Mine are flush with the end of the adapter. Off hand I would say the compression ratio is the same or slightly higher, as you don't have the volume a spark plug has around the center insulator.

My engines seem to idle more smoothly, slowly, and reliably on Gas/Glow than on ignition, so I'm not sure which idle problems you might be talking about.

The Gas/Glow setup does require the idle to be a bit richer than one would set it for straight gas and normal ignition.

I hesitate to mention this, but what the hell. Here's a modification to get a better idle and transition on any of these engines, whether they are running Gas/Glow or gas and ignition. Please do not try this if you are not:

A. Comfortable trying the procedure.
B. Willing to possibly trash a carburetor in the name of experimentation.

If your carb has any holes in the throttle butterfly, (Not Choke), solder them shut. Then using a needle file, file a tiny notch in the butterly where the exit holes in the venturi are. These are tiny holes in the venturi where the idle and transition mixtures enter the air stream and a little difficult to see, but you want the notch in the butterfly to line up with those holes when the butterfly closes. Start with a really tiny notch, as if you make the notch too big, you won't be able to get the idle slowed down even with the butterfly shut. Please don't try this unless you are comfortable with the whole idea, and be careful to not get metal shavings into the engine, etc.

AV8TOR