low compression after rebuild

[an]

I just rebuilt my son's kx65 top end, it had about 145lbs of comp. before the rebuild. After having him ride about 20 minutes of easy laps around the yard I checked the compression, its only getting 120 lbs. It starts and runs fine, I'm wondering if this is normal until the engine is fully broke in. If not where did I go wrong, and what do I do from here? He is supposed to race tomorrow, but I'm scared to let him. Please, some advice, thanks.
A

 

[Jasle]

something seems amiss. did you gap the rings correctly? perhaps a knick in the plating. Kaw's are notorious for thing plating. perhaps you just had a bad seal with the tester...trust me seen that one bite me. try again cold. then with a little oil in the cylinder and see if it goes up with the oil. can tell us if the rings are leaking by.
Also are you testing with the throttle fully open while kicking. if not your readings can vary.

 

[an]

Jasle, I did not gap the rings, I figured if they were new they would be right. I also was worried about the thin plating so I took the cylinder to a local shop to have it honed. He barely touched it, once in and out. Would the nick you spoke of be fairly obvious to the naked eye? In answer to your other questions, I did have the throttle wide open when checking the compression, and Ive checked it several times with different gauges. I will try some oil in the cylinder, how much would you suggest? Also I have ordered new rings and gaskets, so I can give it another try if all else fails. As you've probably figured out by now, I'm not much of a mechanic ( this was my first top end attempt), so I really appreciate your reply.

Thanks
An

 

[Jasle]

not much oil...about half a teaspoon of premix oil ought to do it. This will seal the rings and your compression will be higher if they are leaking by.

The reason you need to gap the rings is that the rings are made for about 4 sizes of cylinders. The cylinders are usually marked with a size like A,B,C,D This denotes machining tolerances and where the cylinders fell. That way they can match up a piston. Wiseco errs on the small side so you don't need to order the specific size piston. When ordering OEM you usually will tell the vendor which cylinder you have so they get you the correct size piston.
If memory serves correctly for he wiseco you need .006 gap for every 1 inch of bore. So a kx65 is 45mm. I think that equates to .014gap. Doublecheck my math because its off the top of my head.
You drop the rings in the cylinder and square them up with the piston. then measure the gap with a feeler gauge. If they are too tight then you need to file them down. The reason is that when heated they expand. If too tight the ends will hit and bind causing scoring or possibly siezure. Eric Gorr has some info on his website. www.ericgorr.com His explanation is first rate.

The nick will be plain to see. They usually chip around the exhaust port.
Not bad for a first attempt. If all else fails just pop the head and give a peak. It cannot hurt to check it out.

 

[MikeS]

How did the cylinder look when honed ? Was there spots that the hone did not hit. If the cylinder was egg shaped at some point the new rings are not going to seal well. That happened to us on our RM. When I changed the piston and ring the compression and overall perforamnce got worse. After a few hours it improved some. I ended up having Eric replate it.

Do the oil trick as mentioned. If the compression increases then pul it apart. Also check that gasket thickness are the same. a thicker base gasket will lower copression.

Goodluck

 

[an]

Thanks guys, excellent advice, I will try the oil tonite & probably end up tearing it down again.
An

 

[an]

From the oil tip, I found that the rings were leaking by. I tore the top back down and checked the gap on the rings, both the ones I just put in and the new ones. The gap on both was .008, which according to Wiseco is about right (.004" per inch of bore = .007"). The part that really has me stumped is I found several vertical scores on the cylinder, there is only one that is deep enough to feel with my fingernail and that is just barely. Also there is one single score on the piston itself, it does not coincide with any of the marks on the cylinder though. So where did I go wrong, does Wiseco offer more than one size piston for the 65? Our cylinder is an A, there are some stampings on the top of the piston, a bunch of numbers and then below that a "cd". And most importantly where do I go fom here? Sorry to keep bothering everyone with my problem, but I would really like to know where I screwed up so as not to have a repeat perfomance!
Thanks in advance,
An

 

[MikeS]

I will assume the scores are very narrow as they look like they are drawn with a pencil. These can and are generally caused by dirt, pieces of gasket or carbon that were stuck in a port or had fallen into the crank area when the cylinder was pulled off. It can happen even when you are careful. Another thing is when it was honed. Alot of different ways to do it and alot of opinions out there. The hone could of nicked a port or a piece of hone grit got stuck. I use a undersize ball hone to break the glaze or just a 3M maroon scuff pad. Eric Gorr uses a Hone/reamer that does a great (perfect) job but sending it out is not really worth it for just a piston change.

By chance did you check the reeds? If they are fraying they can cause scrapes/scoring ..

Wiesco pistons do expand more than OEM in general. Make sure you warm it up real good everytime the lil guys takes it out. Alot of racers only use OEM or Nik's because they can get the ABCD variation if needed.

Good luck

 

[stu182]

Did you wash the cylinder in hot soapy water to make sure to get all the grit from the honing out? That grit is some mean stuff.