DHTA power valve

[TOOKOOL]

Hello, everyone

I have a 1978 x/19. It has a dhta carb. I've had it for 21 yrs. It has never been a daily driver because it always had a problem. I have always had other cars so it has never
been an issue getting anywhere. The problem I have is the power valve. The car just sits in the garage, I'll go out and warm it up sometimes, I don't drive it. Ive had 3
power valve diaphragm keeps blowing out causing fuel to be sucked into the intake manifold thereby causing a vacuum leak, will not idle. I think i'll just plug it up cause
I don't want to keep taking the carb off to install another power valve, can't do it with the carb on the car. Any suggestions, advice, warnings, etc. would be very
appreciated. Thanks guys.

Best;
Sammy

 

[Fixed Again]

Hello,
The power valve is only allowing the extra fuel to flow when there is no vacuum applied to it while the engine is cold. It shuts of when the engine is warm and the thermoswitch sends vacuum to it. So most of the time it is off and there is vacuum pulling on that diaphragm. I've had my 1979 X1/9 since it was new, and have never had a power valve diaphragm rupture. Have you rerouted the vacuum source from the original yellow valve and now sourcing it from a larger and more powerful vacuum source? Or perhaps the spring is the culprit, and it has a sharp coil end, or some corrosion, that eventually wears a hole into the diaphragm. (The vacuum is constantly pulling the diaphragm against the spring the whole time your engine is warm.)
Can't think of any other reasons for the diaphragm to fail. When the diaphragms get old and lose their elasticity they tend to just get very hard and would be less likely to rupture.
So, you could just plug up the vacuum port on the valve (as you mentioned), and if the present failed diaphragm is still able to be pushed by the spring into the ball check valve, your power valve will always be on, and constantly be flowing extra fuel, even though you only need it during cold operation. (This would be the easiest fix as you don't have to remove the carb to do it, assuming you've already determined your vacuum source isn't too robust and you've rerouted that.)
If you don't want the power valve to be on all the time, you could remove that spring, and seal off the vacuum port. But you'd have to take the carb off to get to it. But if you do that, you could check that spring for rough edges and file it smooth, or replace it, and with a new diaphragm be good to go again.
Good luck,
Darryl

 

[LarryC]

By "blowing out" do you mean it is ruptured? That's interesting as I had had a DHTA carb for decades and while I have rebuilt it a couple of times, I have never had a power valve rupture or otherwise cease functioning. I wonder what is causing that in this particular case?

 

[johnph]

This is very sad. You have a perfectly good car that you cannot use. There are little plastic 4 cornered star tabs on the vacuum port on the carb and on the vacuum source to help the mechanic hook things up properly. Fixed suggests properly tracing the Vacuum source. You may need a shop manual or service bulletin that shows how to correctly route the vacuum lines. The vacuum line diagram may be in the Owners Manual if you have that. Midwest Bayless sells manual reprints. You might contact them and see if they can send you the vacuum hose diagram. Is your car California? Does your car have air conditioning? I have a new in box Weber DHTA you can have for $50.

 

[79X1]

This is very sad. You have a perfectly good car that you cannot use. There are little plastic 4 cornered star tabs on the vacuum port on the carb and on the vacuum source to help the mechanic hook things up properly. Fixed suggests properly tracing the Vacuum source. You may need a shop manual or service bulletin that shows how to correctly route the vacuum lines. The vacuum line diagram may be in the Owners Manual if you have that. Midwest Bayless sells manual reprints. You might contact them and see if they can send you the vacuum hose diagram. Is your car California? Does your car have air conditioning? I have a new in box Weber DHTA you can have for $50.

If TooKool doesn't want that DHTA, I'll give you $50 for it. I'm rebuilding mine but it's awfully tired.

 

[LarryC]

Yes, I was going to say, the only time my power valve caused problems was when the yellow thermovalve quit working. Or you could have the vacuum lines switched. One is open after warm up and the other is closed after warm up. Very bad to switch those two! The liile star shaped things on the vacuum ports show which thermovalve to connect that port to.

 

[79X1]

Thanks for posting that, Larry. I'm about to replace my carb with the one I just bought from johnph and, while confident this is no big job, it's comforting to have the diagram so I can be sure the PO didn't get things wrong. Generally I prefer cars made before all the emission control BS, but I've taken a serious liking to the X.