Low compression after rebuild

[Jake Lindahl]

First time making a post. But So my chain popped the day while riding and cracked the case, still rode it with the crack for awhile after I just would add oil before every ride. Eventually I blew the top end.
So I ordered a new top end kit off eBay which came with new gaskets, new cylinder, piston, rings, spark plug. I also ordered a used left case to replace the one that the chain cracked.
I’ve nvr done a bottom end before do this was a first for me. So I pulled the motor and split the cases swapped the cracked left case with the used one I got off eBay and used grey rtv to seal the cases. ( I know I should use three bond, but I didn’t have any). Everything seemed fine crank moves nice and everything seems as it should.
I then rebuilt the top end. Unlike the bottom end I have done about a dozen top end rebuilds and nvr had a problem until now. So I rebuilt the top end put it back one the quad. And could not get it to fire.
Pulled the plug and it was dry so I check the compression and it had 65psi with throttle open. So I pulled the top end back off and put new gaskets on along with another new piston and rings and still no change. Still only had 65psi. I removed the carb thinking maybe it wasn’t getting any air flow and still no change. I even tried using a old head I head laying around to see if anything changed but nothing Did.
I’m stumped and need some advice thx guys.

 

[Larry's Shee]

Have you done a leak down test?
https://www.blasterforum.com/threads/more-info-on-the-chinese-blaster-cylinder.66076/
me thinks you're another bit by the ChingChong junk

What brand compression gauge you use?

 

[Jake Lindahl]

Have you done a leak down test?
https://www.blasterforum.com/threads/more-info-on-the-chinese-blaster-cylinder.66076/
me thinks you're another bit by the ChingChong junk

What brand compression gauge you use?

I have not done leak down test. And I got
A innova compression gauge off amazon but I do have a second old gauge that read the same

 

[Larry's Shee]

Leak down test would show leaking head.
Compression tester needs to have Schrader valve at END of hose. Otherwise you also have to also fill the volume of hose plus head volume.

What was ring end gap? Are rings right side up?
Read thread about JUNK Chinese kits?

 

[Jake Lindahl]

Leak down test would show leaking head.
Compression tester needs to have Schrader valve at END of hose. Otherwise you also have to also fill the volume of hose plus head volume.

What was ring end gap? Are rings right side up?
Read thread about JUNK Chinese kits?

So I made a leak tester yesterday and it’s leaking at right side crank seal , intake, and and exhaust but the head and base had a good seal and didn’t leak. Would that cause low compression? And I think the rings had .012 gap if I remember correctly. And yes compression tester had valve.
I’m really thinking the cylinder that I got off eBay is the the problem

 

[Larry's Shee]

Might as well replace both seals. Use a new gasket on BOTH sides of Reed cage, very common "repair" for this problem. Tighten screws incrimentainc in a criss-cross pattern to 7 ft. pounds.

Ring gap sounds right, so that isn't the problem. Seeing that you tried a second piston/rings only thing I can think of is that the cylinder is out of round. Though leaks might keep it from running, but have no effect on compression.

Recheck/clean carb, especially pilot jet. Even though some say not to use starting fluid, it's helpful as a diagnostic tool. MAYBE if you can get it started maybe getting the rings seated maybe compression might come up some.

Hopefully if you didn't scrap old cylinder you can rebuild it.
No offense but if you can buy a cylinder, piston, gasket, and sometimes carb and head for less than a good piston (Wiseco, Worsner, etc.) should tell something.

 

[Jake Lindahl]

Might as well replace both seals. Use a new gasket on BOTH sides of Reed cage, very common "repair" for this problem. Tighten screws incrimentainc in a criss-cross pattern to 7 ft. pounds.

Ring gap sounds right, so that isn't the problem. Seeing that you tried a second piston/rings only thing I can think of is that the cylinder is out of round. Though leaks might keep it from running, but have no effect on compression.

Recheck/clean carb, especially pilot jet. Even though some say not to use starting fluid, it's helpful as a diagnostic tool. MAYBE if you can get it started maybe getting the rings seated maybe compression might come up some.

Hopefully if you didn't scrap old cylinder you can rebuild it.
No offense but if you can buy a cylinder, piston, gasket, and sometimes carb and head for less than a good piston (Wiseco, Worsner, etc.) should tell something.

Ya I agree with you on that, the top end that was on it that I blew up was a Ebay special but I got lucky and it ran great for awhile but i didn’t get lucky this time lol.
Can I just hone the old one? I’ve heard you can’t

 

[Larry's Shee]

I doubt that would solve anything. It could be out of round or tapered from the factory. But, then again it wouldn't hurt to try. What's the worst that could happen?

Boreing to next oversize wouldn't be worth it. If you read the post by Ken, he found voids in the liner casting.