JR50 top end assembly and injection to premix conversion

[JCBrewer]

I'm a long-time car guy. These two strokes are new to me. My son's JR50 recently ate a piston. I'm thinking, failure of the oil injection system. Anyway, got a new cylinder, piston, pin, bearing, clips, etc. Any tricks on putting it all back together so this doesn't happen again? One specific question, what should I torque the head bolts to? I figure they should torque down in a criss-cross pattern stepping 10 foot pounds every increment. I'm guessing maybe 40 foot pounds total. Wouldn't mind having some real numbers though.

I'm also thinking about going with premixed fuel. I've never trusted the oil injection systems. With premixed, if it's running, you're lubed. I'm guessing that'll require rejetting the carburetor to get it to run right. Anyone have any experience with that?

 

[GhostRider32]

If you remix, you don't have to worry about injection failure. I'd premix and then see if I need to rejet.

 

[Senior KX Rider]

I would call a dealer or find someone with a manual for torque specs. 40 ft lbs of torque would prolly snap any bolt on the JR. My guess would be about 15

 

[Skitspuce]

We have an '86 JR50 that I bought last year. When I got it home, I noticed that it had no oil in the tank, so before I fired it up I took the head and cylinder off. It still looked OK so when I put it back together I tightened everything up to "snug". It ran all year like that, ridden hard and no probs. (5.5:1 compression and a 1.6" bore = little stress on head studs :) ) Haven't had any trouble with the injection, either, and thats on an 18 year old bike. I did see a couple of posts about removing the injection...search for LT50s on a quad board - engine is the same. Apparently if you just let the pump run with no oil it will burn up and lock up. You can either remove it entirely and rig a block-off or I remember some mention of taking off a plastic drive gear and reinstalling. If you go to bikebandit.com you can look at the OEM section at the schematics and it'll give you a good idea of how it all goes together.

 

[Skitspuce]

Found link to LT50 discussion....

LT50 Power, etc

 

[Jasle]

the injectors rarely fail. What does your piston look like. I'd guess it siezed around the exhaust port. These bikes don't like being ridden hard. Run loose clearances if your kid rides it hard. make sure ring end gap is .010-.012

I'd think the torque would be like 10ft/lbs.

 

[jeddclampette]

I'm a long-time car guy. These two strokes are new to me. My son's JR50 recently ate a piston. I'm thinking, failure of the oil injection system. Anyway, got a new cylinder, piston, pin, bearing, clips, etc. Any tricks on putting it all back together so this doesn't happen again? One specific question, what should I torque the head bolts to? I figure they should torque down in a criss-cross pattern stepping 10 foot pounds every increment. I'm guessing maybe 40 foot pounds total.

If you torque it those 10mm nuts down to 40 or even 30 ft lbs, you may shear the studs from the head. Torque no more than 25ft lbs. max. I'd go w/ 17ft lbs and leave it at that for starters. After the bike's runned in, you can go bak and retorque to 22ft lbs max. This engine's merely one step away from a push lawnmower engine.

As far as tricks go on putting it back together, 2 things come to my mind. First, locate the arrow on the piston. Be sure that when you go to assemble the rod and bearing into the piston, the piston arrow points to the exhaust port.

Second, there's two piston rings that go on the piston groove. Be sure the gap opening of one piston is not colinear w/ the opening of the other one. overlap/cross them so that there is no loss of compression. Also, be sure your ring gap opening is twisted away from the exhaust port. The gap could catch onto the exhaust port and break. same caution applies to the intake port as well.

Disclaimer: I'm recalling from memory...I can be wrong in the details. So please do your own research as well.

 

[TEEEE]

Found link to LT50 discussion....

LT50 Power, etc

To re-hash an old thread...

I'm thinking of switching my son's JR50 to premix, just for simplification. I run Dumonde Tech Z1 in my jetski at 40:1 and am thinking of doing the same in the JR50. I've heard of others mixing at 40:1, but is 32:1 a better idea?

Also, there was a post in the ATV thread that is linked above (that is a great thread, BTW) that says the LT50 oil injector is run off the clutch so while it is idling, there is no oil being added. I can't belive that would be true but haven't even started to tinker with this JR50, it's still brand new. Can anyone shoot that down for the JR50?

 

[YZ165]

Even at idle the clutch is still turning.