Carb decision

[fireguyfire]

So I’ve got the rebuild kit for my Carter BB carb from my 1940 flathead 6; about as simple a carb as the is being single barrel, manual choke and no vacuum advance.

I’ve got a 58 and a 58 Fargo 1/2 ton parts trucks here, both with flathead 6 engines in them.

 

They are both Carter BB’s as well, but have more “modern” features being 18 years newer.

So I’ve been torn between rebuilding one of the later carbs with vacuum advance, and installing it on the 40 engine, which would mean I would have to swap the distributor as well for the vacuum diaphragm.

So my questions are, is there much benefit from going away from the factory setup to the later vacuum advance setup, and if so, would the later distributor bolt right in to the earlier engine?

 

Also wanted to say thanks to everyone for all of the great info, and to wish everyone here a safe and merry Xmas, and a happy new year!

[Los_Control]

Just curious if your 1940 6 is a 25" or 23"?

I have read that all Canadian vehicles came with a 25" motor? Was on the internet so much be true.

 

I know on my 1937 23" 218 it had a older distributor in it but it also had vacuum advance. And the old farm truck was a 1949.

Just saying I do not know what year the distributor was. It had a early design where the points plate rotated on ball bearings.

They were rusted and needed cleaning to get them operating, I did not like the way the plate was fastened.

 

A newer distributor out of a 1952 seemed like it was built much better & was my choice to use.

 

But I only worked on the 23" engine, I know they will interchange over the years. I know nothing of the 25" engines.

[fireguyfire]

All 3 are Canadian trucks, the 40,58 and 59 are 25” blocks

[Los_Control]

Well you do have them, is easy enough to pull them and compare the shaft length & gear.

I was hoping to get that info here so others that do know can answer.

 

I really would not be surprised if they will interchange.

The mopar 6's were good about sharing parts to keep production cost low. Would not be surprised if the 25" & 23" distributors interchanged with each other.

A common upgrade on the 23" distributors is to purchase a slant 6 electronic distributor. Swap the guts & shaft out and have electronic ignition on your flat 6 with parts available at local parts store. ..... They are pretty versatile.

 

 

 

[keithb7]

Looking at engine compression ratios climb over the years mentioned, I suspect the distributor’s were tweaked to account for improved pressures. Fuel improved as well. Combustion flame travel speeds changed.  More HP and torque were able to be wrung out of similar displacement motors as the years passed. I assume distributor curves were adjusted to take advantage of this new found power and fuel economy.  Were Mechanical advance weights made heavier or lighter as required? Return spring rates tweaked too I suspect? Springs on both the mechanical weights and the vacuum advance unit? 
 

I am only guessing here. Just a hypothesis.  Did they tweak these timing variables while dyno testing at the factory?  Trial and error until they nailed it. Squeezing out a sweet spot of HP and torque without pinging or too much heat?

 

Would a later 50’s distributor, stock form, be hard to tune-in on a 1940 engine? I suspect so but have never tried. I’d love to hear your tales on these topics. 

Edited December 23, 2021 by keithb7

[Los_Control]

One thing I am waiting to hear .... when did vacuum advance arrive on mopar automobiles. I suspect it was before 1940?

 

Was vacuum advance a upgrade you could select at the time? When ordering the vehicle?

 

Or is it possible it is not the original engine, a replacement engine that came from a industrial application that did not have vacuum advance?

 

I just question this because my 1937 engine did have vacuum advance .... there is more to the story here.

[Young Ed]

Should be 2 different things- vacuum advance goes on the distributor. 

[billrigsby]

 

My understanding is, at least for my era (1948)

VA was on 1/2 to 3/4 Ton, not on 1 Ton up (USA)

 

 

[fireguyfire]

2 hours ago, Young Ed said:

Should be 2 different things- vacuum advance goes on the distributor. 

That is correct, but it also means the carb has a vacuum port at the base which is missing on the non VA carbs so they are linked.

[Young Ed]

27 minutes ago, fireguyfire said:

That is correct, but it also means the carb has a vacuum port at the base which is missing on the non VA carbs so they are linked.

I'm following now. I believe you'll find the boss is there but it's not drilled on the no advance setups. 

[fireguyfire]

2 hours ago, billrigsby said:

 

My understanding is, at least for my era (1948)

VA was on 1/2 to 3/4 Ton, not on 1 Ton up (USA)

 

 

I think you have got it right!

The carb off of my original (seized) engine is long gone. I’ve sourced a good running 1942 flathead that apparently came out of a grain truck.

The carb and distributor on the replacement 42 engine do not have vacuum advance. I assumed because of this, and that both of my later flatheads do have vacuum advance that it was a later addition.

So I just went out in the snow and had a look aren’t my 1940 original seized engine, and the distributor is there, and it does have a vacuum diaphragm on the side, so the carb would have had the vacuum port if it was still there.

So I believe you are correct that tge 1/2 tons and probably the 3/4’s got vacuum advance, and the bigger trucks didn’t.

 

[48Dodger]

Vacuum source: Ported(carb)("timed vacuum") vs Manifold(fulltime vacuum) ......

 

Smog laws gave us the advance systems most understand today concerning carbureted engines.

The first advancing systems were needed(@1930's) when engines rose above @2000 rpm under load.

 

The reason? Burning of fuel moves slower than the motion of the pistons, reducing V.E. (volumetric efficiency).

Most V.E. formulas are expressed as .8, or 80 percent of fuel burned. The unburnt fuel is "raw" or pollution.

The job of the catalytic convertor is to burn that last 20 percent of fuel.

 

 

48D

[Pete]

I know that Plymouths used distributors with vacuum advance as far back as 1936. Probably earlier.

 

First thing to check on your carbs is the bore size. BB carbs came is several different bores. My '38 truck has a 1.25 inch bore and my '39 Plymouth has a 1.50 inch bore. The truck is a 25 inch Canadian engine and the Plymouth has a 23 inch engine.

 

Distributors without vacuum advance were usually used on bigger trucks.

 

Pete

 

[1949 Wraith]

Yes, your later distributor should fit your truck and probably the later carb. The distributor on my '42 Fargo developed a faulty vacuum diaphram. I switched to a later 50's industrial with no advance and it ran fine, I purchased a '56 rolling half ton and am now running that distributor with advance on my '42.

 

The carb should fit fine as only my '38 sedan has a smaller intake to fit its Stromberg carb. Worst case is would swap the intake manifold.

 

People forget that all Chrysler sixes and Desoto engines are 25" blocks and a good source for parts.

Edited December 24, 2021 by 1949 Wraith