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Z400 90psi Compression

7K views 11 replies 4 participants last post by  amcf18 
#1 ·
Hi all,

My 03 z400 has been sitting for about 2 years, but was running rough before it was put up. It is very hard to start, but will run if you push it off as long as it is on full choke. It runs rough, and will not idle, but it does run. A compression test came back with 90 psi. We took the head off, and I'll post some pictures. What do you guys think?
 

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#2 ·
Its probably bad piston rings. Take the jug off and check the cylinder and piston if they are good, order new rings and a new gasket set. Be sure that the valves are not leaking.
 
#4 ·
With the head off, pour a capful of gas in each intake and exhaust hole. The valves should hold the gas without leaking into the cylinder area.
 
#5 ·
Thanks for the response, I poured gas from the bottom of the head and the valves didn't seem to leak. I'll try from the top as well. The cylinder walls felt smooth, and the piston rings were slightly loose? feeling. I'm not sure how tight they are supposed to be, but it feels like you could easily just pull them off. Is that normal? If not, do you think the exhaust valve being .19 instead of .20mm would cause that low of compression? It wasn't smoking, it would pop and backfire though.
 
#6 ·
As long as there was some valve clearance, it shouldn't bother compression. If it's at zero, then its possible the valves are not closing fully.
 
#8 ·
If it is not valves, then it has to be ring to cylinder or gasket leaking, provided the spark plug was tight.
In a perfect world, the cylinder should be honed for the new rings to seal better, but they don't recommend honing the Nikasil that these have. Search honing and hang on... A lot of us just by the cheap kits that have everything fresh, and go from there.
Price stuff out and decide..
 
#9 ·
I installed the new rings, all of the gaps were correct. The piston was pretty difficult to get into the cylinder, but it did go in. When I broke it down, the piston was pretty loose in the cylinder. With the new rings, it is pretty tight. Is this normal, or should it be loose like before?
 
#10 ·
As long as you bought the correct rings and you measureed the ring gap to be sure its in spec then everything should be fine. Remember to not put it in dry!
 
#11 ·
So I remeasured to make sure it was right, and put the piston back in the jug. Installed bottom gasket, installed head gasket, and reinstalled the head. Without the cams in the head, I rotated the crank to get back to TDC. For some reason, the piston will move smoothly and then abruptly stop. You can feel a good bit of resistance, keeping it from getting to the timing mark. What could I have done wrong? This is starting to be a pain..
 
#12 ·
The timing chain may be caught down in the gears...Take the slack out of it.
 
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