Yamaha Zuma with poor compression

sc00ter · 06-06-2020, 05:31 PM

Are you 100% sure the piston ring(s) didn't break during reassembly? Could the snap-ring for the piston popped out (Be certain its FULLY seated before reassembly!) as it will pop out and tear stuff up. We use "G" style snap-rings because their easier to install.

I doubt its the reed cage. If the reeds are not broken then all is good there. Reeds rarely sit tight on the cage.

Again, compression test. Make sure everything is tight, as compression testers are a PITA to use sometimes. Do 3 test, from start to finish, and average the numbers.

No matter what the results are, you need to determine what caused the problem(s) to begin with or they will just repeat.

We see air leaks in the system (mostly around the reed cage/case seam) that will eventually lean seize the engine. Poor torque values on the cylinder head can warp the head, causing a air leak. Torque wrenches are a must!

I'm sorta done with 2 strokes as a whole. And I think my riding buddy is selling all his stuff as well. I might hang onto my 98 pre-bug Zuma but its staying mostly stock/Stage 1. But that being said, keep us posted and I'll attempt to help as much as possible.

Side note: Did you ever get the carb setting back to factory? A lean setting on the carb will slowly lean seize the motor, eventually killing compression.

06-06-2020, 05:31 PM	#6
sc00ter Join Date: May 2017 Location: Norfolk, VA Posts: 531	Are you 100% sure the piston ring(s) didn't break during reassembly? Could the snap-ring for the piston popped out (Be certain its FULLY seated before reassembly!) as it will pop out and tear stuff up. We use "G" style snap-rings because their easier to install. I doubt its the reed cage. If the reeds are not broken then all is good there. Reeds rarely sit tight on the cage. Again, compression test. Make sure everything is tight, as compression testers are a PITA to use sometimes. Do 3 test, from start to finish, and average the numbers. No matter what the results are, you need to determine what caused the problem(s) to begin with or they will just repeat. We see air leaks in the system (mostly around the reed cage/case seam) that will eventually lean seize the engine. Poor torque values on the cylinder head can warp the head, causing a air leak. Torque wrenches are a must! I'm sorta done with 2 strokes as a whole. And I think my riding buddy is selling all his stuff as well. I might hang onto my 98 pre-bug Zuma but its staying mostly stock/Stage 1. But that being said, keep us posted and I'll attempt to help as much as possible. Side note: Did you ever get the carb setting back to factory? A lean setting on the carb will slowly lean seize the motor, eventually killing compression.
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