Weber 32/36 vacuum advance question

[visionaut]

3 hours ago, '76mintgrün'02 said:

There's a vacuum advance curve in the book to go along with that one.  Do you have that handy too, Tom?

 

[Dick R]

This is regarding the question about liquid gasoline migrating up the vacuum advance tube. 

 

In my opinion (without any supporting data) this isn't a problem because the tube is only dealing with vacuum pressure pulling toward the manifold, not positive pressure pushing gas toward the distributor.  Nor is there any flow since the distributor end, while allowing some movement, is pretty much capped.  

 

I don't know enough about MAP sensors to have an opinion about them other than to say looking at some googled pictures of them showed a pretty large opening to be connected to the intake manifold.  Maybe that has something to do with what you mentioned. 

 

Further opinion - I've never experienced any evidence of gas migrating to the distributor on any car, nor heard of this being a problem on anyone else's car with a vacuum advance.  I suppose that using a clear plastic line would provide some actual evidence, but it seems like vacuum lines are always black rubber or some other material that deals with a gasoline environment.   And one more thought - I've never heard of this being an issue on a vacuum gauge, either permanently installed in a car or as testing gauge. 

[John76]

1 hour ago, jimk said:

How do you guys with the vacuum advance tube connected to the manifold, deal with the liquid gasoline that migrates up the tube. 

The vacuum advance pod on the distributor is an enclosed diaphragm, so there is no flow of air or gas.  Just pressure or vacuum.

The spigots on top of the intake runners are used to connect to "dead-end" valves/diaphragms, otherwise you would have a vacuum leak.

That secondary crankcase spigot on the manifold plenum (below the carb) is a flow-through port, designed to suck fumes from the charcoal canister and valve cover...but for some reason (way beyond my comprehension) it does not create a vacuum leak.

Worst case is about 1" Hg of vacuum max! Must have something to do with the offset pull of the primary crankcase ventilation also connected to the hose that is sucked into the air cleaner. 

[visionaut]

On 3/26/2024 at 12:06 PM, Son of Marty said:

Sorta like a 2002 with duel Weber's.

I’m actually exploring running a MAP with mine so I can run a 3D timing curve… 

[jimk]

1 hour ago, Dick R said:

Nor is there any flow since the distributor end, while allowing some movement, is pretty much capped.  

 

When the throttle is quickly opened to WOT, there is an in-rush of air/fuel into the tube towards the distributor.   Later when the pressure in the manifold decreases under partial vacuum after the fuel has dropped out of suspension and/or recondenses due to the increasing pressure, the air moves slowly towards the manifold and the fuel is left hanging on the tube walls.

 

Go for it and report back in about a year of the liquid in the tube/dashpot.

[jimk]

1 hour ago, John76 said:

The vacuum advance pod on the distributor is an enclosed diaphragm, so there is no flow of air or gas.  Just pressure or vacuum.

If there is no flow into the tube, the diaphragm wouldn't move.

[Dick R]

Quote

Go for it and report back in about a year of the liquid in the tube/dashpot.

Another year for me will add up to about 7 years of manifold vacuum - maybe more.  The tube remains bone dry.  I didn't take the distributor off to attempt to shake out any liquid therein. I'm satisfied that no gas has or will migrate to it. 

[TobyB]

I never found fuel in the diaphragm- while technically it's a possibility, it's never been a practical concern.

 

There are several vacuum pods out there.  The 6 degree one's quite common, and if I recall, has 2 different springs.

 

There is one with more travel (10 degrees?) and it has an intermediate spring with pressure between the 2 6 degree springs.

I never had a fully functioning push- pull pod to test.

 

I think I like running less advance at idle than others do...

 

t

never was all that much bothered about most of it.

[John76]

19 minutes ago, TobyB said:

I think I like running less advance at idle than others do...

Toby, 

You probably run richer mixtures and higher compression than some of us (non-racecar folks). 

I like the analogy of a densely wooded forest (rich mixture) will burn much faster in a wildfire than a sparsely wooded forest (lean mixture).

John

['76mintgrün'02]

 I found this set of motorcycle tuning vacuum gauges at the Habitat store and borrowed two of them to put in my console.   
 

 

 

I found a piece of already-black aluminum in the scrap bin, trimmed it a quarter inch shorter and drilled four holes to mount them, using the original as a template. 

 

 

It tucks in nicely between the console and dash with the ashtray removed. 

 

 

Now I can watch ported and manifold pressure at the same time. I also reinstalled the gauge under the hood and moved the switch to the middle bracket.  It is T-ed int the distributor feed, showing manifold vacuum. 
 
   
 

I’m looking forward to today’s 100 mile test-drive. 
 

Tom

 

[John76]

3 hours ago, '76mintgrün'02 said:

I’m looking forward to today’s 100 mile test-drive. 

Hey Tom,

Let us know your manifold vacuum reading at idle rpm (fully warmed-up).

Would also like to see if ported and manifold match at any rpm above idle.

Thanks, and happy motoring.

John

[1lostcause]

I've been following this thread with interest since my car is almost an exact copy of thehackmechanic car. Bone stock 73 except for a 32/36. I had the same dead and worn out retard distributor and swapped in a repaired 71 advance style. Being an old Mopar guy I used ported vacuum since that's how they were set up. After reading the article Dick R posted I decided to switch my car to manifold vacuum. Now granted this is seat of the pants but wow what a difference. It idles better and pulls a whole lot better through the gears. I had to richen the idle screw a turn and a half but did not touch the timing. It's still at 35 total. I plan on switching my old Mopars to manifold vacuum. 

[rstclark]

DickR is right

['76mintgrün'02]

There is a lot of overlap between m&p.
 

P cuts out during deceleration (foot off the gas) while m pulls full advance. 
 

It has 19”at idle. 

 

Based on the new tool I found today at the Habitat store, I have perfect carburation.  

                         

The AFR was just over 12 when I first did the distributor swap but it climbed up over 13 and started hunting, so I also had to enrich the mixture.  It seems odd that it changed. 

Edited March 30 by '76mintgrün'02
Added a photo

['76mintgrün'02]

Okay, I'm glad I added the photo of the gauge under the hood, fuzzy as it is.  That red wire with heat shrink on the end fell off of the vacuum switch, cutting off the signal to the distributor.  That's why the idle AFR changed.  I'll bet it will read too-rich once I reconnect it. 

 

I did not need to adjust the idle mixture; which was a bit of a surprise, since I'd swapped distributors and switched to manifold vacuum at the same time.  Advance at idle wasn't drastically different between the two, (but one was static and the other vacuum).

 

Moving the switch over used up all the slack in my vacuum lines as well as that wire.  I'll have to fix that.  

 

Sometimes it seems like there's no end to the demands of these pesky old cars!