gearheadbill
Senior Member
I’ve been told by a life long Model A friend that a Model B Zenith carb would be considered an upgrade for an A. Can anyone confirm this? Comments welcomed. Thanks
Model B Zenith carb...upgrade?
- Thread starter gearheadbill
- Start date
Jim/GA
Well-known member
What benefit are you looking at getting from it?
If that is the only thing you change, I would not expect much of an improvement in torque or HP.
The B carb is capable of breathing a bit better with its bigger bore, but to get that benefit, you need to bore the intake runner out to match the B carb bore diameter, and you want to put slightly larger valves in. You want to use a valve cam with a B profile and lift, again to breathe better. Do all this and you gain HP. Add a higher compression head and you gain more.
This is a good example of how you need to think of an engine as a balanced system, where you change all of the various pieces in a complimentary way to get the most out of it. Changing only one thing seldoms gets you as much.
On a Model A engine, you get the biggest performance "bang for the buck" just from putting a higher compression head on, with no other changes.
Hope that helps.
If that is the only thing you change, I would not expect much of an improvement in torque or HP.
The B carb is capable of breathing a bit better with its bigger bore, but to get that benefit, you need to bore the intake runner out to match the B carb bore diameter, and you want to put slightly larger valves in. You want to use a valve cam with a B profile and lift, again to breathe better. Do all this and you gain HP. Add a higher compression head and you gain more.
This is a good example of how you need to think of an engine as a balanced system, where you change all of the various pieces in a complimentary way to get the most out of it. Changing only one thing seldoms gets you as much.
On a Model A engine, you get the biggest performance "bang for the buck" just from putting a higher compression head on, with no other changes.
Hope that helps.
gearheadbill
Senior Member
Thanks for that. Perhaps I should clarify. I know NOTHING about these antiquated old systems. I know cars but not these.
Im currently trying to use a Marvel Schleber/Allstate carb. I replaced the needle/seat and set the float using a device I purchased from Renner's. I couldn't get the float bowl to seal using a new gasket so Ive carefully gumpuckyed it with Aviation gasket cement. Sealed now for sure. Also the bowl drain wouldn't seal using the replacement gasket so I reused the original...seals fine. I reinstalled it and having fixed the bowl seal issues I noticed that no matter what I did...choke/no choke...GAV closed or open a bit... throttle set/pulled back etc.. I always end up with quite a bit of gas sitting in the bottom of the air intake forward of the choke flap when I'd crank it over.
Fooling around with it on the bench I noticed that whenever I'd blow some air into the carb inlet that the gas would come out of the angled tube, so I assume that that tube is a siphon that responds to air speed/volume and allows gas to be sucked out of the float bowl and injected into the air stream heading up into the intake manifold.
My friend told me that the 'newer' 32 B carb was abetter design and would perhaps help with the wet carb I now have. ????
Im currently trying to use a Marvel Schleber/Allstate carb. I replaced the needle/seat and set the float using a device I purchased from Renner's. I couldn't get the float bowl to seal using a new gasket so Ive carefully gumpuckyed it with Aviation gasket cement. Sealed now for sure. Also the bowl drain wouldn't seal using the replacement gasket so I reused the original...seals fine. I reinstalled it and having fixed the bowl seal issues I noticed that no matter what I did...choke/no choke...GAV closed or open a bit... throttle set/pulled back etc.. I always end up with quite a bit of gas sitting in the bottom of the air intake forward of the choke flap when I'd crank it over.
Fooling around with it on the bench I noticed that whenever I'd blow some air into the carb inlet that the gas would come out of the angled tube, so I assume that that tube is a siphon that responds to air speed/volume and allows gas to be sucked out of the float bowl and injected into the air stream heading up into the intake manifold.
My friend told me that the 'newer' 32 B carb was abetter design and would perhaps help with the wet carb I now have. ????
Synchro909
Well-known member
I agree with what Jim has said here. To get more out of an engine, there are the three "Cs". Carn, Cam and Compression. A small amount on each will give dramatic results. A lot on only one will give nothing.
MAG
Well-known member
If you can find one of these you can get the benefits of the B Carb. without boring the intaking manifold. I have one in my parts somewhere but never installed it.
Following is from Vince's site.
This particular Zenith Model 124 1/2F O-6989 carburetor does not have a Ford part number and is slightly different than the Ford production Model B carburetor.
This carburetor was specifically made for the Model A's gravity fuel system and smaller intake manifold size.
The Model 124 1/2F O-6989 venturi is a smaller diameter size number 20 compared to the larger internal diameter Model B size of number 22.
The Model 124 1/2F O-6989 float valve orifice is a larger number 54 compared to the smaller Model B size of number 44. This difference is due to the difference in fuel line pressure between a lower Model A gravity system and the higher Model B fuel pump system.
The Model 124 1/2F O-6989 main jet is a smaller number 18 instead of number 19, and the compensator jet is a smaller number 17 instead of a number 18. The Model B used the larger orifice size jets.
Following is from Vince's site.
This particular Zenith Model 124 1/2F O-6989 carburetor does not have a Ford part number and is slightly different than the Ford production Model B carburetor.
This carburetor was specifically made for the Model A's gravity fuel system and smaller intake manifold size.
The Model 124 1/2F O-6989 venturi is a smaller diameter size number 20 compared to the larger internal diameter Model B size of number 22.
The Model 124 1/2F O-6989 float valve orifice is a larger number 54 compared to the smaller Model B size of number 44. This difference is due to the difference in fuel line pressure between a lower Model A gravity system and the higher Model B fuel pump system.
The Model 124 1/2F O-6989 main jet is a smaller number 18 instead of number 19, and the compensator jet is a smaller number 17 instead of a number 18. The Model B used the larger orifice size jets.
Jack Shaft
BANNED
The model a engine suffers from a lack of fuel and compression first.One of the biggest mistakes you see is raising compression without raising the fuel air flow.does raising compression alone improve performance? yes but the engine still suffers,the engine is capable of about 160 CFM air flow stock,yet a Zenith only flows about 80 CFM,all the compression in the world doesn't change that..the B flows higher,about 90 CFM or so with the proper intake...the best?the weber 32/36 sold by the vendors or a single stromberg 97 downdraft that almost exactly matches the CFM of the engine (Cubic Feet per Minute,the measurement of air flow in an engine).Never run a high compression head with a standard model a distributor,the damaging effect of spark knock rises with compression,use a B or an FSI for precise timing at all rpm ranges
The B cam has better torque rise and builds torque to 2800 rpm instead of the A's 2400,yet still has a nice idle and is quite driveable.The stock exhaust manifolds is well suited for mild gains,provided you open the muffler side up some,using a 2" straight through resonator works well.I grew up with and respect stock engines and restored cars,nothing like one,my next build will be pure stock..but the current one is for fun,to see what a flathead model a engine with balanced performance parts will do..sanely,for longevity on the street.
The B cam has better torque rise and builds torque to 2800 rpm instead of the A's 2400,yet still has a nice idle and is quite driveable.The stock exhaust manifolds is well suited for mild gains,provided you open the muffler side up some,using a 2" straight through resonator works well.I grew up with and respect stock engines and restored cars,nothing like one,my next build will be pure stock..but the current one is for fun,to see what a flathead model a engine with balanced performance parts will do..sanely,for longevity on the street.