Glowing exhaust

cosworth

cosworth

Coast to Coast
Thumper yamaha 250 engine.

Engine is timed mechanically and ignition is timed dead on. Motor had a 47.5 pilot jet stock but has a 50 in there. Airbox removed. On rev and overrun it is more than a little black. Plug reads rich. Bit of a hard start and the idle screw is set pretty deep to keep it idling I think.

Exhaust is glowing red hot at the muffler just idling. Timing is right and it's not running lean, only thing I can think of is that the valve clearances are way too tight and the exhaust valve is opening way too early. No? It's late and I didn't get to popping the valve covers off. Taking a text shortcut so my morning might be easier.
 
I don't know about the SR motors, but Suzuki motors can be a real pita to even start if the valves are too tight. Even with the larger pilot, could it possibly be a low float height causing it to get that hot? Or maybe a substandard spark leaving you with a dark spark plug?
 
Lots of fuel, plug is gapped at 0.030 and she's a hot spark. It's gotta be the valves. If the float was buggered she'd be running lean.
 
valves being a little tight wont do that,it wont effect cam timing much at all
how long is it running before the pipe is red? what kind of pipe single wall?
 
It was idling at the end of the driveway to charge the LiOn battery a bit. Maybe 10 minutes tops. The pipe is a repacked dunstall copy with a swiss cheese baffle.

It would make sense if it was lean or the timing was way off. But it's not.
 
Can't ride it currently, seat is getting mounted. It's repacked with a little course steel wool around the baffle to keep the flame away from the FMF dirt bike packing.
 
aha i had an inkling !
steel wool burns don't ask me how i know ::)
but seriously you need stainless steel wool if you are using it for packing
 
Solved.

The recommendation for a 50 idle jet is wrong for the bike. The stock 47.5 works. Since it was too rich at idle and won't idle right, the slight idle adjustment to get it to idle made it run lean. There is a fine line between rich and lean within 1mm of throttle plate movement. When you have the wrong jet. The mixture screw did nothing, that pointed it out pretty fast. I just thought the oring was dead.
 
If it isn't red all the way from port, you have a restriction and excess fuel/carbon burning where it can get some air. I would re-check the end of pipe, baffle and exhaust entry
 
There's no restriction. The baffle comes out and has come out. The idle jet is way too big, the mixture screw does nothing.
 
Re-set float level +1mm while your in there, service manual will have a +/- spec, make sure your at max height
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light

    • Spread the Love

    Back
    Top Bottom