Running lean....leaking vacuum advance??

[Sam Buchanan]

For the past couple of years I've been chasing a slight random miss under light throttle between 55-60 mph. It seems applying some choke clears up the miss so I've surmised the engine is running a little lean at light throttle (high manifold vacuum). I removed the carb, checked for play in the throttle shaft and re-installed the carb paying close attention to good sealing on all the base gaskets. This did not clear up the issue.

 

Just to cover some bases I removed the distributor so I could closely inspect the points and the little lead that often chaffs and shorts against the distributor body. To check the vacuum unit I pulled some vacuum and heard air being pulled through the diaphragm. Ah Ha! Since the advance (it actually retards timing, not sure why it's called an advance....) is plumbed directly into the carb base it seems logical the mixture might be leaned at light throttle opening (high vacuum).

 

The car runs great without a functional vacuum unit so I removed the line on the carb, plugged the port and test drove. I think we have found the culprit. I use an initial timing of 10-12* BTDC so maybe that is compensating for the lack of the vacuum advance. But the engine doesn't ping under heavy load.

 

So I'm left with deciding whether to get a rebuilt diaphragm or just run with centrifugal advance only. Any one have a source for a vacuum unit that you are pleased with? Anyone have experience with leaving the vacuum unit on the shelf?

Edited March 23, 2023 by Sam Buchanan

[FarmerJon]

My car had a totally blown Vacuum advance when I got it. While age of the diaphragm was a factor, I am sure, the plate that it moves was also totally seized up with dirt and old grease. I luckily had 2 spare parts distributors that I was able to piece together one that had a functional advance. With everything clean and functioning, the car felt significantly perkier than it did before.

I built a second distributor, with dual points, for when I add my performance goods to the engine. I sent it's vacuum advance to Terrell Machine. He did a great job, and was very pleasant to deal with. Cost the same as the unknown (china) ones from the reproduction places.

 

I havent used them, but there is a shop called "Advanced Distributors" that can rebuild and set a custom centrifugal and vacuum timing curve for our distributors. 

 

Gratuitous distributor picture:

Edited March 23, 2023 by FarmerJon

[D35 Torpedo]

8 hours ago, Sam Buchanan said:

For the past couple of years I've been chasing a slight random miss under light throttle between 55-60 mph. It seems applying some choke clears up the miss so I've surmised the engine is running a little lean at light throttle (high manifold vacuum). I removed the carb, checked for play in the throttle shaft and re-installed the carb paying close attention to good sealing on all the base gaskets. This did not clear up the issue.

 

Just to cover some bases I removed the distributor so I could closely inspect the points and the little lead that often chaffs and shorts against the distributor body. To check the vacuum unit I pulled some vacuum and heard air being pulled through the diaphragm. Ah Ha! Since the advance (it actually retards timing, not sure why it's called an advance....) is plumbed directly into the carb base it seems logical the mixture might be leaned at light throttle opening (high vacuum).

 

The car runs great without a functional vacuum unit so I removed the line on the carb, plugged the port and test drove. I think we have found the culprit. I use an initial timing of 10-12* BTDC so maybe that is compensating for the lack of the vacuum advance. But the engine doesn't ping under heavy load.

 

So I'm left with deciding whether to get a rebuilt diaphragm or just run with centrifugal advance only. Any one have a source for a vacuum unit that you are pleased with? Anyone have experience with leaving the vacuum unit on the shelf?

It's called vacuum advance because it advances with vacuum... high advance at idle helps it run cooler. As vacuum drops the mech advance takes over.

[Sniper]

7 hours ago, D35 Torpedo said:

high advance at idle helps it run cooler

 

Except there isn't any vacuum advance at idle.

[kencombs]

10 minutes ago, Sniper said:

 

Except there isn't any vacuum advance at idle.

Only if using ported vacuum, tapped above the throttle plate.  If tapped below them there will be advance.  I think Mopar used ported, but lots of other 'off brands' used manifold.

My 56 uses ported so no idle advance.  

The only real benefit, IMHO, of vacuum advance on our flat sixes is fuel economy.  Without light load advance MPG will suffer.  That's why trucks seldom have vacuum advance, they seldom encounter light loading so rely on RPM to determine advance.   Lots of off road applications do the same.

[Sam Buchanan]

9 hours ago, D35 Torpedo said:

It's called vacuum advance because it advances with vacuum... high advance at idle helps it run cooler. As vacuum drops the mech advance takes over.

 

No, in this application the device retards timing under conditions of low manifold vacuum such as heavy throttle. At idle or high vacuum conditions the device is inactive and timing is dependent on static timing and the centrifugal advance. It's only purpose is to retard timing under heavy load to prevent detonation but I suspect our modern fuels make it kinda irrelevant.

 

I run 12* static timing which gets max timing with centrifugal advance where it should be. Since the engine doesn't ping under load it is happy with this "high" initial setting even though there is no vacuum retard.

[kencombs]

Can you describe your application?  Sounds unique

[Sam Buchanan]

Ken, it is totally stock per the Mopar engineers. Here is a scan from the service manual.

 

But regardless of how it works....mine is busted.  ?

 

Edited March 24, 2023 by Sam Buchanan

[kencombs]

Gotcha Sam.   We both understand how it works but describe it differently.   At cruise and moderate vacuum the timing will be advanced to promote fuel economy and cool running.  Upon application of throttle vacuum drops and the spring retards the timing.   As soon as vacuum rises advance is added.   So it does retard timing but only by removing that which it had added.  
 

too high rear gears or hot rod cams can drop cruise vacuum to the point that the advance never happens ruining mileage.    
I elected to do the slant six mod and use the later advance can.   Much more available and affordable. 

Edited March 24, 2023 by kencombs

[Sniper]

11 hours ago, kencombs said:

Only if using ported vacuum, tapped above the throttle plate.  If tapped below them there will be advance.  I think Mopar used ported, but lots of other 'off brands' used manifold.

My 56 uses ported so no idle advance.  

The only real benefit, IMHO, of vacuum advance on our flat sixes is fuel economy.  Without light load advance MPG will suffer.  That's why trucks seldom have vacuum advance, they seldom encounter light loading so rely on RPM to determine advance.   Lots of off road applications do the same.

 

Yes, I am not going to delve off into what other, lesser brand's, foolishness consists of, lol.  In our case it is ported vacuum.

 

There are some out there that claim that the only reason ported vacuum was ever used was for emissions purposes, lol. 

 

 

[Sam Buchanan]

Just to close the loop on this thread--the P15 ran nicely without a vacuum advance but I decided to install a new one since that is how the distributor was designed. I purchased a rebuilt advance from Bernbaum and even though I don't have numbers to support my observation (placebo effect?), the engine does seem to have a bit better midrange response. I didn't change the 10* BTDC static timing. The light throttle lean condition that started this thread is now gone.

[James_Douglas]

Make sure you double check the little step up piston in the BB Carb. Every year I do after it gummed up some years ago. The water vapor in the fuel causes an issue there. See my old posts on that. If it hangs up you can get a rich or lean condition.

 

James

 

 

[Sam Buchanan]

2 minutes ago, James_Douglas said:

Make sure you double check the little step up piston in the BB Carb. Every year I do after it gummed up some years ago. The water vapor in the fuel causes an issue there. See my old posts on that. If it hangs up you can get a rich or lean condition.

 

James

 

 

 

Thank you for the tip, James. I checked the piston when I was looking for an intake leak and it seems fine. But the defective advance unit was the problem, the diaphragm was completely blown out and was an open vacuum leak to the intake manifold.

[FarmerJon]

Glad to hear that it is running better!

Did you clean and check your advance plate and weights for free movement when you installed the new advance can?

[Sam Buchanan]

39 minutes ago, FarmerJon said:

Glad to hear that it is running better!

Did you clean and check your advance plate and weights for free movement when you installed the new advance can?

 

They were free, the mechanical advance was working fine, verified with timing light.