Mark IV Starting Carburettor Thermostat Confusion

My Mark IV starting carburettor thermostat seems to be working in reverse. From what I understand, it provides a ground for the starting carburettor solenoid when the engine is cold. The solenoid should open and provide additional fuel for the engine to idle at approximately 1200 rpm. When the engine warms up, the bi-metallic spring in the thermostat is supposed to pull the contact away from the head of the screw which the solenoid is attached to, opening the ground circuit. With no ground, the solenoid closes and the additional fuel from the starting carburettor is cut off and the engine idles back down to about 650 rpm. Mine is working in reverse. It does not provide a ground when the engine is cold, making for extremely hard starts and warm up. I removed the thermostat and tested it with a heat gun. When cold, there is no contact, but when heated, the bi-metallic spring bends and makes contact with the screw head, completing the ground circuit. There doesn’t seem to be an adjustment or any way to reverse it. Thoughts? George

I think it sounds like a scrapper so you’ve got nothing to lose by trying to get inside to see if you can repair it.


I’ve got it removed and the brass cover removed from the back, exposing the bi-metallic spring. Nothing much there to look at. All that’s there is the head of the screw, and the bi-metallic spring with a contact on it. Doesn’t seem to be anything to repair or adjust. When I heat it with a heat gun, the spring bends and makes contact with the screw head, completing the ground circuit. It should be working the other way around. It should be making contact when cold, and then opening the ground circuit when heated. There’s no way to reverse the spring direction without drilling out the rivets, with the risk of destroying the spring in the process.

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Did it work correctly in the past for you? If not it sounds as if it is actually a thermostat intended for some other purpose. Could you post a couple of photos, one of the unit with its cover on and another showing the interior?


Include a close-up side view so we can see the profile of the spring and its relationship with the contact. The spring is a diaphragm type, not bi-metal, so it relies upon the ‘pre-set’ of the main section.

Also, can you tell if it has been opened before?

Peter L

Hey Peter S & Peter L. Yes, it previously worked okay. No, it didn’t look like it had been previously opened. Can’t do pictures right now. Car is in an enclosed trailer. With near-hurricane force winds here in Florida right now predicted to get much worse over the next couple of days, I’m not going to try to do anything with it. So more to come when the wind and rain subsides.

Okay. Power restored late last night. Still no internet…working off a cell phone hot spot. Here’s what it looks like. Contact is open with it cold. When I heat it with a heat gun, the contact closes.


I wonder if the bimetalic element has come apart at the outer end. Is it spot welded?? If the two layers were not attached it might result in the behaviour that you are experiencing. Take the element out and see if the layers fall apart. As to repair I guess you would need to find someone with a spot welder.


They seem to be completely fused together. No fraying at all and otherwise in excellent condition.

I have included a few pictures of one I rebuilt last week. This unit came in a bundle of parts and had a completely shattered insulation board. I have scraps of resin insulation board for such jobs. This item is not Mk IV or V that I’m aware of and I hope someone can advise me what vehicle it might suit. I had to re-tension the contact strip after rebuilding, to get it to switch at 35° and it is now spot-on.

Back to your problem George. I think this is the fault with yours, ie the diaphragm strip needs curving upward to ensure the contact finger is sloping down and makes firm contact. Note the upward curve in the picture and the way the finger ‘throws over’ to ‘off’ when pressed down slightly.

As a guide to bending the strip, make sure it is not bending at its base. Hold the unit with your thumbs on the outer corners and your forefingers pressing lightly on the two side parts of the strip at the base. Then lift the end slightly with your thumbs and ‘stroke’ your fingers along the two side strips, pushing down at the same time. Do this until the finger is making firm contact. Test in some hot water first - say drinking temperature. This is to confirm that it is switching off. Adjust the tension until it switches. Then test at about 35° (95°). You can use a medical thermometer for this range if you have one in the medicine cabinet.

I have a theory about the service life of these parts. In normal continual use, the contacts could oxidise like normal distributor points, and the spring tension could change, both conditions causing failure . These would have been treated as scrap and dealers would have made money on replacing these every few years.

In our hobby we are fortunate that we can open these carefully and fiddle with the cleaning and adjusting to make them good to go for another how many years. Just one hint - don’t re-fold the tabs fully after your work as you might need to open it again. Just bend it enough to lock it and you don’t need to bend all.

Thanks Peter L. I was reluctant to try bending the strip as it appears to be spring-type metal. I’ll give it a shot. I believe I found another one…somewhere in your part of the world. Now I need to get the guy to give me a price and send it to me…along with several other things he has I’m interested in. And yes, this is the second time I’ve opened it up, so I didn’t bend the tabs over completely when I put it back together the first time. The first time was for testing to try and figure out what was wrong, and now this time for pictures to post here.

Until I get it fixed, I ran a wire from the 3d carb solenoid to under the dash. I ground the wire when starting the car and then after it runs for several minutes, unground it. Yeah, I know, it’s a tractor-mechanic fix, but it works until I can either fix the thermostat or get another.

Thanks again, George

Hi George, how is your car doing after a hurricane?
Hope she is in better shape than the McLaren shown in the picture!

Minimal flooding a block down the street from me. Water probably 8-inches deep in the street. Didn’t get into any homes or garages. Mine all okay. Can’t say

the same for this Superbird. It was somewhere in the Fort Meyers area. I’ve been trying to get a hold of a friend there with a 1940 Packard. He and his wife are not answering their phones. I’ve heard of other CCCA members who’s houses and garages are under water.

Peter L. I tried bending it. After running it through a heat - cool cycle, it went back to staying open when cool. The bi-metal spring is acting like it’s got lamination separation, but it’s not visible to the eye.