MGB & GT Forum

There was some discussion while back of idle speed dropping toward stalling when the a/c compressor kicks in. This concerns cars with a/c installed, of course. This typically happens when you suddenly come to a halt (getting off the freeway or at a stop sign). The idle speed drops so fast the car stalls. The solution recommended by some was to install a "throttle solenoid switch" of some kind. Apparently (if I understand this correctly) this will kick in when the a/c compressor turns on and it will somehow physically raise the idle speed, though I can't imagine how that works on an MGB. What does it push? Wouldn't it have to contact the throttle lever on the carbs somehow?

Has anyone done this?

Looking at throttle solenoid switches on Amazon, at least one of them is described as not having nearly enough force to make the idle speed increase while another gets better reviews but costs about $150. Here's the one that gets rated well but is expensive:
http://www.amazon.com/Edelbrock-8059-Solenoid-and-Bracket/dp/B00062YCWY/ref=sr_1_3?s=automotive&ie=UTF8&qid=1442809690&sr=1-3&keywords=throttle+solenoid

Where do you even attach such a switch? I'm confused. Currently, I use the edge of my shoe on the gas pedal (the rest of my shoe is on the brake pedal) or pull out the choke partway. Neither works well and both are awkward. I realize these cars weren't exactly designed for a/c, but the Moss system I have installed works great. Except for this one stalling problem which happens in very hot weather when I come to a stop and the car stalls.

---

Drew Maddock, So. Calif. USofA



Edited 1 time(s). Last edit at 2015-09-20 11:29 PM by DrewM.

Drew,

There's a useful reference on air conditioning MG's from Bruce Ibbotson (PFT 000). Bruce is a member on the MGC forum.

Here is the link to his article re. Fitting air conditioning to his MGC;

http://www.them-g-c.com/index_files/Page2706.htm

Bruce, used a vacuum actuator to lift the choke lever at idle.

Try searching "idle control solenoid " on Google

Re attachment options, the link shows a photo with a vacuum actuator attached to the heat shield.
.
Safety Fast,
Neil

Thanks, Neil. What a great article. I'm in awe of that level of ability, cleverness, and just plain tenacity to make an MGC into virtually a modern car. His ideas about installing an a/c throttle solenoid are what I'd like to do on my car . . . somehow. I may need to talk to an a/c shop to see if they can think this out a bit with me as to where you install the plumbing, etc. and how it would operate.

Sure wish it was a simpler plug in option rather than inventing a whole new way to do something simple like raising the revs when the a/c compressor turns on. A modern a/c system on an old car like this has some minor issues like this problem of stalling under certain conditions. Using the choke lever sort of works but if you forget, you stall. Coming to a stop, I don't much like to have to reach for one more lever as I'm shifting, braking, looking at traffic, etc. It ought to happen automatically.

---

Drew Maddock, So. Calif. USofA

Put one on my TVR. Worked OK. ( 5.0, Edlebrock carb) Another thought is if you have a fully programmable ignition system, you put a little trough right at the preferred idle speed. So, if it slows down, it gets more advance and is more efficient, speeding up. If it over shoots, it runs out of gas and slows down. That's the theory at least. You would want a cranking retard though.

---

Cogito ergo sum periculoso

Drew, I can't remember what kind of carb you have? I was looking for the same info a while ago and ran across this for a Weber:

http://www.jeepforum.com/forum/f12/those-ac-weber-carbs-quick-fix-idle-speed-1188723/

there is also a thread somewhere on here with this info, but i can't find it at the moment (i'm sure i have it bookmarked like the above). it explains and shows pics of a vacuum advance, different than the one pictured in the above article. hopefully someone will post it

i should add that i USED to have this issue, but no longer. after a carb rebuild, new plugs, adjusting my mixture/timing and cleaning my air filter, i only drop about 200 rpm. i don't hesitate to turn on my system anymore and anytime. the attached pics show the difference between off (1st pic) and on (2nd pic)



Edited 2 time(s). Last edit at 2015-09-21 11:42 PM by curtis7420.

Curtis, it's a '74 MGB roadster with HIF 44 carbs.

That's a neat way to do a Weber carb on that Jeep article you linked to. And best of all I think I understand it! A lot of this stuff I don't really understand well yet. I'm not quite sure why and how power is sent from the a/c compressor when it turns on, to a vacuum or solenoid switch to blip the throttle up. The rest, it seems to me, is just mechanical. Things like finding some way to attach the plunger, pin, or hook on the solenoid to the throttle lever so it pushes/pulls to rev up a bit. And finding a place to attach the solenoid which would have to be near the linkage. Would you want to have a solenoid pull up the accelerator lever like the throttle cable does? That would mean installing it above the throttle cable. Or would it be better for it to push up the lever from below? And so on. And wouldn't the solenoid have to be pretty strong to pull the MG's throttle cable? When I lift it with my finger it takes some effort.

Not sure if an a/c shop or a British car shop would have a more experienced and logical answer. The a/c shop might know how throttle solenoids work, but the MG shop would know how an MG runs best. I'll ask around when I get time.

My carbs were rebuilt (by me) about a year ago, and everything else (air filters, etc.) is in pretty good shape. Maybe I should look into "new plugs, adjusting my mixture/timing and cleaning my air filter" more, though. Until recently, when the a/c compressor turned on, my revs only dropped a little -- just like yours -- from about 1000 to 800. Now I'm getting occasional stalling. Maybe all I need to do is change plugs and clean things up more to avoid stalling? That's a lot easier than inventing an entire compressor powered solenoid system. Maybe that's the place to look first? If a new set of plugs solves this problem, that would be kind of neat.

---

Drew Maddock, So. Calif. USofA

Drew,

Here is a link I found while searching for information on my air conditioning install in my GT. There is a little write up on a solenoid and a picture of it on his car. My installation is still not finished yet, so I may have to look into what he did. Here is the link to his pdf.
http://www.fbccsc.org/tech/coolgt.pdf Hope this helps.

Ken

Ken, thanks for that link. I'd seen it before, but forgotten about it. The small pic of his solenoid is interesting as locating it somewhere had always confused me. Apparently right below the accelerator lever would work fine if a bracket was made to hold that solenoid there. Good stuff.

---

Drew Maddock, So. Calif. USofA

Drew, did you ever figure out how to increase your idle speed with the A/C on. I have exactly the same problem and even though I have set the idle at 1200 it still cuts out occasionally and especially at high elevations. I have a 74 B GT and the A/C works great and it was a kit from Retroair in Texas.

I agree that an idle up solenoid is the answer but have you worked out which one and how to install it.

Thanks Andy

You could use a cable I guess. I used the Edlebrock one on my TVR, but then again, I had an Edlebrock 4bbl.

---

Cogito ergo sum periculoso