Yellow QT50 rebuild

Yellow QT50

I sold the 1985 red qt50 and received this yellow back in partial trade. The most recent owner had been a little rough with it. I replaced several things so far:

New piston and rings;
new spark plug;
new air filter;
new ignition coil;
changed out transmission oil.

I still have a bit of a to do list:

Cleaning carburetor now;
replace speedometer;
replace tires?;
new seat;
clean out gas tank;
new fuel filter;
solve nonworking headlight mystery;
add reflectors.

I’m using the oil tank as a reserve gas tank (running premix). I connected the old oil line and gas line with a fuel T. I am cleaning the carburetor now but am wondering if the fuel filter might be clogged. I need to post a picture of the old piston.



Here it is. I’m pretty sure this is the one. I had put it in my old parts box along with four other old pistons but narrowed it down to this one. The scratches in the piston cause a loss of compression making for difficulty starting and loss of power. Fuel also gets past the rings and the bike will surge as a result.

Of particular note, I’m running a 3mm(?) rev plate on this one. So I’ve pushed the cylinder forward to allow complete opening of the exhaust port. Of course, this changes the timing so we’ll see how it does.

8/9/16 – I’ve been heat cycling the engine as part of the initial break-in process. So far so wonderful! It’s still early and I hate to jinx myself but she’s running really well. I generally do 5 heat cycles and then 4-5 rides of about 8 minutes each. First ride is no more than 1/4 throttle, then a few at 1/2 throttle and then 3/4 throttle and then one with a little full throttle thrown in. After that, I don’t ride it too much at full throttle until after a couple hundred miles.

Heat cycling involves no riding but letting the engine idle and occasionally blipping the throttle and doing so until the engine reaches operating temperature (about 325F). From there, I shut it down and let it cool completely and repeat. The break-in process lets the new rings properly seat with the cylinder. I’ve found I can get it to cool down to air temperature pretty quickly by directing a fan on the cylinder head; however, usually I’m not in a hurry to get through the heat cycle phase. I’m rarely able to do it all in one day.

Yellow QT50 running well!