Tecumseh Flathead Carburetor and Air Filter Upgrades
Article by Mark Trotta
The final part of the Tecumseh H25 overhaul included carburetor and air filter upgrades, and finally engine start-up.
Crankshaft, connecting rod, valve seats, cylinder bore, piston and rings are covered here
Sheet metal renewal and gas tank repair are covered here
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Replacement Carburetor
There were several types of carburetors fitted to Tecumseh flathead engines. Some may have chokes, and some may have an idle adjustment screw.
Smaller horsepower engines (Tecumseh H25, H30, H35) may have neither of these.
Finding A Replacement Carb
The first replacement carb I bought had a primer bulb, but no choke, and was non-adjustable. It fit correctly but the motor wouldn't start. The primer bulb was working, as gas was getting into the float bowl, but it just wouldn't start the motor.
After searching online for a solution, I found a forum member with the same problem on an old snow blower. He remedied his situation by switching to a carburetor with a choke.
Choke-Style Carburetor
The second carb I purchased was a replacement for carburetor numbers 631921, 632284, and 631070A, which are commonly found on Tecumseh H25, H30, and H35 engines.
Applications for this carb include many Tecumseh-equipped snow blowers, which operate in cold climates and need a choke to help it start. It is also a popular replacement carburetor for Troy-bilt roto-tillers.
NOTE: If the choke lever on the new carb is too long and is causing fitment issues, you can (carefully) trim it down with a hacksaw or cut-off wheel.
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Air Filter
All gas engines need some type of air filter to keep dirt out of the carburetor, and small engines are no different. With the exception of initial break-in and troubleshooting, you should always run your engine with an air filter.
Replacement choices include a conventional factory round-style or a clamp-on pod filter.
I was missing the original air filter, so I installed a high-flow air filter that I took off another small engine.
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Start Up Engine
After the carburetor and air filter upgrades, a new Champion RJ19LM spark plug was installed before start-up. Correct gap is .030"
After about 10 pulls, the motor started up. It smoked for about a minute, then cleared up. I let it run for just a few minutes, then shut it off to let it cool down. Not sure if that was necessary, but it's a habit from rebuilding air-cooled motorcycle engines!
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Notes and Misc
"Upgrading the carburetor and air filter on a Tecumseh flathead or small-displacement overhead-valve (OHV) engine can restore lost power, smooth out rough idling, and fix persistent fuel-delivery problems
Keeping track of engine disassembly with pictures proved to be invaluable later on, particularly while reinstalling the governor and carburetor linkages.
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Related Articles:
Tecumseh Flathead Engine Build - Part One
Tecumseh Flathead Engine Build - Part Two
Tecumseh Flathead Ignition Troubleshooting and Parts Replacement
