Yz80 no compression

Wont get at AutoZone. Check motion pro or other bike specific tool companies.

 

What is it called?

 

2 stroke leak down or 2 Stoke pressure test set

 

Thank you

 

Yes, the cranks and bearings are NOT immersed/oiled- 'cept for what's in the gas, as in 2 stroke oil?

 

Don't see how reeds or crank seals will change compression considerably, surely after the engine is running it will change fuel mixtures and you'll see other symptoms. Make sure exhaust is open, have seen pipes nearly drooled shut in the past...

What was the reason for the teardown in the first place? Was this a running bike prior? New parts used? Any measurements taken? Something seems out of place.. Did you oil the cylinder at all? Ring end gap checked?

 

Here's a very nice pressure tester. You can make one cheaper, but probably better value for this kit if you build multiple varieties of engines.
https://www.amazon.com/Motion-Pro-08-0071-2-Stroke-Tester/dp/B000GUYSQG

Reeds have to seal to help contain crankcase pressure, which leads to combustion pressure. Not a direct 1:1 with compression, but if the reeds don't seal then almost no air is captured in the crankcase, which would then be pumped into the combustion chamber. So, little volume of air in crankcase directly affects compression.

 

Running i see it being an issue with power and mixture, kicking speed i cant see it . I could be wrong however. How much pressure is actually made in the case? I wish i had a 2t here just to pull out the reed block and see if compression numbers changed.

 

Reed's can change it, as the piston falls and push air up transfer ports if it's not sealed at reeds and crank it can push air out instead of into combustion chamber.

 

I’m going to look into both. I checked the reeds but it can’t hurt to put a new set in.

 

Main thing is get a piston and bore measurement, ring gap measurement and see what your working. Don't keep throwing parts at it until you know what you need. If the shop can't do simple diagnosis pick it up and don't go back.

 

tell the shop you found a plumber to adjust your valves...lol

 

50-90psi though?

 

I'm not saying reeds are the problem, one of many suggestions to look at. I'm saying go through some diag, measure some things and fix it. He mentioned OEM stuff AND big bore stuff. Somewhere in there is a problem I bet. Don't know internet mechanicicing ain't always easy.

 

Facts

 

I bought the bike with the same issue it’s having now. Cylinder was oiled.

 

Let me see if I understand: You bought the bike with "low compression," and had a new top end installed to cure the low compression, correct? Here are my questions:
Did the bike run when you bought it?
If not, when did it last run?
Did you buy it knowing that it has "low compression"?
Who diagnosed the low compression symptom?
Have you or your mechanic started it, or attempted to start it after the top ends were installed, or did you check compression, see that it's low, and stop there? Do you have spark? Fuel?

Here's where I start to think that it's something other than "low compression": Two strokes with high RPM porting (such as a YZ) will have poor kicking compression, and you need to determine captured volume to determine combustion chamber pressure. You can't accurately measure compression on two stroke engines because of the way that intake and exhaust porting interact with the piston rings. Also, these small bore engines kick over really really easy, to the point where a four stroke guy would say "this thing has no compression!" It will almost fall through the kick stroke and appear to have a huge hole in the cylinder.

My advice: Start the bike and if it starts, it's good. If it won't start, check fuel and spark. If no one has attempted to start it because they think that the compression is too low, I believe that you are chasing an issue that doesn't exist. If it starts and runs, well, it starts and runs. If you haven't even tried to start it, then you will never have a running bike.

Just go for it. It's too simple to have it blow up in the first 30 seconds, unless something is REALLY wrong, and then it won't be a compression issue.

 

Yeah, you're at the point of going back and checking everything. I'd pull it apart and start measuring. Maybe the kid putting parts in the box grabbed the wrong piston and it's 10 under or something.

 

Big bore cyl + std piston = poor compression!

 

Oh I’ve kicked it nothing. Mechanic has tried nothing. I bought it with the problem thinking it would be simple.