Yz80 no compression

It is. You're skipping the basic diagnostic routine.

 

I doubt that you can achieve 60-90 psi with a standard piston in a big bore cylinder.

 

I had 2 set ups. Oem and big bore with matching pistons for the cylinders

 

This the rundown on what’s going on to give a insight on the problem. I bought the bike the guy said he couldn’t get compression on the new big bore set up. I thought that ok easy fix he didn’t install it right( maybe piston installed with the arrow to the intake). I took the top end off inspect it and reassembled. Sane result. I bought a oem cylinder off eBay and a standard bore wiseco piston and replaced the big bore sane result. I took the bike to a shop they got the same results from both.

 

This is difficult. If it's building some pressure, then it should be building all of the pressure unless the rings are that "wrong." Stumped, I am.

 

I don't know if this would help you.... but I have a YZ85 motor in a box taking up space in my garage. You'd have to confirm whether it would fit but I think it might, it is a 2002 motor (I think - I can send you serial number info if you want). I'd sell it to you for $75 plus shipping (or you can pick it up if you are in So CA area). I bought it for a project bike but never installed it. It is a Darcy DeCoste built motor in freshened condition. I'm told that it would fit in all YZ85 motorcycles and probably in YZ80s up until mid 90's, you'd have to verify if it will drop in or not. Let me know if you want more info. There's an M4 kart exhaust that can go with it, too, if you have a use for it.

I know the sell price sounds fishy but I'm in garage clean out mode and just want it gone, the price is just to cover my time in getting it out and ready and taking it somewhere to ship. I can get you references as to who I am so you know it's legit.

 

No no, we can't start internet fix it shit and not fix it. We demand some closure.

 

No thank you. I have a spare bottom end that I would try before anything else.

 

Calling robert stack...

 

Maybe a hairline crack in the head or spark plug thread issues.last part you did not change...since we are shooting darts.

 

This is a '94 with original crank seals? Very likely the problem. I would put new seals in it.

 

Soo pulled crank drain plugs on an old polaris today and got same comp. numbers as with them in....

 

I would assume you have pulled the carb and cleaned it, made sure there isn’t a wash cap on in place of the air filter and that the reeds aren’t stuck shut and will open?
It has to get air in before it can build compression.

I’m assuming the ring moves freely in the ring groove and you placed it in the cylinder and checked ring gap and for even contact around its perimeter?

It takes 90 PSI or so for it to run, if it is only getting 60, it is either the gauge reading wrong, a leak allowing air to escape the combustion chamber or a restriction not letting air in. I would do a leak down and make sure it isn’t pushing pressure out the radiator or past the rings first. Then pull the reed block and inspect the intake tract if it is sealed up.

 

People have been saying do a leak down since page 1. Not sure why he won't do the one thing that will help the most.

 

Give him time, I think not having the tool is the hold up and apparently the shop he took it to doesn't know how or doesn't have tool either.

 

Yeah, most are talking about vacuum testing it. I’m saying lock the piston at TDC and put some air pressure to it and see if it is leaking out.
Vacuum testing doesn’t have much to
Do
With not having compression, unless it burned a piston from being lean...

 

A pressure test doesn't even require TDC on a 2 stroke. You can plug the intake and exhaust and pressurize from the sparkplug if you have the piston at BDC. Six psi and a spray bottle with soapy water will tell you everything you need to know. Heck, you could put a YZ85 engine in a shop sink filled with water and watch for bubbles like a tire leak test.

 

Yeah, you are talking about. A low psi pressure test to look for crankcase leaks.
I’m talking bout putting 100psi in the combustion chamber and seeing where it goes, or doesn’t. That type leakdown can be very valuable when diagnosing low compression.