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By Wheaters
My road & off-road trials car used to run on a tuned 850 Reliant engine which initially had, by accident (!) a 13:1 CR. It was OK on the starter motor (but it hammered the starter ring on the flywheel). However, it was almost impossible to bump start it, especially off road, because the back wheels locked before it went over compression, even with those small cylinders.

You stated that you have AVGAS in the fuel tank. It's less volatile than pump petrol and using it neat may make starting more difficult. Have you tried starting it on super unleaded? My trials car ran on it well enough although I did put the odd cheeky gallon of AVGAS in it for competition work and not surprisingly, it loved it.

You might find that AVGAS adversely affects certain types of rubber (e.g. carburettor seals, possibly the VITON rubber nose of the float needle).

AVGAS also contains a LOT more lead than you might imagine - even though these days it's advertised as "LL" = Low Lead. It's likely to foul the spark plug.

I used to put one gallon AVGAS in for every four of Super unleaded in my trials car's tank. By my calculation that gave it the same amount of lead as the good old "4 Star" had in it.
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By Wheaters
I meant to add... half a century ago I learned to ride on various grass track racers. One of them was an ex-speedway J.A.P. 500 single which ran on methanol and castor oil.

To start that it was necessary to put it in gear, pull the bike backwards to find "reverse" compression, then get a helper to push it forwards to running speed and dump the clutch whilst jumping on "side-saddle" to get weight on the rear tyre to give it grip..
By Count Johnny
I more recognise what Finepitch seems to be describing, which is the piston simply bouncing back against the compression, with not a spark in sight.

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