New rebuild help

openupyoureye

New Member
Feb 9, 2022
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Been doing a lot of reading over the past few months but now it’s time for me to post.

I have just completed a rebuild on my sons blaster. When I bought it it seemed tired and the only mods I knew of was a fmf pipe. Stock air box and I feel like the reeds are in good shape. His piston blew up at the dunes and I suspected lack of oil. I am fairly mechanically inclined but this is my first engine rebuild.

I completely broke down the motor and split the case. I cleaned it very thoroughly. I ordered a wiseco crank and rod assembly. I replaced all the seals and mated the case with yamabond. During reassembly I did the oil pump delete. I replaced the jug with a new one and put a wiseco piston in it. Both the jug and piston are stock bore.

I got the motor back in and all hooked up. Filled the gear box with transmission fluid. I drained the fuel and refilled it with premixed 32:1. I use redline oil with 98 octane. It fired up to my surprise on the third or fourth kick. I was super happy. I drove it around the block a few time and noticed it was smoking like a train. Thick white smoke. I drained the fuel and went leaner on the oil to 40:1. It helped a little with the smoke.

The issue now is it feels very responsive on the low end but when I try to give it much throttle it bogs out. It feels like it runs out of fuel. So next I went through the carburetor and cleaned it out. Put it back in and another slight improvement. I ordered a jet kit and upped the jut to a 280 if I recall from a 260. Again a small improvement but it still feels like something is off. Lack of mid range power. Second issue is I can’t get it to idle. It’s either almost running away at 5k rpm or stalls out. I’ve adjusted the air fuel screw with not much change and adjust the tors screw a little and that’s when it wants to run away on me.

so I’m not sure where to go from here. Do I need to try a leak down test or what? Any advice would be greatly appreciated. Thanks!
 
A leak down test is not something that you “just try”
It’s something you DO any case work.
It’s also recommended as routine maintenance 1-2 times per :eek:year. It ensures no air leaks, which is a death sentence for a two stroke !!
White (stinky) smoke is a prime indication of a leak burning trans oil !!
All the things that you did were like turning up the radio when your car makes a noise :eek:

First thing you need to do is list all known mods. What pipe, air filter, jets in carb? Any porting? What air filter, lid on or no lid?

One mistake is the Chi-NA cylinder, especially with a Wiseco piston. You probably don’t have the proper clearance. Why didn’t you just bore the old cylinder?

You need to learn how to jet properly. You can’t just throw any old jet in and be right. That’s why you’re having trouble.
 
Understood! Pulled to motor and found the leak. Took it in and had the chip welded up. Got it all back together and will leak test tomorrow. I bought a new jug because when the piston blew up it scratched it pretty good. I figured how could you screw up a jug? I still have the old one. Is there a way to tell if it’s original? I didn’t get out a micrometer but it doesn’t seem to be sloppy.
 
Left side of cylinder by the reeds there would be a Y-1 or Y-2 cast in cylinder.
Have it looked at and measured at an automotive machine shop. They can tell you if it can be bored.

IIRC you can bore up to 2.25 mm
If it can be saved the shop will tell you what size to get. You get piston and have it bored/honed to fit THAT piston, they all vary slightly.
 
Test held pressure and got it back in. Getting it broke in rich on oil and I’ll fine tune the jetting now that it’s running properly. So the “China” jugs are bad why? Is it the tolerances or is there other issues?
 
Test held pressure and got it back in. Getting it broke in rich on oil and I’ll fine tune the jetting now that it’s running properly. So the “China” jugs are bad why? Is it the tolerances or is there other issues?

Poor Materials for sleeve- soft- casting slag left behind in ports could come free and cause damage, Improper port timing and port dimensions compared to oem, Its china....