Hi,

Today I was lucky enough to spend some time with Don Hancock from the Brant Flyers in Ontario. Don is a very experienced modeler and very knowledgable with engines. Don had a problem GMS .47 engine from one of his student pilots and invited me to be there when he took it apart.

The symptom is: the engine seems OK, airplane takes off, then the engine quits. The pilot has also noticed air bubbles in the intake fuel line on the ground, which may be caused by fuel tank vibration. This engine also has an exhaust extension/deflector and a rear (remote) high speed needle valve.

The one thing we all should remember is that we are working with three different styles of GMS engines. The first is an older style front needle valve engine with the "shiny" muffler, the second is the new style front needle valve with the "finned" muffler and third is the new style rear needle valve with the "finned" muffler.

Before dissassembly, Don noted there was some air leakage through the front bearing, but not where the carb mounts into the engine.

The motor was completely dissassembled, and except for some minor rust on the rear crankshaft plate, the engine seemed OK.

It was noted that when the brass insert for the front fuel intake was removed, that the brass insert is under cut (grove around the insert) where the fuel comes in. The insert has fuel intake holes on both sides of the insert in the under cut.

This means that it really doesn't matter if the holes under the intake valve nipple line up or not. The fuel will flow through the intake nipple and around the brass insert (using the under cut) and can find its way in to the carburettor through either of the two holes. Basically this means drilling the hole under the nipple probably does not help. However, it would not hurt anyone has already done this.

This leads us to believe that the problem is probably one of two things with this particular engine. One could be fuel foaming due to an air leak or tank vibration. The second would be low exhaust pressure (we know this carburettor has poor fuel draw).

In previous posts, we know modelers have got the GMS .47 to run by increasing the hole size in the exhaust pressure nipple, by drilling one drill size larger (we may even need more) and adding an exhaust deflector/extension causing a little more pressure.

Don will keep us up to date on how the engine works, and any other modifications he may have to make.

If anyone still has problems with their GMS, they may want try and analyse the problem and let us know the results by:
1. Check for air leaks at the carb, fix if necessary.
2. Check to see if the fuel line air bubbles at "start up" clear quickly, if not, chances are the tank pressure is not enough.
3. Then drill the exhaust pressure nipple larger for more tank pressure, again check to see if the "start up" air bubbles clear quickly. If this fixes the problem, great!
4. If not, try an exhaust deflector/extension to help with fuel pressure.
Note: I did not have to add an exhaust deflector/extension on my engine to make it work OK.

If anyone does the above, please let us know your findings. Thanks for your help.

Fly4Fun,

Wayne Miller