Carburettor vacume acuator

[DanZac]

Does anyone know anything about carburetors (I can't even spell it which show's the extent of my knowledge).

The Mrs has got a 1990 Subaru Brat which runs and idles fine, but has no acceleration. Its a twin choke carburetor with the secondary choke being vacume actuated. Having done a bit of investigation the second choke never opens up, which is why I guess it's struggling?

I took off the vacume pipe to the actuator and that operates correctly and freely, but sucking or blowing down the carburetor side it appeared blocked, which is where I guessed the problem lay.

I've stripped it down and put it through the ultrasonic cleaner and now I can suck through the vacume drilling but not blow (it's like there's a one way valve in there).

This to me seems backwards, I'm expecting there to be a vacume in the carburetor, which sucks open the vacume actuator, opening the second choke (Which is the direction it won't work in). If the carburetor was pressurised it would work, but not under vacume.

I guess the question is, am I just stupid and missing something really obvious, or can anyone point me in the direction to go to possibly fix this?

Appologies for the gibberish explanation, any guidance would be much appreciated.

 

[Erie Fred]

Try using a hand vacuum/pressure pump with a gage, might be mouth pressure & vacuum is not enough,
diaphragm has a spring behind it ?

 

[DanZac]

Try using a hand vacuum/pressure pump with a gage, might be mouth pressure & vacuum is not enough,
diaphragm has a spring behind it ?

The actuator works fine, both under mouth and vacum tester. It's the drilling into the carburetor itself that's the issue.

What sort of vacum values would I expect to see at the carburetor during operation? I'm guessing not a lot?

 

[Blobber]

Dont you get less vacuum with more RPMs? I would assume that the vacuum holds the flap shut until you get more RPMs and less vacuum to allow the flap to open??

 

[DanZac]

Dont you get less vacuum with more RPMs?

I assumed more RPM = More airflow / airspeed through the carburetor = more vacuum created. (Bernoulli theory, increased airspeed through a venturi causes reduced pressure?).

I would assume that the vacuum holds the flap shut until you get more RPMs and less vacuum to allow the flap to open??

If that was the case would'nt the flap (throttle valve) fully open with the engine off and no vacuum?

I'm in way over my head here.

 

[Gareth_16V]

It depends where in the venturi the vacuum take off is drilled, and if it's connected to both venturis. This could make it operate on a pressure difference between both chokes.
The fact you couldn't get any air movement through it previously but now you can might be a sign that something is "better".
You could also try another run through the ultrasonic, with occasional sucking/blowing through the drilling to see if it improves any further?

 

[dave h 2]

Might pay to check all the vacuum hoses for cracks and splits while the carb is off.

 

[DanZac]

It depends where in the venturi the vacuum take off is drilled, and if it's connected to both venturis. This could make it operate on a pressure difference between both chokes.

Ahhhh, now you've got me thinking and wondering. It's drilled into both venturis so a pressure differential would make sense. Further investigation on what's supposed to happen required on my part.

Might pay to check all the vacuum hoses for cracks and splits while the carb is off.

Im planning on giving everything a good clean and inspection whilst it's in bits and accessible.

 

[Blobber]

I think a lot of the cars from that period used vacuum secondaries like Pierburg, Holly, Rochester so it may be worth doing some googling to see how it works

 

[dave h 2]

No vacuum choke plates, weird thing.