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Farmall M Carburetors

I

IHpuller20

Member
I have an M that I built that has a stock 450 motor in it. I have everything complete but my question is with my carburetor. It is a M carburetor that I put a rebuild kit in and I put a 806 venturi in. I've done all the adjusting I could do on my two screws on the carburetor and yet when you throw back the throttle, it acts like it is lacking on fuel. I know it will run perfect because I took another carburetor off a running tractor so I could time the distributor. Any thoughts on the idea would be helpful.
 
A vacuum gauge will be your friend here. If you drop below 1" vacuum there's not enough drop to pull in the fuel. Meaning the venturi is too large. There are several venturi sizes available for those carbs.
 
Venturi too big. Use the stock 27, or a 29 Venturi. Too big a venturi is like a 1000 cfm carb on a low rpm small block. It will stumble and fart and be impossible to tune. Best case, you may get it to run decent at WOT, but never at part throttle.
 
My data contradicts both previous posts. I have tested a 27, 29, 33, and 36 venturi on the dyno on a C-281 with a really good governor and the 36 has the highest horsepower at 540 PTO but at the same time with that big of a venturi it loses "lugging" power. Your symptom "acting like it is lacking on fuel", what does that mean? I can take a factory M carb and put it on a C-281 engine and it will run fine but the horsepower won't be there without drilling out the main jet and upsizing the venturi.
 
Are you using the original discharge tube? I have found that using one out of the same carb you got the 36 venture out of works the best.
 
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