Flathead Problem


So i recently bought a used flathead engine, at a race this past weekend it would go down the straightaway and suddenly just lose all power. It would drive just fine then all of a sudden cut out and have no power. I have so far put a new diaphragm, checked screens for clogging, adjusted mixture screw and jet sizes with no luck so far, looking for anyone with flathead experience that can help. TIA


Site Supporter
Piston rings worn out, loses compression after motor temp goes up during race conditions? Try a cylinder leak down test?


Not exactly sure on jet size, my parts guy played with a few different sizes before deciding the original was better, it runs on alky


2A supporter
Sounds like the pickup tube sucking down against the bottom of the tank.
How many gaskets between the carb and tank?
If it is shutting off at peak rpm, then it is jetted too rich.

Could have closed the plug gap.
Pick tube too close to bottom of tank.
Tank over-filled with fuel (only fill to seam at the back of the tank.)

🏁Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cutz
Carlson Motorsports on Facebook
32 years of service to the karting industry ~ 1Cor 9:24
Linden, IN


there a several quirks to flatheads that you must be aware of.
1 you cannot fill the tank all the way up or it will cut off around the corners. Up to the seam as mentioned above
2. the spark plug must be indexed to the valve or the valve will smack your electrode closed.
3 the main jet can and should be manipulated on the track.
there is a mixture screw running perpendicular on the flywheel side of the engine. turn it until closed (carefully as not to damage anything) and then back it out 1.25 turns. you can ad a knurled finger adjuster or a remote cable adjuster to this screw if you can find one.
you will need to run head temp and lean or enrich your carb with that screw for a target head temp of 400 deg F.
12 -14 oz (someone please verify) of oil and you should change it frequently. i change my oil before feature and dump at the end when the motor is HOT. not the next day.
make sure the valves are closed as the methanol vapors from the tank will etch your cylinder and rings. and drain your tank at the end of the night. methanol should be fresh as it absorbs water when left out.
good luck and welcome to the world of Flathead racing.


in the old days carb adjustments were reach under the pipe on a stock strait pipe with a 15 degree mount and over the pipe on a limited with a flat mount. i burnt my arm a lot in those days going from one to the other.
Another quirk of the flathead is getting shocked by the coil wire when i would reach over my motor to shake my competitors hand. Witch we used to do in a double file line up and wish people did more now days (not double file) but acknowledging a little respect to your fellow competitors . I always make a point of giving people i race against a thumbs up and if they passed me clean i would congratulate them with a nod, a thumbs up or telling them so post race. a win at all costs is not in my playbook we all have to work the following week.
Saw too many fights and screaming matches this week end .