newly rebuilt 717 runs like a demon for 15 minutes......then dies and will not restar

[Nathan Mayo]

There were originally some problems with water in the gas, but I THINK I have gotten that beat using addition of "dry alcohol" to gas tank. The port engine in my '97 Speedster is a bit weak, but cranks and runs well. The new premium rebuild from SBT is the prob. They both crank immediately in my parking lot. Off to the lake and they are a little balky at first, but then Wham and I am a happy dog........They both run well at all speed ranges and have no problem restarting..........for 10-15 minutes. Then starboard dies during running fine and refuses to restart. I limp home on the remaining engine.
Plugs appear to be burning clean. It is as if the right motor is not getting any gas.......but where did it go? Has happened 3 times now and makes for a bad day. When I try a restart in the parking lot the following day everything is all OK again. Could I be getting some kind of heat related vapor lock situation that only shows up after things are hot? I am baffled. I figure if it was some sort of blocked filter or carburetor problem then it would not start in the first place. HELP ! Rebuilt has maybe 1-2 hrs of total run time on it. When it works, it works great! Now what?

 

[Sportster-2001-951C-Stock]

Hopefully you disassembled the carburetors of the new engine and cleaned all the internal fuel passages? This is arguably the #1 reason for 2-stroke engine failure, running too lean will roast a piston.

Do everything you can to make sure it's getting clean and clear fuel. If the factory fuel pump is divorced from carburetor type, those are known for becoming weak as they age and the replacements are slightly larger. Usually though I think a fuel pump just can't keep up with running hard but still will idle and run ok low speed. Thought I'd mention looking at fuel pump though.

 

[Rich_]

Possibility is the fuel filters are getting clogged up from running. When you let it sit for the night all the gunk falls away from filter and you are good to go again for a bit. As said above, figure out why it starves for fuel before you fry it.