Anyone use full vacuum advance (manifold source) for the distributor?

[rnch]

My '65 352 gets it's distributor vacuum advance from above the throttle plates, on the passenger side of the carburetor.

Would tapping into a stronger, full time manifold vacuum source be beneficial for eliminating the just above idle hesitation/stumble this car has?

I notice that this stumble is less with the current vacuum advance source hooked up than without any vacuum advance.

Would more vacuum advance be better?

[Y-Blockhead]

Originally Posted by rnch

My '65 352 gets it's distributor vacuum advance from above the throttle plates, on the passenger side of the carburetor.

Would tapping into a stronger, full time manifold vacuum source be beneficial for eliminating the just above idle hesitation/stumble this car has?

I notice that this stumble is less with the current vacuum advance source hooked up than without any vacuum advance.

Would more vacuum advance be better?

Basically what you're suggesting will do the opposite what you want. When you open the throttle manifold vacuum will drop off, decreasing advance.

The ported vacuum will increase as you open the throttle and continue to increase as the flow thru the carb increases. giving you full advance.

If you have a stumble just above idle I would look at carb tuning.

[scicala]

Once the ported vacuum hole is revealed (opening throttle slightly), the vacuum there is the same as the manifold vacuum. You're hesitation/stumble may be caused by the accelerator pump linkage being sloppy or internal problem with it. You should see the accel pump nozzle under the choke plate squirt as soon as you slightly open the throttle (engine off).

Sal

[40cpe]

These are musings from my experience and thoughts. With full manifold vacuum the engine will idle faster than with ported, so you will have to reduce the idle accordingly. True, manifold vacuum will drop off with throttle opening, but the ported vacuum will never give you more than manifold vacuum anyway. Ported vacuum is metered, it starts off at low vacuum and increases with throttle opening, but never above available vacuum in the manifold. It won't take much time to experiment with it to see for yourself if it will help. As suggested above, check carburetor condition. Does the accelerator pump shoot an adequate stream of fuel?

[rnch]

Originally Posted by scicala

....You're hesitation/stumble may be caused by the accelerator pump linkage being sloppy or internal problem with it. You should see the accel pump nozzle under the choke plate squirt as soon as you slightly open the throttle (engine off).

Sal

Yes, I do recall seeing squirt gasoline when opening the throttle, when I first installed this rebuilt-by-a-reputable-ebay-seller, about a month ago.

Just now, when i shined a flashlight down into the carb, I don't see any squirts at all; not matter how many times I open the throttle or how forcefully I open the throttle.

I took the 4 screws loose and removed the cover, the spring and accelerator pump diaphragm were all in their proper places.

Could the accelerator pump diaphragm have worn out THIS quickly? I haven't driven this car more than 75 miles since installing the new/rebuilt carb.

NO WONDER it's stumbling, with no off idle accelerator pump squirt!

[Y-Blockhead]

Sponsored Links (Register now to hide all advertisements)

Originally Posted by 40cpe

With full manifold vacuum the engine will idle faster than with ported, so you will have to reduce the idle accordingly.

It's idling faster because your getting full advance at idle with manifold vacuum. With ported vacuum you get little to none advance at idle.

[scicala]

[QUOTE=rnch;1268523]Yes, I do recall seeing squirt gasoline when opening the throttle, when I first installed this rebuilt-by-a-reputable-ebay-seller, about a month ago.

Just now, when i shined a flashlight down into the carb, I don't see any squirts at all; not matter how many times I open the throttle or how forcefully I open the throttle.

I took the 4 screws loose and removed the cover, the spring and accelerator pump diaphragm were all in their proper places.

Could the accelerator pump diaphragm have worn out THIS quickly? I haven't driven this car more than 75 miles since installing the new/rebuilt carb.

NO WONDER it's stumbling, with no off idle accelerator pump squirt!




Was there fuel in the carb bowl when you took the top off ? Kind of a silly question, but thought I would ask. If there was fuel in there, and the pump wasn't squirting, there must be some issue with the pump circuit in the carb. If the diaphragm was bad, it would be leaking fuel on the manifold.
If your carb has the red rubber umbrella check valve behind the diaphragm, maybe there is something holding it open. If so, the fuel will discharge back into the bowl instead of out of the squirters.

Sal

[WestCoast]

slow timing will cause the engine to hesitate when accelerating, don't trust the timing marks on the engine, put a screw driver thru the spark plug hole and turn engine over by hand till the piston reaches top dead center, then check your timing marks

[rnch]

Originally Posted by scicala

Was there fuel in the carb bowl when you took the top off ? Kind of a silly question, but thought I would ask. If there was fuel in there, and the pump wasn't squirting, there must be some issue with the pump circuit in the carb. If the diaphragm was bad, it would be leaking fuel on the manifold.
If your carb has the red rubber umbrella check valve behind the diaphragm, maybe there is something holding it open. If so, the fuel will discharge back into the bowl instead of out of the squirters.

Sal

I removed the 4 screws holding the accelerator pump linkage on the front of the carb; not the screws holding the top of the carb. I'll do that next.

It does have the red rubber umbrella check valve behind the diaphragm.

There was gasoline between the rubber umbrella check valve and the black diaphragm; the gas didn't drip out until I took the diaphragm loose.

Perhaps a piece of dirt or trash is holding the red rubber umbrella check valve open, allowing the gas to not go into the squirters (as you indicated)?

Should I blow some compressed air around the red rubber check valve and look for trash or dirt holding it open??


(This if my first experience dealing with an Autolite 4100, as you can doubtlessly tell.)

[scicala]

The red umbrella is very soft. You will be able to roll the outer edge up to see if anything is behind it. You won't damage it unless you get brutal. An air hose might be too strong. You should be able to see an edge of the rubber being raised up before you even touch it, if it's the problem.

Sal
Sal

[rnch]

Originally Posted by WestCoast

slow timing will cause the engine to hesitate when accelerating, don't trust the timing marks on the engine, put a screw driver thru the spark plug hole and turn engine over by hand till the piston reaches top dead center, then check your timing marks

I quite agree! I did the "thumb over the spark plug opening" to find TDC, pleased that the timing marks on this unmolested, 58K car are accurate.

I started off with the factory timing setting, then played around until I found the "sweet spot" of the least amount of hesitation & smooth curb idle.

[rnch]

Originally Posted by scicala

The red umbrella is very soft. You will be able to roll the outer edge up to see if anything is behind it. You won't damage it unless you get brutal. An air hose might be too strong. You should be able to see an edge of the rubber being raised up before you even touch it, if it's the problem.

Sal
Sal

Thanks, Sal!

When the rain stops here in New Orleans tomorrow I will take off the accelerator pump linkage cover (AGAIN) and remove the carb top and see what I can find there that shouldn't be there.