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Old 09-12-2007, 07:51 AM   #1
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Carburator diaphragm question

We figured out why my car was not running right, my vacuum advance hose was crimped from my vacuum gauge install . Turns out the hoses were leaking also so it prolly hasn't been working right even before that.

I got the hoses replaced and noticed the vacuum hose on the side of my carb going up to a diaphragm hooked up to the choke linkage. I figured I would see if it was working. It wasn't. Turns out the plastic casing is cracked, not sure if it has ever been in tact, it may have been cracked at the junk yard when I picked it up.

So my question is, does anyone know what it is used for?

Here is a link to my carb

The part I am talking about is number 58, it looks like with vacuum (idle) it would be forcing the choke plate on the carb OPEN. Which seems odd to me.

Any input on this would be appreciated, I have to decide whether I want to replace this or not.

EDIT: Was going to fix the spelling in the subject line but can't, you guys will have to live with my inferior spelling! I guess spell check doesn't work in that area.

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Old 09-13-2007, 04:41 AM   #2
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i'm affraigth i won't be that much help as i'm not really a carb expert myself, but i'm trying to learn what i can about them since i'm stuck with one of these wonderfull mashines myself.

on my carb (a pierburg 2e3) there's a similar setup and from what i understand about it there's something called a "thermotimervalve" that controlles the pressure on the membrame. wich like you said is there to open the choke plate once the engine has warmed up enough wich is determined by this thermotimervalve or whatever it's called.
(at least that's how i think it works)

i know that when my temperature gauge reaches a certain temperature (wich is interestingly marked on the gauge) i suddenly seem to have more power so i suppose that's when the choke fully opens.

the method to test this setup on my carb is to have the car idle and close the choke valve by hand. is should close without resistance untill there's about a 3mm gap left, that you should feel some resistance. if you can close it all the way without resistance the membrame is ruptured or there's a vacuum leak
(now that's what som repair manual says) might not be any help to you...

anyway. i suppose the best thing to do is find a replacement part or just a complete carb. i got mine for free once as the shop owner said it was taking up shelve space for years. also if you do find a complete unit, it might be best to swap the complete unit. if you pull it appart some old gaskets might need to be replaced. the replacement carb was from a different engine, and slightly different, but it works fine (even better than before.... more power + better FE )

i hope this somehow helps you


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Old 09-13-2007, 06:44 AM   #3
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On my carb the choke is opened/closed according the the heat on the exhaust manifold by a mechanical "bi-metal" spring. Then the diaphragm is also there for some reason. I plugged the hose with a pencil for now as I figured there may be a vacuum leak from the diaphragm since it is cracked.

I guess I will be doing some digging today to find out what it does. Was just going to find out if someone had a quick answer.
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Old 09-13-2007, 12:37 PM   #4
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The item your describing is an actuator for the automatic choke plate. When you open the throttle, the vacume to that hose/diaphragm pulls the choke plate open, against the thermal spring which is in the choke housing. If you don't have it hooked up, you can end up using more gas and having the car run poorly, while it's warming up, if your accelerating. The less you peddle to the medal, the less noticeable it would be.

When the choke thermal spring gets warmed up, it should open the choke plate, to full vertical, so that the choke plate doesn't cause more fuel to be drawn into the carburetor.

That carburetor design goes back a long ways. I used to have a 1953 Chevrolet with one. I ended up disabling the choke 100% and I would just pump the accelerator to get it going in the winter.
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