Breaking in New Top End & Seizing

[Bobbyutton]

So I recently bought my kymco people 50 2T and it had been sitting for a while, the engine was not maintained very well, so I upgraded it to a 70cc, a charmo 47 mm big bore kit. Well I've been breaking it in for a couple days. I have not been exceeding 35-40 mph while trying to break it in, but today I gave a friend of mine a short lift to his house because frankly I was his only hope of getting home. I shouldn't have since I'm breaking in a new top end, but I did it anyways. Well right as I got to his house, and was pulling into the driveway, my moped just died as I was coming to a stop. I only did 35 with him on the back, but I'm guessing his extra weight pushed the new top end too far? I let it sit for a while before trying to make it home, and on the way home it died once more as I was doing 40 mph down the road, the engine just cut off, no jolting, nothing, so I pulled off into a parking lot. I still had about 2 miles left until I made it home so I let it sit again for about 15 minutes and then started it up again and went home. I stayed at a generous 30 but no more than 35 for the rest of the ride. I don't think I had as good of compression at this point because I could feel that i shouldn't be exceeding 35, earlier when I would ride I would stay around there just to be safe, but I knew it could go faster if I tried. But that was before I gave the kid a ride home. Well as I pulled into my driveway and was coming to a stop, it died once more just before I came to a full stop. I'm not sure what's wrong exactly, I have the slightest of ideas, but I definitely do not know what I should be doing in order to prevent any more damage and/or fix any damage already caused. I really don't want to have to replace the top end again and I don't have the money to do that anyways. Please help me

[chaz35]

Hello Bobby, welcome to forum.  I guess you installed the kit yourself?  I would pull the cylinder and see what you have.  If it's still starting after it cools down, probably can clean up cylinder with sandpaper and will run OK.  Be sure to clean out all the grit, before putting it back together, any grit will destroy what's left of your engine.  Shouldn't cost anything to clean it up, remove any rough spots on cylinder/  piston, and put it all back.

BBK's take a lot of easy miles to break-in.  I have a Malossi and they recommend 500 miles, which seems excessive, but apparently yours is like that too.  More $'s, but CHT instrument (cylinder head temp) will help you monitor engine temp and help you keep it alive.  See below link for Trailtech TTO CHT that I use, shop around, you can get for less $'s.  Good luck.  Cheers

http://www.trailtech.net/723-et3

[Bobbyutton]

Thank you, I will try this and hopefully be back on the road!

[Bobbyutton]

So, I cleaned out everything, sanded down the piston, but the engine died when I took it out on the road after a mile or two. I don't think it will start back up this time. Can I just replace the piston, or do I have to purchase a whole new top end?

[CROSSBOLT]

Did you set/check the piston ring end-gap? Real important since too little end-gap will lock the piston in the bore when it warms up. Unlocks when it cools down.

Karl

[Bobbyutton]

I did not set the piston ring end gap, could you elaborate what I would need to do? I'm not exactly familiar with everything on the moped yet. I've been working on cars for years, I can work on a moped, I just don't know what I need to be doing necessarily.

[CROSSBOLT]

Setting the ring end gap in a two-stroke is like in a four-stroke the only difference may be in how much of a gap is required by the manufacturer. First, you put just the ring in the cylinder without the piston. Then, "square" it in the cylinder by pushing it a little farther in the cylinder with the piston. Use the end of the piston which presents the largest area to the ring; not important which end just so it is flat to the ring. Finally, with a feeler gauge, determine what the end gap is. Grind off a little if the gap is too small as I suspect in your situation. Be mindful that two-stokes have a little pin in the piston which keeps the rings from rotating. This keeps the ends of the rings from rotating into an open port. You may have to grind a little off each end to make sure there is enough clearance for that little pin.There will be an arrow stamped into the top of the piston usually pointing forward to aid you in getting the piston installed correctly. Make sure the rings are installed on the piston right side up, usually any makings on the ring will be positioned up.

Karl

[chaz35]

Bobby, what did cylinder/ piston look like?  Post some pics if you can.

Crossbolt's info about ring gap is good info.  Do you have instructions for what ring gap is supposed to be?  I am not familiar with your kit.  Actually, I am still trying to get my Malossi BBK to run right, but I did check ring gap like Crossbolt suggested.  Actually, I have installed 2 Malossi BBKs and rings in both kits were proper gap.

Once you run engine and stick it, your ring gap is probably over spec anyway.  You are just trying to get it to run at that point IMO.  Yes, you can hone cylinder and install new piston/ rings, if you can find piston/ rings and have it properly honed (another expense)?  I can get new complete kit for not much more than what piston/ rings and hone cost.  If you can't get what you have to run acceptable, I would probably just get another kit and start over.

Do you know it's recommended to chamfer the ports?