MGB & GT Forum

Just throwing this out there (from the MGA Forum). Has anyone successfully installed an idle up arrangement on their air conditioning installation? To keep normal idling revs at around 750rpm, the carby adjustment needs to be around 1200rpm when you switch the a/c on. Anything less than that can create stalling unless the foot on the pedal keeps the revs up.

I done quite a bit of research on early British cars with twin carbs that had A/C installed from new, but most (like the Rover SD1) had large torque engines, and the changes in revs were not very noticeable.

Cars with single carbs (Webers) are no problem, with a range of solenoids available (Edlebrock etc.)

Any suggestions?

Hi Gary,
Bruce Ibbotson has an idle up solenoid on his MGC airconditioning installation.
When the compressor is turned on a solenoid provides vacuum to a vacuum actuator to rotate the front SU throttle shaft.
The vacuum actuator is connected to the throttle shaft via an Su choke lever.

Heres a link to a thread re Bruce's A/C installation. One of the photos shows a top view of the vacuum actuator mounted on the carb heat shield. There are photos elsewhere on the www showing a front view of the Actuator with the SU choke lever attached to the throttle shaft.

https://www.mgexp.com/phorum/read.php?48,2099389

Safety fast

I just raise the idle a bit using the choke control. I have the choke set up to raise the idle before enriching the mixture.

Many US cars, from the '60s, used this system. You should be able to track down a step up solenoid at your local auto parts store. RAY

I used a generic solenoid from Edelbrock when I did AC in my TVR. You will need to make your own brackets.

Some new cars cut off the compressor at idle. Not great in parking lot traffic jams.

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Cogito ergo sum periculoso

Rebumping this topic in hopes someone will provide a good solution for an MGB.. out of the parts bin idle solenoids are not cheap from a parts store...

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David Terhune

My Blog http://mymgcars.blogspot.com/

1970 MGB-GT, 1974 MGB and 1957 MGA Coupe

On my TVR, I used the one from Edlebrock. Of course, they had the bracket that worked on the big 4 bbl. Some modern cars turn the compressor off at idle instead.

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Cogito ergo sum periculoso

For HS4’s, this works perfectly.

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1973 MGB: Blow through supercharged, intercooled, 10psi boost. VP12 cam, aluminum flywheel, 8.9:1 compression. Black label OD, Saab 15” minilites. Goes a lot better than it looks.

Using the choke is interesting.. I have HS4s... but what is meant by "I have the choke set up to raise the idle before enriching the mixture." How is this done.. maybe would be more meaningful looking at the carbs and linkage....

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David Terhune

My Blog http://mymgcars.blogspot.com/

1970 MGB-GT, 1974 MGB and 1957 MGA Coupe

Assuming you have HS4 carburetors (HIFs may be similar), the choke control performs two functions. It raises the idle speed and it enriches the fuel-air mixture. To raise the idle speed, the choke control opens the throttles on both carburetors. To enrich the mixture, it pulls the jets downward from the carburetor bodies. Usually it increases the idle speed before changing the mixture, but it should be easy to verify this. Turn off the engine and remove the air cleaners. Reattach the choke cable bracket to the forward carburetor with a couple of short bolts. Have someone sit in the driver's seat and pull the choke control while you observe. You should see the throttle linkage move before the jets move. Ask your assistant to lock the choke control at the point just before the jets begin to move. Measure the distance from the choke knob to the dashboard with your finger, and that's how far you can pull the choke out to compensate for a/c compressor drag. If you need more idle speed, you can back off the choke adjusting screws to allow more speed before the jets are pulled down. Study the linkage for a while and you'll see what I mean. You don't want to enrich the mixture at all while the engine is hot, because that will stall it.

I like my GT to idle at about 1150 rpm normally. A lower idle seems too rough. When the a/c is on, I bring the idle to about 1800. When the compressor comes on, it brings it down to 1300 or so.