stop, I may have to feather the pedal to get it to start. It definitely helps to turn the AC off when trying to restart.

Where should I start trouble-shooting this problem?

[Just call me Sean]

There is supposed to be a solenoid that kicks up the idle when the a/c compressor is running. Usually missing when someone changes the carb with a non stock one.

[MJN]

Sounds like you're missing the idle speed solenoid like Just call me Sean stated.

Heres a screenshot from the service manual- when the compressor clutch is energized this solenoid is powered at the same time and the plunger pushes on the throttle raising the idle. When the compressor is off the idle returns to normal.

Summit Racing has them available, this is the Holley version
https://www.summitracing.com/parts/hly-46-74

[Dead Parrot]

AC vehicles came from the factory with the idle speed solenoid for just this reason. Often lost during a carb swap or when someone removes "All that emission crap".

That said, if it is a stock or mild cam, you should be able to get it to idle in gear with the AC on and not be too hard on shifts if everything else is in good shape.

1. Make sure your idle mixture screws are adjusted correctly. A lot of folks have them set rich because "Rich makes more power". Idle mix screws have very little to do with anything once the vehicle is moving. In many ways, you are often better off adjusting them a 1/4 turn lean as this helps keep the plugs clean during long idle sessions.

2. Make sure you have enough initial timing. Most close to stock SBC motors like lots of initial timing. 10-12 degrees BTC with the vacuum advance pot disconnected is a good start.

3. If your vacuum advance is on ported vacuum, experiment with connecting it to manifold vacuum. Ported is often the emissions setting, manifold usually works better.

When 1 -3 are finished, try your AC at idle. If this didn't fix the problem, you may need the idle solenoid.

My 75 C10 350/TH350 with AC works fine without a solenoid. Stock motor except for Performer intake and Eddy 1405 carb.

[sick472]

Great! Thanks for the inputs.

I have owned this truck for 7 years, or so, and it has never had the idle speed solenoid, but she may be getting worn out enough to need one. I'll see if I can tune the problem out first.

I have leaned out the idle mixture screws and that has helped a fair amount (Thanks DP for the tips). They were just a 1/4 turn from making it run rough, so I gave them another 1/4 turn to the lean side. This has improved the dying situation. It's still there to some degree, but much better.

I need to tweak the choke a bit to improve cold starts, or should I say summertime cold starts. It seems that Missouri seasons demands a couple choke adjustments a year and, for me, that means a week or so of dialing it in for the season. It's frustrating that the motor warms up so quickly that you only get one chance to adjust the choke before needing to let it cool back down to check the results.

I have not found the timing marks on the old engine (the bracket that should be on the timing chain cover). I think it's missing, but I'll look again this weekend. I may be just bumping the dizzy and test driving.