now i am totally confused

[hipshot]

i have read ,in different threads and posts, that the carb. diaphragms,DO have a direct bearing on "engine idle."

i have also read, here, that the diaphragms have NO effect on idling. so which is correct?

also , today, i read a thread or two that mentioned "enrichment diaphragms"! first time i've ever heard of them. where are they located, and do THEY have any influence over "smooth idling".?

just jt

[Gearhead]

Here's what I have surmised; may be correct, may not:

 

SLIDE DIAPHRAGMS

They seal off the top of the slide from the rest of the carb. As the throttle allows more air to flow, more vacuum develops in the venturi at the bottom of the slide, which is transferred to the top of the slide through a hole. That vacuum then pulls up on the slide against the slide spring. Strictly speaking, slides, needles, etc don't affect the idle. But I'm not totally sure what the effects are of a leaky diaphragm (yeah, there's a joke here...). It would lose some of the vacuum that's supposed to be pulling the slide up. I kinda don't think it would affect the idle either, but not totally sure.

 

COASTING ENRICHMENT DIAPHRAGMS

When you let off the throttle in gear (called an overrrun), the mixture tends to go lean causing misfires; the resultant unburned fuel in the pipes is what causes the popping often associated with an overrun (most noticable with louder pipes). This doesn't really matter except for bad emissions (and would be bad for the cat converter if we had one). So for emissions reasons they installed the "coasting enrichment system", which is a second pilot air jet with a vacuum-operated, normally-open diaphragm valve. Under most running conditions (including idle, I think) there is not sufficient vacuum to close the valve, so the pilot (idle) circuit gets air from two air jets for its normal mixture. Under the very high vacuum of an overrun, the diaphragm pulls the valve closed, blocking the path of air from one of the air jets, enriching the mixture which prevents the lean condition and misfires. If the diaphragms are bad, I think it would allow the engine vacuum to leak to an open air source, causing a vacuum leak on that cylinder, and that WOULD affect the idle (carb sync and mixture). It would also prevent the enrichment system from operating correctly.

 

Personally, I don't care one way or the other if the enrichment system is working, but I do care if I have a vacuum leak, and I don't really want to spend another 100 bucks on 4 new diaphragms. I think if I were to plug the vacuum source the carb simply would leave both air jets open all the time and function fine, just as if it didn't have that system.

 

Oh, BTW, that diaphragm is on the side of the carb under a metal cap with 3 screws. Except for one of the caps, which is plastic and seems to mount some of the throttle hardware. They don't appear accessible without removing and splitting the carb assembly.

 

Jeremy

[Marcarl]

Can't argue here. It all seems to be very true.

[BradT]

Can't argue here. It all seems to be very true.

 

I thought Marca said you would argue with anyone !

 

 

Brad