low compression

mike11743

mike11743

New Member
Feb 12, 2008
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So I just did a compression chsck and am coming up with about 68 lbs of compression. The cylinder wall looks clean and free of scratches. I just had the motor rebuilt a little over a yaer ago so the piston is newer. Do you think I can get away with just new rings?? The piston is a stock yamaha piston that has one year/90 hours on it. Is this useable and what rings will fit it?? I know sometimes wiseco rings only fit wiseco pistons and pro-x rings will fit stock. I am also hearing a ticking niose at idle but not when I hit the throttle even lightly which I have no idea what it is. What do you guys recommend??
 
i would just hone out the cylinder and buy some rings. if it was me i would buy a wiseco piston kit cause i wouldn't keep reusing that stock cast piston.
 
be careful of what kind of compresion tester you use you need one that has a schreader valve in the end right were it screws into the head cause if you dont and you use one with a long open tube with the valve at the gage end it actually gives a false reading cause it acts like a bigger combustion chamber i made this mistake and the other tester actually showed that i had 130 instead of 100
 
I thinnk im just gonna buy the rings this time and then buy a forged piston the next time my compression starts to go down. Like said before I dont think that this cast piston will last much longer than 200 hours anyways. Another question I had is Are blaster kickstarters really esay to kick over compared to others?? Ive owned a CR 125, 250sx, and 250x and they are all significantly harder to kick over than the blaster. It has been this way since I got it, even after the first rebuild.
 
if you ahve a factory yamaha piston buy factory yamaha rings. make sure to check teh piston for any cracking.. inside and out.. if thats all good.. then re reing it and hone the cylinder.. IMO hold off on that wiseco swap til you need a bore.. really you should check that cylinder for out of round and taper wear... if either are out of spec.. you need a bore.
 
X-2 so far. If you need piston get it bored. Did a bike once that had cracked piston, nominal wear on cyl, just new weisco, no prob, just wish I'd bored also.
 
mike11743 said:
I thinnk im just gonna buy the rings this time and then buy a forged piston the next time my compression starts to go down. Like said before I dont think that this cast piston will last much longer than 200 hours anyways. Another question I had is Are blaster kickstarters really esay to kick over compared to others?? Ive owned a CR 125, 250sx, and 250x and they are all significantly harder to kick over than the blaster. It has been this way since I got it, even after the first rebuild.
Click to expand...

My friends daughter has over 300 hours on her Blaster of which 99% is at the dunes and has good compression and the the cylinder/head has not been removed. He took the oil injection off before the engine was even started and gutted the stock silencer and runs a 270 mainjet and changed the clip position to make it a little richer in the middle and on top. I would think that your piston will be okay and that you should only need new rings. I have maybe 75hrs on one of mine and I ride it WOT to even try to keep up with everyone and have never had the engine apart. I used a trick that Angus told me about to check the rings (look throught the exhaust port) and they look good to me. Every Blaster that I have ridden has been very easy to kick over. Since I switched to 110 octane fuel w/ Klotz my Blasters will practically start themselves. I am impressed with the combination.
 
a bigger combustion chamber wont change the psi. checking cylinder psi is just a measurement of what kind of shape the engine is in and how its sealing. Your confusing compression ratio (cylinder from its largest capacity to its smallest capacity) with the psi. something WAS wrong with your first gauge though. reason being is you still keep kicking untill the cylinder can hold as much pressure. it doesnt matter how much area its filling with air, its going to reach 130 untill your rings will not contain any more pressure


98blaster said:
be careful of what kind of compresion tester you use you need one that has a schreader valve in the end right were it screws into the head cause if you dont and you use one with a long open tube with the valve at the gage end it actually gives a false reading cause it acts like a bigger combustion chamber i made this mistake and the other tester actually showed that i had 130 instead of 100
Click to expand...
 
combustion chamber size definatly affects how many psi compression you have what happens when you mill your head your compression goes up why cause you made the combustion chamber smaller thats why when you whack .035 off your head and havde to use higher octane gas cause the compression is higher and the lower octane will light under the heat from more compression (aka detonation or pre ignition) and the race gas takes more heat to make it light so it will still light only when the plug fires it
 
you are correct in the sence that your compression RATIO will change since your changing the size of the cylinder..PSI will remain the same.

98blaster said:
combustion chamber size definatly affects how many psi compression you have what happens when you mill your head your compression goes up why cause you made the combustion chamber smaller thats why when you whack .035 off your head and havde to use higher octane gas cause the compression is higher and the lower octane will light under the heat from more compression (aka detonation or pre ignition) and the race gas takes more heat to make it light so it will still light only when the plug fires it
Click to expand...
 
compression ratio and psi are directly related to each other your right the combustion camber does change volume and comp ratio but with a higher comp ratio comes more psi and the rings have nothing to do with comp ratio or psi (unless they are beat and the air is leakin past them in this case you would do a leak down test to check engine state of health)or you want to get real specific and you change the ring land location on the piston raising it will make less volume and also raise psi and lowering will creat more volume and lower psi
 
PSI has absolutely nothing to do with A) compression ratio B ) type of fuel and C) it will not change b/c IT IS a factor to see what state the engine is in...nothing more. do some research. The only other thing that will raise PSI is carbon build up preventing blow by or chaing the port timing...i know these things...
 
so what your saying in all your i know these things knowledge is that no matter what you do to a cylinder or combustion chamber your compression psi is going to stay the same
 
unless your rings, valves arent sealing or you change the port timing or a cam. if you mill the head, you change the ratio, but it doesnt mean that the PSI will change...it does nothing more than tell you how well of condition the engine is in. think of a bike pump..you pump it over and over just like you would kick an engine over untill it cant hold any more air and starts to leak...thats your max psi.. i dont know how to explain it any better. if you dont beleive me dont.
 
ok mister wikapedia if you keep reading it states that your psi will be 15 to 20 times greater than your comp ratio hense the 10:1 215psi of your bike and the reason you can runn lower octane fuel is because of the head design and the cylinder design its all in the shape of the chamber and quench area directing the charge as to prevent detonation
 
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