Hey guys I have another one for ya. While working on the clutch assemblies with the engine chocked up in my bench vise I was spinning the engine over which is all together (except the exhaust, reed, and carb) I put my finger over the spark plug hole and was suprised at how low the compression seemed. I put my compression tester on it bolted on the starter and spun the engine over to see only 50-60 psi. I had the engine bored for the new Wiseco piston and I checked the skirt clearance before going together and it was at specs .003-.004. I coated the cylinder with some 2 stroke oil before sliding it down over the piston (was this a mistake?) I am sure I have the rings installed correctly and I gapped the rings to the recommended specs. So I am befuddled. Will the compression come up after the rings seat? I had a very good pulse signal on the inlet (reed side) and outlet (exhaust side) Again any help is greatly appreciated. The bore job looked good, I dont know if they used a torque plate or not but it had a nice cross hatch pattern. Thanks.

 

I do almost exactly what you do (bench test) and I can crank out 180+ compression at times.(after modification)...
The only real mistake that you have made here is buying a crappy Weisco piston.But it'll get you by. [img]i/expressions/face-icon-small-tongue.gif[/img]
Maybe the starter is not turning it fast enough?! But prolly just needs run and seated. Check the comp while the motor is hot and see what ya have.
I got a motor in the other day w/the piston installed backwards...make sure that the arrow is toward the back of the machine/exhaust [img]i/expressions/face-icon-small-happy.gif[/img] It ran fine but took a chunk off the exhaust port. <EG>

 

Strokit, if you didnt have the reedcage and reeds installed in the motor, your compression will be very low. Your not able to trap the primary compression in the crankcase to get pushed up into the cylinder. Your just compressing "dead air" on top of the piston. Put the reeds back in it and take another reading. Bubba

 

Thanks for the advise guys. I did not have the reedcage and reeds installed so I will check again after it is all together. I keep hearing the same things about Wiseco pistons, live and learn I guess and try to keep my mistakes down to one time only. Thanks again guys, appreciate it. Lt80 I made sure the arrow faced the back of the engine, the one mistake I did make was not comparing the two pistons making sure the the dome was the same distance from the pin centerline, I decked the cylinder about .010 but by the looks of the deck height I had I could have taken .030-.040 and still been ok. How much do you usually remove from the cylinder and/or head?

 

No Problemo [img]i/expressions/face-icon-small-smile.gif[/img] Glad to help.
Good thinking Bubba,I didn't pick up on the reed cage!! [img]i/expressions/face-icon-small-blush.gif[/img]
On a stock motor looking to bump the comp. I recommend .030 off the head. This still allows for the real head mod needed for a ported cylinder.
Pistons:The top to centerline distance "should" be the same as stock. I've never actually measured it,but when I've got to the bigger bore (1.5 and 2.0 we have no choice but to go weisco) I've not re-chambered,and not had a problem.
The main difference between the stock piston and a Weisco is that the Weisco needs to be fully warmed up.AND a FEW good warm ups and cool downs at initial starting of the motor. I also drill and champfer 6 holes (3 each side) below the ring gaps. This helps drag a lil more oil onto the cylinder wall. [img]i/expressions/face-icon-small-smile.gif[/img]
Have fun [img]i/expressions/face-icon-small-happy.gif[/img]