Scooterotica

Right firstly I'm not good with 4 strokes so I'm struggling with this. Gts 300 engine carburated, runs fine for 10 to 15 minutes but once up to temperature it starts to backfire at full throttle. I've changed the ht lead spark plug & ht cap, this seem to help but has not eradicated it completely, had me thinking it could be electrical but I'm clutching at strawas. Any ideas from anyone.

Get in touch with PM Tuning, try and speak to Paul Melici if you can.

When you say misfire, are you sure the rev limiter isn't cutting in? If your belt/rollers are worn they speed at which the limiter comes in will drop.

Talk to an expert first, but sounds like a CDI/coil issue. I've had coils that cause misfires and poor running after a few minutes when they get warm

It's not the rev limiter, it's a proper backfire and loss of power, gonna try & swap out the rectifier see if that makes any difference. Someone has mentioned something about valves lapping? Exhaust valve closing too early but wouldn't this be controlled by the timing belt on a 4 stroke, would have thought that would happen no matter what the temperature?

Could be fuel starvation.

Is carb gravity or pump fed?

SF

fuel pump is my guess

There's no fuel pump it has a vacuum fed tap, which I have recently changed. Any other ideas?

sounds to me a like a coil breaking down when it warms up, seen it on bikes a couple of time, but normally older vehicles.