NU50 Ran around the block too lean

Just finished cleaning up an Urban Express I picked up on craigslist. I ran it around the block before I did anything and it ran ok but no brakes, broken shock, etc I didnt go far on the death trap. Anyway, I got it ready to ride and changed the belt to a BX31. Instead of grabbing a stock box from the other NU50 I just used the K&N the guy had on it. The bike blasted 10 times better than I ever hoped and much better than the stock Iowa NU50. On the fourth time around the block (it was too much fun) I heard a clunk and slowly lost power and then when I went off the gas it died completely. Walked it home. Pulled the plug and it was obviously running way too lean. Won't start at all now and it was starting on one kick.

Any ideas? I suppose I could have jacked something up with the kick start gears when I put in the new belt... but that doesn't really explain why it stopped running unless thats interfering with the belt. I haven't seized a bike before but this didn't seem like even a soft seize, but I wouldn't know.

Re: NU50 Ran around the block too lean

Wrist pin clip.

Pull the top end and lets see pics.

Re: NU50 Ran around the block too lean

Will do. Might be a couple hours, realized its way past dinner time.

Re: NU50 Ran around the block too lean

Pull the plug and cover with your thumb, kick it over. If it doesn't blow your thumb off no matter how hard you push, your rings are toast. I vote wrist pin clip as well though. Although you did make it lean, the auto oil injection keeps things mostly safe on stock or very lightly modded setups.

Re: NU50 Ran around the block too lean

Well... it looks fucked.


Re: NU50 Ran around the block too lean

Would say so.

Re: NU50 Ran around the block too lean

..alittlethin .. /

60 cc kit from treats




Re: NU50 Ran around the block too lean

I am trying to understand how this happened so I don't repeat it with a new piston. Was this likely caused by improper installation of the wrist pin clips by previous owner? Or because I ran it too lean and it got too hot?

This tragedy has got me considering a kit. It will be my first kit on any bike except one (already had it when i bought it). I have read loads of posts and wiki articles about it but there are still a few things I was wondering about.

I see lots of posts of people complaining about shitty clips in kits and saying not to use old ones. But I don't see any recommendations or links for "good clips".

I am considering the

Malossi cast iron 70cc


DR Kit

Any others I should consider for the NU50?

Can I use my stock head on these kits (above) or is that a bad idea.

I think I read that the jet on the stock carb is 85. I see people posting good results with DR kits and 90-95 jet in stock carb. I was planning on just getting a 90 and 92, should I get more jets in a larger range to find the right one?

Re: NU50 Ran around the block too lean

alittlethin, I like the price of the kit you linked much better... but I was worried about the quality being an issue. Have you used that kit? I will add that to my list of possibilities. At that price I can afford to get treats for some other bikes or maybe 2 kits so I can put one on the hobbit.

Re: NU50 Ran around the block too lean

That was not due to lean running in any way. That was mechanical failure. Either the clip was reused at some point (don't do that) or a new one installed improperly. OR, the bike was free revved to the moon while on the stand. Could be a combination of all three.

Re: NU50 Ran around the block too lean

CONEMAN (you will call me Coneman.) /

snag the DR and romp. its a solid solution. I love mine and treats is having a memorial day 20% off sale.

Re: NU50 Ran around the block too lean

The DR is a killer kit. Loved mine, killed it losing a clip (revved on stand). Bought another, chunked a piston, honed it, new piston, ate a clip, honed it, new piston. The thing will not die.

Re: NU50 Ran around the block too lean

what kit was on there? stock urban cylinders don't have the decomp passage. see the oily circle on the bottom left corner of the head mating surface, you gotta plug that on the cylinder or you can get a head gasket airleak. 90ish with stock airbox, 95 for the hi flow filter should be a good starting point.

Re: NU50 Ran around the block too lean


Thanks for noticing. I don't know what kit it is, i can check for any markings and measure it. Not sure how else to tell since I never bought any kits until yesterday (treats sale got me). I didn't realize it wasn't stock. I just got it on craigslist and the dude didnt know a damn thing about it. He tried to put the front end back on and didn't hook up the brakes at all. He also didn't know what the autochoke was and hooked the battery drain hose to the CARB! so the battery box was filled with oily residue (no battery tho).

The fact that its not stock certainly explains why it was so much faster and better than my stock Urban. I threw the BX31 belt on it and took off so I thought it was just that and didnt consider that it came kitted. But also explains the failure... someone sucked at putting in a piston and they should have plugged that decomp. Hope I can do better. (edited)

Re: NU50 Ran around the block too lean

Judging by the look and size I think its

Do you think the cylinder is too scratched up to be used? If not then I suppose I can just get the piston

Re: NU50 Ran around the block too lean

Jesus G. saldivar /

How fast was it running man since I'm undecided on the kit 60cc or 70cc. I dont want to change the carb.

Re: NU50 Ran around the block too lean

The needle went beyond what the speedometer can show. I didn't gps it or anything. I don't know how much of the performance increase was the 60cc kit or how much was the belt but it was really fun.I haven't checked to see what jet is in there but I just ordered a few different sizes. I also ordered a pipe for it and I can report back with the results when i get everything together.

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