B Carb on an A?

[G Baese]

Regarding the breaking of B carb mounting ears: Using a gasket that is thicker than the thin one supplied in the gasket kits, such as the laminated copper one or thicker fiber gaskets puts too much strain on the ears, popping them off as the center section of the gasket does not compress equally, especially when using the copper gasket with the compression ring around the throat. Maybe that's why none of the vendors have the B size copper gasket?

 

[Gene F]

I have a B carb, and a B intake. Seems like I get the best performance when I floor it, then after a second or two I let up on the throttle just a vary little bit. That the way a B carb works, or is something not quite right?

 

[Gene F]

I saw this on another post. Thinking about this as something to try. (change the valve)

"FYI. When I first started driving my Model A many years ago my local club experts warned me to not use the Grose Jet type of ball float valves. Their reasoning was simply based upon their own experiences. They all told me that it would limit your top end by not supplying the same fuel flow as the original Henry float valves.

I have always used the Henry valve that has been honed into the seat to make a leakless seal. Lots of miles over the years with no issues or leaks. Lots of top end smooth running speed using Zeniths, B Carb, Tillys, and Marvel Scheblers with this same float valve setup."

At any rate, I have never heard this before.

 

[BrianH]

I have a B carb on an A manifold that I had machined to match bore size and it works great.

 

[PotvinV8]

The Model A intake doesn't need to be reamed or machined to suit the Model B carb, all that needs to be done is a simple port match so the smaller ID of the Model A intake matches the venturi of the B carb.

 

[jack backer]

1.25” drill bit, square it up on a drill press way easier than a cone carbide burr on a die grinder.. set up is all the work.

 

[Don S]

Question on the b carb gav housing that is pressed in, how is it removed for cleaning out the passages where the spring is etc. ?,also my carb is the B1 with the round float,worth rebuilding ,or should I be looking for the square or rectangular float B2 ?
Thanks Don

 

[Will N]

I've always thought that the engineers who designed the Model A engine probably knew a little bit more than I do about how one thing impacts another. I suspect they intentionally chose the restricted throat size in the A carburetor for a number of reasons, one of which was to limit the max rpm. You don't want it to breath and therefore rev too freely when you don't have a counterbalanced crank and you have babbit bearings and an unpressurized oil system. The Model B engine has larger main bearings and a counterweighted crank and is therefore able to withstand higher revs, which is why the Model B carb and manifold have larger throats. I would personally not risk a stock Model A engine by putting a B carb on it. Oh, you say you won't push it and over-rev it, then why bother with the B carb?

 

[PotvinV8]

1.25” drill bit, square it up on a drill press way easier than a cone carbide burr on a die grinder.. set up is all the work.

I guess it depends on your skill level with a die grinder. Only takes a couple minutes on a bench to match the intake to the gasket. I don't like the drill bit/end mill approach as it removes more material than is necessary. When one port matches a cylinder head (OHV) to an intake, you don't machine the entire length of the intake port, just where the two components meet in order to ensure a smooth transition. That's all that is being accomplished here as well.

 

[Terry NJ]

Potvin, For what it's worth, using the gasket as a gauge, your hole is not out to size, which for me is about 1.281 after polishing. Will it make a difference? I don't know. I use a Bridgeport and a pin and get near perfect centering at the top of the hole and the bottom. The hole is about 4" deep and drills and long burrs can walk off center and come out the side of the bore. Also, I polish the manifold for the length and smooth out the side for maximum turbulence reduction. I have a extended polishing tool which does a great job of cleaning up tool marks. Also, when you are using the carbide burr, you must bore the walls straight down for the full length or you're creating a second venturi which is actually a choke point to no advantage. Otherwise it looks pretty good.
Terry

 

[PotvinV8]

Not everyone has access to a Bridgeport or even a drill press, so my response/approach was aimed towards the guy in his garage that wants to do the swap. My point is, it's super simple and does not need to be over thought. If the gasket matches the bottom of the carb and it matches the port on the intake, you're good to go. It's not rocket science.

I would argue against polishing the inside of the manifold as the casting surface helps fuel atomization and a little turbulence is actually a good thing. Take a look at the inside of this intake...

There's a reason it has that waffle pattern at the bottom.

Not trying to be a jerk, just keeping up a good conversation!

 

[jack backer]

I've always thought that the engineers who designed the Model A engine probably knew a little bit more than I do about how one thing impacts another. I suspect they intentionally chose the restricted throat size in the A carburetor for a number of reasons, one of which was to limit the max rpm. You don't want it to breath and therefore rev too freely when you don't have a counterbalanced crank and you have babbit bearings and an unpressurized oil system. The Model B engine has larger main bearings and a counterweighted crank and is therefore able to withstand higher revs, which is why the Model B carb and manifold have larger throats. I would personally not risk a stock Model A engine by putting a B carb on it. Oh, you say you won't push it and over-rev it, then why bother with the B carb?

Harold Hicks, the engineer who got horsepower up to 40 from 22 during engine development pointed out the potential for center main bearing failure.. Sheldrick ignored him, if that speaks to Ford engineering prowess.The engine even at 40hp is detuned, the Zenith flows at about 80CFM, the engine flows air at about 160 CFM.. compression was lowered for poor fuel quality and drivability. A B carb increases hp by about 10% (about 4 hp ) not enough to warrant concern.

 

[jack backer]

My skill level with a die grinder wouldn’t let me use a cone or taper the runner..the idea is not creating velocity with a tapered match port,the idea is full 1.25” flow capability

 

[Terry NJ]

Not everyone has access to a Bridgeport or even a drill press, so my response/approach was aimed towards the guy in his garage that wants to do the swap. My point is, it's super simple and does not need to be over thought. If the gasket matches the bottom of the carb and it matches the port on the intake, you're good to go. It's not rocket science.

I would argue against polishing the inside of the manifold as the casting surface helps fuel atomization and a little turbulence is actually a good thing. Take a look at the inside of this intake...

There's a reason it has that waffle pattern at the bottom. ot trying to be a jerk, just keeping up a good conversation!

Good for you! I despise jerks but love good discussions! If I'm full of ... Ah you know, say so! Ok, I know about the atomization argument, But I disagree with the reasoning. I don't think the waffling is there for that reason. I think it's some engineers brain fart thinking that the exhaust gases that I think are passing below it will warm up the gases before they get to the valves. And the waffling will help with the heating thereof . " Yeah! we don't have to do all that polishing, The rough sides will actually help with improved atomization!" Suppose we look at the shape of the flow cylinder. Suppose we then take a look at the core of this cylinder. What's doing the atomization of the fuel in the center of the flow? As far as I can see, nothing! Or it's getting batted and bashed into the walls of the manifold and the the head and lastly, the valve surfaces. Let me close by saying, for decades guys have ported and Polished manifolds to prevent there being any turbulence to slow the flow of the gases. Iv'e seen some modern cars with factory polish jobs. I'm old school ! I polish!
Terry

 

[PotvinV8]

I will say that any time one touches a manifold, head, etc. with any sort of porting tool, it can become a slippery slope! LOL

 

[jack backer]

It’s maybe a 10% increase in power.. roughly 4 hp, with an engine at 4.2 to 1 compression and Siamese intake port design the biggest benefit from a B carb is a port/starboard mounted float…so hog away with whatever you want, don’t like the results? Intake manifolds are cheap

 

[Terry NJ]

It’s maybe a 10% increase in power.. roughly 4 hp, with an engine at 4.2 to 1 compression and Siamese intake port design the biggest benefit from a B carb is a port/starboard mounted float…so hog away with whatever you want, don’t like the results? Intake manifolds are cheap

OK Jack, I hear ya! and you're correct, it is only slightly less than 10%. It really doesn't make a whole lot of difference. But as someone pointed with better numbers than I have, the model A engine is starved for gas. I've even thought seriously of boring the throat of the carburetor. Some where, Someone must have tried boring the venturi. I don't have a lathe at my disposal at present. so that will have to wait, but if anyone knows of any experiments with this, I'd like to know exactly what was done and the results.
Terry

 

[jack backer]

What I understand Terry is a stock model a engine flows air at about 170 cubic feet per minute, or CFM.
Matching CFM of carburetor to engine yields the best performance, a ‘maximum’ use of air enriched with fuel at 14 to1 ratio air to fuel,according to guys way smarter than me.

Model A zenith flows about 80CFM
Model B zenith flows about 90 CFM
Holley 94/Stromberg97 down draft V8 carb up to 175 CFM
Weber 32/36 flows about 220CFM

The Weber is offered by the vendors and is on the high side of acceptable. The Holley/Stromberg is dead on, the A+B zenith underfuels.

Now before anyone gets out of hand these numbers do not reflect increases of volumetric efficiency of the engine, or high rpm racing applications, it is a good thumbnail for figuring street efficiency/ fun..