Cylinder pressure....compression.. Old YZ 250

[Rcannon]

When a person is tuning a motor, is there a desireable pressure that needs to happen in the cylinder?

I know folks talk about milling heads, compression ratio, etc. In my mind this is all trying to achieve a certain pressure inside. Is there an idea range.

I am at higher altitude. My pressure would be less than sea level. My brothers old YZ 250 looks like it has been ported by some school children. The bike is very easy to kick start despite having a new bore, piston and rings. The bike is virtually impossible to start when cold. I am wondering if milling the head might boot pressure enough to help this?

I have looked at everything else. All electrical connections are good. Reeds are fine. Carb is clean enough to eat out of. I can think of no other reason this will not start besides the child porting exercise.

 

[David Trustrum]

Ignore the compression tester meter. It is possibly a bit irrelevant. The compression ratio can be raised to the limit of what the fuel you are using will stand in your engine. Head design & several other factors influence the limit. But unless you are running extra base gaskets or something the ratio should be just fine.

When the engine was rebored, was it measured after? & did you for instance check the ring end gap. Perhaps it was bored with too much clearance?

Carb is clean enough to eat out of

Maybe the temptation was too much & there is food stuck in your carb?

I am almost tempted to say maybe you should look at the pilot jet, perhaps it has been damaged in a past life with someone poking wire through it?

Air leaks? Around reed block, base gaskets, crank seal. Check for oil.

Squirt some gas down the spark hole with a small syringe to start when cold. Choke circuitry? Ignition may be weak if separate low speed coil on that model?

I believe in the early eighties the hot set up WAS to get a schoolchild to port your YZ. :eek: