Power valve on 79 carb

[bobcity]

I've got a problem with the power valve on my 1979 ADHA car.

It's allowing fuel to get passed the diaphragm and was sucking it into my intake manifold.

I swapped the diaphragm with one I had on a spare carb, and it started doing the same thing.

Why to I have fuel on the wrong side of the diaphragm?

I know the power valve was not on all the spider carbs, but has anyone who has had one ever had this problem?

Can anyone explain the function of the power valve?

I think that it appears that vacuum keeps the plunger from pushing on a ball and opening(?) a fuel(?) circuit. So when no vacuum is present the circuit is open.

I'm getting a hesitation when accelerating, could this be the cause?

Thanks,
Bob

[So Cal Mark]

power valves open at very low vacuum (heavy load or full throttle) and supplement floe thru the main jets. A hesitation is usually related to an accelerator pump problem. I don't recall an external power valve

[bobcity]

power valves open at very low vacuum (heavy load or full throttle) and supplement floe thru the main jets. A hesitation is usually related to an accelerator pump problem. I don't recall an external power valve

This is the image of one of the carbs, the yellow collar is on the vacuum fitting of the power valve.



If you take off the cover there is a spring, and diaphragm with plunger. Then a small steel ball that is set inside a jet and kept depressed by the plunger.

Is this the main jet?

[majicwrench]

There should not be fuel coming out of that vacuum nipple. Something is wrong, most likely diaphram.
Keith

[bobcity]

There should not be fuel coming out of that vacuum nipple. Something is wrong, most likely diaphram.
Keith

Ah ... quite right! That's why I changed the diaphragm with one the was know to not be leaking. But, It still leaked. I also switched the covers to see if there was some warpage or something. It still leaked. That's why it's blocked off now.

I'd just like to know, and maybe Mark has answered this, what exactly it does.

Is it taking main jet fuel out of the loop when vac is applied?

[majicwrench]

Going to find me a breakdown of that carb when i go back out to the shop, but just looking at it, it does not look like a power valve. If it WAS a power valve, and you had it blocked off, then yes, you are reducing the amount of fuel flowing into the engine at full throttle or acceleration. It looks to me like an auxillary accelerator pump. And there is something wrong there, and it should not be that tough to find, fuel should not be coming out of that fitting. It's not rocket science, soemthing is leaking.
Out to the shop to find a breakdown....
Keith

[majicwrench]

Found a breakdown of carb. Yup, that is a power valve. Could not find vacuum diagram that showed that vacuum nipple and line. But the power valve will function WITHOUT vacuum going to that nipple. Vacuum is supplied to that side of the diaphram thru a drilled passage in the carb. That nipple likely bled off some of the vacuum when desired by the emmision controls. But still, there should not be fuel of that side of diaphram!!! And if there is, it is likely being sucked into the engine thru the passage in the carp that supplies vacuum to the valve. Get yourself a new diaphram and be nice when tightening the screws.
Keith

[bobcity]

I think your right the only hope is to order new diaphragm, which I have done.

You've helped me to get a better Idea about how this works. It will function, as you said, because It's function, is to get fuel from the bowl and pass it to the vac accelerator and the mechanical accelerator pump. This is its normal state, full of fuel on the inside and passing it in 2 directions.

My Fiat repair manual shows the vac line to the power valve on ADHA so I know that's right.

Now when vac, is applied that flow is temporarily stopped.

Perhaps it's just not very important that this flow is shut off like this. Just some crazy pollution solution.

Also, everything you said makes sense except, on my cover that passage you referred to does not go through. I can see it drilled on one side but blocked off on the other. On the inside view it is above the screw hole, but it goes nowhere, passage deadends. Maybe in 79 they decided not to use it.




BTW, Is there any way to find out exactly how these carbs were intended to function?
Is there any weber book that would tell me about them?
Thanks a lot for your help
Bob

[majicwrench]

Bob,
Good detective work. Knowing the hole is not drilled thru makes clearer what the hose attached to the valve is for.
The power valve has fuel on one side of diaphram, and fuel on the other side. Normally, the vacuum jcomes thru the drilled hole. In this case, they did not drill the hole, and provided the neccessary vacuum via the hose and nipple. So in the case of your carb, without the hole driled thru, the valve WILL NOT funtion without the hose and the vacuum it provides.
You are not quite correct about the function of a power valve, it has nothing to do with the accelrator pump. Under normal cruise, sufficiant fuel flows thru the main jet itself. Under heavy load, like acceleration, the power valve loses the vacuum on the one side of the diaphram and then lets an additional amount of fuel into the main circuit. once you let off the gas and return to cruise, the vacuum returns and closes the valve, and the main circuit is again provide with fuel from just the main jet.
The accelerator pump deliver a shot of fuel directly into the carb venturi, to cover the sudden loss of vacuum when accelerating.
Does that make any sense?? ALL carbs work on the same principles, just different ways to get the same results.
Keith

[bobcity]

does that make any sense?? ALL carbs work on the same principles, just different ways to get the same results.
Keith

Yes! now I get it.
Thanks