hello again everybody, i was wondering if anybody knew what amount of p.s.i. i should be getting out of my yfm225 moto4. its a single cylinder 223cc. when i crank it cold using electric start, the needle jumps to 63 every time(revolution) being a 4 stroke. does anybody know what i should be getting hot or cold? and if this is low, normal , or high? thanx for looking and any info. -bean

 

You may have problems, at 60psi thats pretty low. It could be worn rings or a stuck or bent valve.

Generally a good reading is atmospheric pressure X compression ratio of the motor. say the compression ratio is 9:1 and atmospheric pressure is almost 15. Usually you do compression checks while the engine is warm unless you know what the compression looks like cold before any trouble. I would think you would see atleast 100lbs cold after a couple times around.

To find out if its the rings or a valve, add about half a teaspoon of oil in the spark plug hole. then work the starter a few times to coat the cylinder. Then take a reading again - make sure you hold the throttle open. If the reading goes up alot, or near 100lbs or so I would say you need new rings. If it doesnt change it may be a valve.

 

Oh, I forgot that motor may have a decompression mechanism. You may have to disable it somehow or get it to crank really fast like with a jump from the car. But I would still expect more PSI than 60.

 

ive now got a couple opinions saying that thats low. i now need to check it when its hot and see if theres a big jump in p.s.i. yes it has a decompression mech. soooooooooooooooo, the needle jumps for every compression stroke but then releases on the exhaust stroke back to zero. im gonna check it tomarrow with the throttle wide open for maximum air intake. (more to compress=higher compression p.s.i.) does anybody know the spec psi for that cc cylinder? thanx... its a 1987

 

Are you just having problems trying to start it? Do you have a good spark from the spark plug? I had a 200cc 4-stroke 3 wheel atc, it sat up for three years and the decompression cable from the pull start was keeping the bike from starting. Readjusted it, then sold it.

 

Hey YAMABEAN, Sounds to me like you need a new compression gauge or at least a new valve in it! It shouldnt release back to 0 psi untill you release the valve. So it is not letting it build compression like it should.

 

Are you opening the throttle all the way while cranking? This makes a difference. You should hold it wide open.
Are you using the electric start? I don't think the decompression release is activated with the electric start.

Out of control is right. If your valve in the tester is bad, the engine will pump up then pull vacuum so it wouldn't be a correct reading.

 

thank each one of y'all for the info. im fixin to go try again. ill try it by hand (flywheel) and with electric start---wide open. my guage doesnt have a release button- its pressure as its applied, wont build up. but if i did get one that had a check valve in it... wouldnt it just keep building up pressure till it hit the red zone on the guage? thanx again and have a great weekend!

 

You have a good weekend as well Yamabean! Let us know what you find.[img]i/expressions/face-icon-small-smile.gif[/img]

 

No Yamabean , it will only build as much preasure as the engine will. It reads preasure not volume.