Jonway YY250T out of commission again.
 

Idle speed dropping. Dropped down to ~1100rpm. Battery voltage dropping below 12V during usage.

Sprayed the carb exterior with starter fluid, bike roared to life from an almost-stall.
Removed all the plastics; the intake manifold is split.
Ordered a new intake manifold, and trying to find a place that sells the Helix carburetor support brackets. The YY250T doesn't have the support bracket. Does anyone know of a source that has these in stock? (As a kit, with all the associated mounting hardware.)

Clamped a good DVM on the battery:
12.024V when bike is off.
Start the bike, after it finally kicks over, is at 11.920V.
Voltage slowly climbs as I rev the engine to 3000RPM. I turn off the bike after the battery gets to 12.100V. The charging system is obviously working, but the stock battery is on its last legs.
Replaced with an AGM battery from Autozone.

maybe you can try other
http://www.chinesemotopartsonline.co...ical-parts_c71

Thanks for the link.

When it rains, it pours.


Last Sunday, took the YY250T out on a trip, about 10 miles in, I see the temp gauge pegged at "H". It's never been above 1/2 before, no matter how many miles I put on it on the highway. I slow down, and keep seeing the gauge go from 2/3 -> H and back again... not instantly (like a wiring problem would do). I pull off, open the gas cover, and see some coolant by the filltank cap. I drive the bike home, and it makes it. I ordered a new thermostat and replacement front brake rotor.


Yesterday, on the way home from work, my car's transmission decided that it didn't want to shift out of neutral. I ended up getting home 2 hours late, with the car at a shop for the night. I still haven't heard back from the shop with the damages, the mechanic said that it was probably the clutch master cylinder (its a standard transmission and has a hydraulic clutch).


I took the day off from work today, to strip apart the YY250T and replace the thermostat. I knew I was in trouble when I removed the old one and it was opening at around 173F (measured using a type K thermocouple and a thermometer in a pot of water), but I installed the new one any ways. I've heated and cooled it 3 times already, and I'm now pretty sure I have a head gasket leak, since I'm still getting bubbles when the engine warms up, and the coolant line leaving the thermostat is only 165F or so (courtesy of an IR thermometer).


Fortunately, I had already purchased a top end rebuild kit with gaskets. Going to end up spending the rest of the weekend doing this one.

I thought for sure that two of the threaded studs holding the cylinder wall and cylinder head down were stripped. Went around crisscross with the torque wrench and they both felt like they were moving a little bit, even after 8 or 9 passes around after the other two had stopped moving.
I was about ready to call it quits, but decided to go back and check to make sure that the nut itself wasn't stripped. Pulled one of the troubled nuts off, looked inside, no visible thread damage. No damage on the stud, either. Put it back on, and for some reason I decided to repeatedly torque down one nut. I thought, "That's odd, it almost feels like its slipping back". I took out the sharpie, put a line on the nut, the crush washer, and the cylinder head, shone the flashlight on it, torqued it again, and confirmed that yes, the nut and crushwasher were in fact bouncing back to their original position!
I then removed all four nuts and installed a stainless steel washer between the nut and crush washers. This time, everything torqued down nice and tight.
It's quite possible that this was the source of the original leak. The original gasket between the cylinder and cylinder head didn't look in terrible shape. Still re-assembling the bike. Still pissed off that the manufacturer could allow something like this to leave the factory.

Notes on bleeding the YY250T coolant system:
Several people on forums have advised removing the carburetor heating lines as being useless. These are the lines which carry coolant from right before the thermostat, into the carburetor, then from the carburetor, to the fill cap. Even if heating up the carb is optional, the lines proved a way for air to escape from a high point in the closed system.
Another point to note, is that the filling hose from the filling cap to the radiator is routed such that it goes down from the radiator, before going back up to the filling cap. Unfortunately, after you've introduced air into the system (by taking off a hose, for instance), this design ensures that any air drawn into the radiator cannot escape.
Once the low point in the hose has filled up, there is no where for the air in the radiator to go. When you start the bike, the water pump will draw whatever coolant is in the bottom of the radiator, send it through the engine, and on the return trip, the coolant will simply fall down the side (inside) the radiator and pool at the bottom. Any air drawn in by the pump will rise up to the thermostat and eventually be expelled through the carburetor lines. This will allow a little more coolant to be drawn from the reservoir when the bike cools down, but the hard truth is, only a fraction of the surface area of the radiator (mostly near the bottom) is actually cooling down the bike. I get around this by disconnecting the fill hose at the radiator cap, un-routing it , and holding the entire line above the radiator while filling the tube. Once there is no more gurgling and it doesn't want any more, dump the excess from the tube back into your coolant bottle and reroute the hose to the radiator cap.