1990 Yamaha G8 Engine Problems


New Member
My 1990 Yamaha G8 is having some major engine problems. I was running just below the rev limiter on a paved surface and began to go up a hill about 100ft up the hill the engine just quit and would only putt and ran really rough. I thought that if I let the engine turn off then restart it that it would run again. Well, I did that and the starter/generator could not crank the engine past TDC. On a fully charged battery it gets to TDC then the solenoid clicks.
I then pulled off the valve cover and found a pushrod, exhaust, had worked its way loose. I reinstalled it and got the lash about right. When I started it up it still ran really rough.
So I pulled off the head. I found out that the piston is .25mm oversized. I have had this cart for 5 years and have been abusing it and the piston had no carbon at all on it. The valves looked good and only had carbon on them as did the spark plug. The valve springs seem to have sufficient pressure and the lifters look to be lifting.
This cart has been backfiring for awhile now and last week when I turned it off after running it hard it dieseled, in reverse rotation, for almost one full minute. The backfiring has also broken every baffle loose in my muffler and blown out 2 quarter size pieces of the casing.
Today I took the head off again and took out the valves. The valves are not bent neither are the push rods. The engine feels like it is getting decent compression and there is a nice blue spark from the spark plug. I swapped ignition modules with a known working one and it still has problems. The fuel filter is not clogged and the fuel pump pumps and the float floats. The carb is not clogged. I also disconnected the air filter from the carb and it backired like a jet through it.
Is there a way to adjust or retard the timing? I think that somehow the timing got thrown off and that is why it won't run.


I'd start with doing a actual compression test and see what you get. If the engine is still apart and nothing is broken you might as well hone the cylinder and put a new set of rings in it.

Also check the intake spacer between the carb and head for cracks and make sure the air box is sealed tight everywhere.