FBD - Thats a nice looking quickie. I bet it hauls rear-end.


Many people don't realise it, but the GMS engines actually use two orings to seal the carb. there is the one on the carb spigot that you can see and squash down when seating the carb, but there is another one lurking around as well. If you look down the hole that the carb goes into (with the carb removed) you will see another oring sitting in a recess near the bottom of this hole just above the crankshaft. It is important you take note of this oring when seating the carb. I have pushed a carb down to seat it, and the had the bottom of the carb spigot grab this oring (instead of sliding past it) and dislodge it. A bit of oil on this oring will go a long way towards succesfully seating (and sealing) the carb.

Just another thing about GMS engines: they are supplied with 3 shims under the head. These 3 shims look like 1 thick shim ( I think they put 3 sheets of copper together then stamp out a shim from them) but if you look closely, you will see that they can be separated into 2 thick shims and one thin shim. This gives a lot of flexibility for adjusting the compression - the possibilities are:

-all 3 shims
-2 thick shims
-one thick and one thin shim
-one thick shim
-one thin shim
-maybe no shim at all, but I wouldn't recommend this

You should take this into account when tuning you GMS for different prop loads and nitro content. They run very well on low nitro fuel and high compression.
I run my 47 on one thick shim, 0% nitro FAI fuel and a hot A3 plug and around a 10x7 - 11x5-6 Bolly or MA or APC prop. It screams and idles and transitions as well as you could want.