Hello,

I recently bought a 78 TY 250.  It is next to impossible to start cold unless you push it.  Once it is running, it goes great.  I cleaned the carb and checked the compression.  The compression is 90.  I have a new top end off a 2nd donor bike that is new I was going to swap out.  I cannot find the specs for compression anywhere.  Is the 90 compression the problem and if so, I have not been able to find the head gasket and bottom gasket.  Any advice would be appreciated!  Thanks!

 

Don

Was this compression checked with throttle wide open? it should be. I'm not sure about specs for a TY but 90psi sounds low. Newer bikes are closer to 150psi. Good Luck.

Edited July 16, 20215 yr by Glayne

Sounds daft, but have you tried setting the idle to be a bit faster? Just for curiosity give it a fraction of a turn to idle a bit higher and see if it makes a difference. 

Hi, check your compression as above with throttle wide open and look for at least 130 to 140, if that comes out OK I would look at your'e crank seals as lack off lower compression would give the symptoms you have in starting, ty's are prone for LH crank seal failing especially if there is slight play in the main bearing [lift flywheel up and down to check], pull the flywheel and remove points plate, brush a mixture off washing up liquid[diluted] over seal and turn engine over by hand [with spark plug removed] and watch for bubbles, if there is, seal can be changed without stripping if RH seal was faulty it would smoke badly as gearbox oil would be sucked into crankcase. note lack off crankcase compression means lack off fuel supplied to cylinder hence having to spin motor faster to get it going. If this isn't the problem add some more to your'e post and i'll get back to you. regards trialman. ;-]

 

my understanding is that air cooled bikes 120 PSI is about the max cranking pressure