Motorcycle Repair: Bike difficult to start when cold, combustion chambers, hydraulic tensioner

Question
Hi Mark,

I have had my Yamaha FZX250 Zeal for just under a year (I am a learner). In cold weather, the bike has difficulty starting - it doesnt start with the choke on and if it does start with the choke off, it either cuts cuts out quickly or maintains a very low idle before eventually quitting (any application of the throttle causes it to die). I often have to clutch start the bike. In warm weather, the bike will start (without choke) but it will idle quite low. Once it has started and I have control over the revs with the throttle, I can apply the choke and it will then idle at a good speed.

Furthermore, the engine emits a clacking sound while warming up. Once warm, the bike starts first time and idles fine and the clacking noise ceases.

My theory is that the cam chain is loose, which would cause the timing to be out on startup (and would account for the clacking noise?). Once it is warm, the oil pressure activates a hydraulic chain tightener and that is why the bike works once warm.

Do you think this is the cause of the problem, or could it be something else?

Cheers,

Dylan

Answer
Hi Dylan,

The hydraulic tensioner may be defective and contributing to the noise problem. I doubt it would develop enough slack to affect valve timing.

Perform a cylinder compression check with engine cool and warm. Follow with a leak-down test. These tests will determine the sealing properties of the combustion chambers (condition of the engine).

If the motorcycle was stored any periods, I would suspect the fuel in the carburetors has varnished.


Respectfully,
Mark Shively