Picked up a 2004 LTA50 for the kiddo for a bargain, the seller disclosed it didn't like to run when hot- no problem so i thought and handed him $500 bucks, brought it home- gave it a once over and fired it up, ran great until it got hot- and hot he wasn't kidding, it exceeded 350F on both the iron barrel and head in under 5 minutes (via my quality IR thermometer) of running and stuck when i left of the throttle, holy crap i thought the auto lube system failed so I sucked the fuel out and put in amsoil 32:1 from my cr250r and fired it back up after draining the carb bowl- nope, same problem. Next step was to check jetting, plug is nice light grey and it carbrated clean threw the entire range but I pulled it apart and yanked the intake and exhaust restricters along with the 1st pilot jet and upped the main to a 65. put it all back together, big power increase but same problem with it hitting 350+F in a few min. Switched to a colder #6 plug, plug color darkened but still got hot. Mixed up a fresh batch of VP110 octane at 32:1 and this got me almost 5 minutes of run time before it stuck. Ok, so i thought time for a fresh top end- yanked it apart and the piston was badly scored (as expected) did a oversize wiseco piston and put it back together. Nope, still rocketed up to 350F+ so I pulled it apart again, and filed the rings to max gap, double checked the piston to cylinder clearance and everything is perfect! Nope, no go- Pulled it apart yet again, last step was to sand blast the paint off the cylinder fins and repaint with a very light coat of engine paint to give max cooling- now i get almost 10 minutes before it hits 350F in anger i pushed it with throttle to see what would happen- rattled to a stop @380F so thats no good. Im out of ideas, never seen such a crazy problem- maybe the previous owner installed a hot CDI box? Any help is welcome! Edit, do these things support any 12V output at the key? Was thinking about mounting a WR450 radiator fan on the front frame

 

Plugged pipe?
110 octane is too high imo.

 

Why do you feel 350 is excessive? Looking around it appears 400 degrees is the norm...for 2-stroke cylinder temps that is...

 

I wondered about that too.

 

I guess I'd get concerned if temps got much more than 425 as that's when aluminum, depending on it's composition, starts to anneal (weakens slightly)...


In fan-cooled sleds you always knew when they were getting too hot as they'd loose power due to the excessive expansion and compression loss...

 

I think he's on the right track thinking electronics.

 

I have 2 other friends with LT50's and a JR50 with the same engine, theirs never break 270F ever, plus I have been checking temps on engines for over a decade, never seen anything break 350F unless ridden hard at slow speeds. Most liquid cooled bikes and quads average 180F on cold days to 220F on hot days and boil over around 245F Air cooled 4 strokes range from 150F on snowy rides where snow kicks up on the fins up to 290F in hot weather in tight trails.
I will check that spark arester- good idea LT80 that tank you sent me is working quite well

 

Well...liquid cooling is not air cooling...so I'd hope the temps were lower!! Air cooled 2-stroke motors use 2 methods to keep temps in check 1) heat transfer and 2) the fuel charge itself. I would really fatten it up and see if that helps. I'd also remove the cylinder/head paint as it only inhibits the heat transfer...can't hurt...


Also...what's your idle set at? I'd think a high idle would contribute to your fast heat ramp up...


I'd also get a small fan and have it blow across the fins when at idle to see if the temps drop...they should...


You also mention a "light grey" plug color...is that at WOT or at idle? I'd shoot for that being a medium brown at idle since it's air cooled...

 

idle is set low, but just high enough to where it wont stall when cold. Its pretty fat, plug color on a #4 is grey and a nice brown on a #6 and yes, if i hit the engine with the leaf blower for just a minute temps fall down to 200F and everything is dandy. Here is a video i took back in December 2016, just after the rebuild but before i switched to the colder plug, paint removal and 110 octane- as you can see it totally grabs and locks up as it passes the low 300's

 

Maybe it's that wiseco piston seizing. What clearance was it set at?
Your buddy's got electronics to swap out to check?