Hi Richard
If you're not getting good re-starts with the O.S. 18's, and your battery and plug are ok, then the P/L is either worn out, or the engine is getting a load of fuel in the venturi when filling the tank and on the first flip you're flooded. Usually it's the latter. When that load of fuel from the pit stop hits the piston on the first flip, it shrinks the piston, and you lose your pinch at TDC. A hot flooded engine behaves almost the same as a hot dry engine except the dry engine will sputter when flipping and hot flooded engine won't. I have this problem with several of the engines I've run in Clown Race, and my fix is to dump the fuel out of the venturi after filling the tank, then flip. Usually it works fine, but I do have one engine that likes to start wet, which is unusual for my setups. The tighter the P/L fit, the easier the engine will start hot.
Re: suitability of the Nelson for Clown Race. I have used a Nelson .15 in Clown Race, but not the high timed glow engine. The engine I used was the high timed Front rotor side exhaust Diesel variant with a glow head. The Diesel P/L has timing numbers more in line with what you need for Clown Race (ie. decent speed and laps). If memory serves my lap count on 52's was around 50 laps per tank, and my 15 min. final lap count was around 300.
I haven't run my O.S. 18's in several years. I'm just using my old Moki .15's now.
Les