1) Fuel delivery, blocked line, filter, gummed carb
2) glow plug is bad, even though it glows red, may still be bad. Fuel ignition relies on a catalytic reaction between the platinum coating on the GP coil to react with the methanol in the fuel. Also relies on proper heat of the coil being applied. If the coating is worn or damaged, makes starting with the heater usually ok, but stalls or idles very badly once the heater is removed.
- did you actually take your plug out and make sure it was glowing orange/red when the heater was applied? If not, try it to make sure your plug is in fact "apparently" ok.
3) heater is dead or making bad contact
4) stale fuel, should still start, but may not run well
5) Too cold outside, most engines I've had refuse to start if the ambient temps (and temp of the engine) is below 40F (or around there). Warming them up to 70F by putting them on the floor of my 1:1 car with the heat blasting gets them going in short order.
6) Broken clutch spring causing the clutch to drag which won't allow the engine to get up to proper RPM's to stay running.
7) Worn/binding CB bearings causes the same issue.
8) OWB on starter is seized to the shaft so it won't let the engine rotate freely when it does fire.
9) carb adjustment isn't open far enough at idle to allow air/fuel flow.
10) crank case is cracked causing massive pressure leak.
11) head bolts or backplate is loose causing the same issue.
Basically, all that stuff has either happened to me or my friends in the past. I can't think of much else.
