Reply To: Power – GT125

[Tom]

According to the service manual the compression service limit is 8kg (113psi), it should sit around 10-12kg (140-170psi).  So you are at the service limit and it would indicate that things are a miss somewhere.  Probably just valve clearance, if a valve stays open or does not open properly the compression is adversely affected and you get a rapid drop off in HP.  Combine low compression with a slippy clutch, carb issues or exhaust leaking near the head then bike will be greatly under powered.

How old is the bike?

What mileage has it done?

Do you know its service history?

Service history is important because a bike set up properly with the correct oil etc will last and perform beautifully.  If a bike has been run with low oil, wrong oil or even too much oil it can quickly damage the rings causing compression loss.  Running a lean (hot) bike can also damage the piston and rings, I have seen pistons burned through from running lean.  A discoloured exhaust is often a visual sign of a bike running too lean and hot.

If an owner just ignores dropping speeds and power loss it will mess things up badly, unfortunately the poorly cared for bike is often passed on to a new owner who has no idea a numpty did not care for it.

The first step I would take before assuming terrible things (you have not mentioned excessive exhaust smoke which would indicate rings) would be check the valve clearance.  These are really tough little engines, but they need to be cared for.