Weak or no crank when bike is warmed up

I have recently run into an issue with my '02 vtec. Bike will start strong when cold. Cranks over easily as expected but after riding for a bit and turned off, the bike will crank over weakly or not even crank. I will have to allow it cool off for hours and then it fires up very easily.



Things I've tried: changed battery (brand new and one from a friend's VFR), swapped starter motor with same friend's that works for his bike, ran a direct wire connection from battery negative to starter motor bolt to allow for better grounding.



Problem still persists.



Bike will start easily if my buddy pushes the bike or I hook the bike up to a car's battery (doesn't crank if the car is not running and only cranks strongly if car's rev is raised).

 

cranking with car running.........could damage your charging system. have you checked your charging system output across battery terminals ?

you certain the bike isn't overheating ?? check your coolant level under the radiator cap when engine is cold. if coolant level falls too low due to leaks or bad rad cap, the temp gauge reading will not be accurate.

 

What squirrelman says.......

Potentially charging system issue is my first thought..... check your R/R connections are not fried....... you fire it up and go, then continuously use battery if the charging system isn't working...... when the bike sits, battery is still good enough to recover........ maybe.

Overheating a possibility too, since you mention using another battery works, but it's taking too much effort to turn over..... engine is near seizing hot?? First clue will be the coolant reservoir empty........ if so, rad likely low.

 

the only SURE way to check or top up coolant is under the radiator cap [when the engine is cold] as there are a couple of reasons why the overflow bottle level might not be an accurate measure of engine coolant level, otherwise you're taking chances of damaging the engine.

 

Funny you guys mentioned overheating. Something strange has been happening. The bike doesn't overheat, I ride for hours without any overheating. Though I noticed the coolant level always drops inside the radiator and I have to top up before the next ride.

So I swapped radiator caps to try and fix this, at around 82 degrees (180 F) from a cold start, coolant starts to flow from radiator cap into overflow reservoir and it increases as the temp goes up. After rides, my reservoir holds a lot more coolant. I would expect this excess to be sucked back into the radiator but it doesn't, so I have to keep topping up the radiator level.

Shouldn't it take higher temps to make the coolant expand enough to a high pressure to push past the radiator cap to overflow? Tried 1.3bar cap and it starts flowing at around 101 degrees (213 F) but never goes back into the radiator when cooled down.

Another thing is I can let the bike idle for 3o minutes without it overheating but when I shut it off and try starting, it cranks weakly like a dead battery but if left to cool off or I give it a wash, it starts right up.

 

Update - Problem fixed! I don't know for sure what fixed it but, I replaced the clutch cover gasket and refitted the starter motor, greased the starter gears and problem gone. My guess is the gasket gave the gear pins more room to expand and allow oil flow easily?