I'm converting a Honda GX31, the carb has been changed for a normal butterfly type carb so I put a nipple into the inlet manifold and another on the carb as the impulse ports do not line up. This works very well and now gives me the option of trying several size Walbro's although the one I have right now appears to be ok as the induction port on te head is only 10mm in dia so a massive carb would only flood the engine unless I did a lot of porting. I assume the fourstrokes need a smaller port as they only bang every other rotation. for now I'm sticking with the mag ignition but was wondering if I were to go the CDI route whether the timing would be 28 degrees as per 2 stroke or if it's different on a 4?
I have read these engines will throw say a 16" X 8" prop at around 6,000, I wonder if they have enough torque to throw a larger dia and less pitch for a slow flying 1st WW type thing such as a 1/4 scale Pup or maybe even a 1/4 scale DVII?

Do a search on GX31 and you will find more info. Many people have done what you are doing with success. I have a 40cc, 4 stroke Briggs on gas/glow and it is swinging a 22x10 at around 6700 rpm.

You should be able to swing a bigger prop than a 16". As for EI, 28 degrees BTDC worked OK for my Stihl 4-strokes but I set it back to @26 to make starting (hand flipping) easier and didn't notice any top end drop-off. The EIs I have do have auto advance. As always, your results may vary.

J

Thanks guys, I have been searching for info but most threads seems to fizzle out before any concrete info comes up. Lots of talk about using the standard type carbs, a few mentions about video which I cant find and no definitive prop size/type for slow flying WW1 type aircraft.
I see the easiest method of prop driver using the mag ignition is a basic driver using it as the flywheel nut, this concerns me that it may come off with hand starting and I'm reluctant to put any thread lock on. I think for now I will stick with the mag ignition as I intend to use my XYZ CDI on my Zenoah 62 conversion.

Oh, I forgot to mention the exhaust, it's loud with none at all, does just a header alone quieten it whilst giving rpm improvement?