Yz80 no compression

I have a 94 yz80 that can’t get compression over 60psi. Top end was replaced with big bore and oem parts. New gaskets. What could be stopping it from getting enough compression to start?

 

Just built? Like just picked up from shop? Or you built it?

 

I built it then took it to a shop where it is now. He switched top ends and it still isn’t getting compression. I don’t want to pay anymore money I just want to fix it myself but can’t figure out the next place to look.

 

Sure it is correct size piston and correct cylinder bore size? Crank seals good? It is a 94 model. Cylinder and head torqued down?

 

What piston to cylinder clearance?

 

I had a 47mm std bore piston with the oem cylinder and big bore kit came with everything together. I was thinking crank seals also because the guy I got it from said he replaced them but I was wondering if he installed them correctly I actually have a set of cometic seals. But someone also told me that seals won’t stop compression. But a long time ago someone told me 2 stroke motors compression is in the top and bottom end.

 

If the shop can't figure out need new shop.

 

They will but I don’t want them jumping around fixing stuff then have more money in it than it’s worth.

 

Did they find any leaks when they pressure tested the engine? If they haven't pressure tested the engine yet, they need to. Or you need to find a new shop. First step after rebuilding a two stroke is pressure test before you start it and burn it up.

 

They did a compression test afterwards. The said it came up a few psi but not enough to start. I thought 2 stroke motors were pretty simple so I’m stumped on what could be the problem. Will crank seals stop the compression?

 

Yes crank seals are important, they need to seal. Also reed valve needs to close. They act on tension and they are essentially one way valve.

 

I checked the reeds they’re flat. I think I’m going to replace the crank seals since I have them and see if it fix the problem.

 

Need to check and see what piston to cylinder clearance is and what ring gap is. That is your biggest source of compression problem right there.

 

No way 2 top ends would have that problem. It’s possible but not likely.

 

Pressure test != Compression test FYI. Pressure test involves blocking off the exhaust and intake, and bringing the engine up to 5 or 6 PSI and seeing if it'll hold that pressure over time without leaking down. I really need to build up a pressure test setup for my sumo...

 

So it needs a leak down test not compression test? Because a compression test was done as stated.

 

Compression test just shows there is an issue, pressure test shows where the problem(s) are.

 

That’s good to know. What is the tool called used for this? Do they have them at advanced auto? And how do I know exactly where the problem is from using this?

 

Is it a good video on YouTube using the tester showing how to read symptoms?

 

I have had a lot of people take for granted they got sent the right parts. That's why I am saying to check certain clearances of parts.