Small Engines (Lawn Mowers, etc.): Honda 4 HP engine for snowmobile, drive gears, main bearings

Question
Hello Howard, I have a 120CC Mini z snowmobile and it is running a 4HP Honda engine. I have it maxed out @ around 5600 RPM he gets the usual 15 miles an hour out if it but anytime he goes up any hill the machine almost stops running. he raced this weekend and had to run beside it and get help from the corner spotters to get up the big hills. Do you think this is the engine? I have changed the clutch and drive gears. Main bearings and also added idler wheels to reduce the track friction. My son says that the RPM seem to go down when he starts climbing the hills. please let me know what you thing and also the compression that the engine should be.
THank you so much

Dan Deigan

Answer
Hi Dan,

You should check the operation and measure where the engine starts to loose RPM.

It sounds like there is a loss of power under load.  Check air intake obstructed, Lack of lubrication, improper lubrication, Carb improperly adjusted, exhaust obstructed, improper valve lash, loss of compression (worn rings blown head gasket).

That should get you started,

Hope this helps,

Howard B.