Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Hi all. I am a newbie, having only joined this forum a couple of months ago. I was taken by the M90 hit and miss and ordered one. While it does start up and run, and even though I have never run a hit and miss, and am not sure how it should run, it just seemed not to run well. I was getting a lot of unburned fuel out of the muffler, and if you looked very close at the muffler when it it fired you could see a very fine mist coming out of the 4 holes in the muffler. So it was time to pull the head and see if I could see anything that did not look right.

Everything looked great. The valves seemed to be seating fine. I pulled the piston and that looked good also. The ring was in good shape and the cylinder wall looked good. I then decided to remove the spark plug and check that. That's when things got interesting.

A 1/4-32 plug should require a 5/16 inch socket or wrench to remove. I tried a wrench and it would not fit. A socket would not fit. I knew that some of the guys use a 8mm wrench or socket as it is only a bit larger than 5/16 and also works. That would not fit. I then used a small 4 inch adjustable wrench and removed the plug. Using a dial indicator i measured across the flats and got .322 inch's. While it was only about .011 oversize that was enough that my wrench's and socket's would not fit. I then measured the gap. It was about .006. That seemed very small. I had bought a spare plug by Rcexl when I purchased the the engine and i measured the gap on it, and it was .016. Wow. A very big difference. I installed the new plug leaving the gap at .016 (a 5/16 wrench fit this plug) and put everything back together.

I started it up and it was like a new engine. Runs much better. No mist or unburned fuel coming out of the muffler and it seems to have more power. Has anyone else had a problem with this? Does anyone know what the gap should be?