Zenith throttle plate modification

M

mcgarrett

Well-known member
This might sound a little odd to some here on the Barn, but I was rebuilding some Zenith carbs over the weekend and while re-installing the throttle plate I had a flashback to a modification we used to make on the throttle plates of Holley Bug Spray carburetors we used on VW dune buggies. Since the throttle plates were thick material like the Holley, the modification was made to enhance the throttle transition from idle to acceleration by "knife edging" the throttle butterflies. Many of the performance-minded buggy owners made that modification and were convinced it helped throttle response. I was wondering if that would do anything for our beloved Model A's? Has it been tried? Please forgive the mental gymnastics for a moment...old guys do this!:rolleyes:
 
In short, a sanding belt was used to remove material from the throttle plate as illustrated below. Since the throttle plate material is thick and squared-off the thought was that the blunt edge created some unwanted turbulence as air tried to make it past the thick slab edge of the plate when the throttle was opened. By "knife edging" the leading edge of the plate it would make it more aerodynamic and reduce that turbulence. The "knife edging" needs to be done on the side of the plate that lifts away from the carb throat as the throttle begins to open...you get the idea. I'm thinking I might sacrifice a throttle plate to see if it makes any difference. I'll admit the Holley Bug Spray carbs were a little more sophisticated (they are simply modified Holley 94's) but the principle seemed like it might work on the Zenith. I'm sure I'll have to experiment to see if it works at all, but should be interesting to see what happens.
 

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Wouldn't that turbulence also help atomize the fuel better? What about the air/fuel coming through the other side? Isn't the edge on the Model A throttle butterfly beveled, not a square edge. Would the relatively small diameter of the butterfly even make a difference by doing this?
Not pooing the idea just asking questions.
 
The biggest hindrance to snappy throttle response in the Model A is its 60 pound flywheel. Newton's first law....
 
I'm not sure anything would be gained by making the modification, but I'm gonna give it a shot to see what happens. Will N is right about that 60 lb. flywheel being a deal breaker as far as throttle response! :D
 
when the sides of the throttle plate is worn next to the shaft the plate has to be closed too much to get slow idle , thinning the throttle plate could improve the relationship of the throttle plate to the transfer port , instead of modifying fix the root cause, replace the throttle plate, this will cure a lot of low speed idle problems
 
Although the 65 lb flywheel might not spool up as fast, it stays turning due to all that mass. while a shaved flywheel might get a stock flywheel in the hole shot, it will run out of steam going uphill where momentum is needed. An A engine is all about Torque. gobs of it.
 
redmodelt said:
Wouldn't that turbulence also help atomize the fuel better? What about the air/fuel coming through the other side? Isn't the edge on the Model A throttle butterfly beveled, not a square edge. Would the relatively small diameter of the butterfly even make a difference by doing this?
Not pooing the idea just asking questions.
Click to expand...
The first part of this post has been proven about the importance and effect of turbulence and atomizing the fuel/air mixture. Yes the edge of the throttle butterfly is beveled to, I believe, obtain as close to possible a complete seal of the carb. throat to adjust for an ideal engine idle. I too am interested to hear if there is a noticed in the seat of the pants improvement in acceleration. Good thread...thanks for putting this one up here.
 
The early Model B Zenith carbs (B-1 type) featured a notched groove in the upper surface of the throttle plate adjacent to the idle port hole. This was later eliminated with the change to the B-2 type as it featured an idle port with a vertical slit that permitted a greater range of adjustment for the idle as well as the transition from idle.

I have seen Model A Zenith cabs where the idle port was slightly notched on the upper radius perhaps to allow for the same level of performance.
 
Steve Pargeter's book "ZENITH MODEL A CARBURETOR RESTORATION GUIDELINES" shows pictures of three different shaped idle ports for the Zenith carb. See page 16 in his book. LRF
 
I doubt any improvement will be noticeable. Certainly not in the seat of your pants. An Engine dyno may indicate a small improvement from 40hp to 40.001. On the other side of the coin without testing you won't know if you got a gain or reduced the hp to 39.999.

Lots of people try the small tricks that pro racers do trying to get an edge. Unless you're chasing 1/100th increments of horsepower for that 1/100 of a second edge most of them are not worth doing. I'm always amazed looking at Holley carbs at swap meets as to how many of them have been destroyed by ham fisted hot rodders looking for an edge.
 
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