01 ST4 stalling under load - out of ideas

[doocatiboy]

Friend's 01 st4 stumbles, sputters and then dies under heavy load/acceleration. Wait a minute or two and she goes again. She'll idle indefinitely but once you roll it on hard (in gear, not idle), she's out.

Wasn't bad gas. Electrical health (battery, charging, reg) is fine. Turned out not to be pump, filter, or cracked hoses inside (or outside) the tank. Verified this by swapping the tank w.my ST4's and problem persists, ergo why I'm posting now hoping for other suggestions.

Only other things I can imagine *may* - but not likely - have an effect:

• Oil may be a hair above full, though it's been run 30mi before this stalling began so wouldn't think it's anything to do w.overfilling
• Electrical issue that stalled the bike a couple mos ago - turn sig wires were loose, somehow causing some funky dash lightworks and some tugging seemed to 'fix' the problem, hasn't happened since, and besides - the stalling characteristics differ in that and current case so I doubt the loose wires shorting is it

Any thoughts or advice?

[lefin102]

sounds like fuel problem to me. not enough. hows the air filter?

[doocatiboy]

Originally Posted by lefin102

sounds like fuel problem to me. not enough. hows the air filter?

That's what I thought too, and it could still be fuel restriction. Air filter was blown clean w.compressed air, no change.

I suppose it could still be fuel related but not in the tank - only thing is, nothing on the bike was moved or manipulated in a way to, say, pinch a line somewhere or pull a vent hose free when this 1st began. We put my ST's fuel tank on his bike and it did the same thing so the problem obviously doesn't lie w.in the fuel pump assy.

No fuel is leaking otherwise under/onto any part of the engine or chassis so I'm not looking at other fuel lines as being leaky, etc. Maybe I should be..?

[neno]

Sounds like it could be a faulty TPS.

[demontech]

i had an st2(i know its not the same!) but the water temp sensor for the ecu was bad, thrown of on the fact that the guge worked till i remembered their is one for each. i switched the connectors so they swapped pickups- one it ran better and two the guage would go nuts of and on about the same frequency as the running used too

[duckracer]

I would get a volt/ohm meter and start with the tps, it should "cycle through or build the voltage or loose resistance smothly, if at any point in the range of motion it makes any large jump either up or down in voltage or resistance it is bad. if it is good, then check the other sensors read what the book say's. I did find one time I had a bad plug wire on my race bike, and it did this same thing. would idle all day,but give it throttle and it would stumble. shut off the lights in my garage and tried it and I could see the plug wires arc!!!!....Just a few ideas that might help you out.