Monday, September 16, 2019

Monday Mystery tech corner, Track Speeder carburetor

thanks, Glen!
Talk about obscure... This is a carb for an old track speeder. The engine is a big slow-turning crankcase-scavenged two stroke engine. The float-type carburetor is simple, with choke and just one adjustable main jet. No throttle but are there not one but two vacuum-activated poppet valves in the intake tract. 
Anyone know (or have any guesses) why this arrangement? It appears the speed is mostly controlled by the spark advance.


Modern Garage said...

OK, I'll play. It's a crankcase inducted two stroke so instead of an intake reed it has a poppet valve. That's one of them. Now what if the engine turned so slowly that the venturi effect was unreliable? Maybe the two poppet valve springs are calibrated so the engine side opened before the atmosphere side so it had a little intake vacuum acting on the main jet to slurp a little fuel in with every intake stroke.
Just spitballin' here...

Mister G said...

Thanks for your input! Talking to some hit and miss engine people and right now your theory is the general thinking. Someone says these engines run basically one speed, and possibly the idle is achieved with spark advance (actually retard I guess.)