Topic: QT50 Yamaha, official pit bike of Performance Marketing Drag Team.

[Don R]

 My son in law saw a pic on facebook and thought it was for him. He was going to look at it once but never did. Now I need a set of them.

[Don R]

 I got the new petcock and it still dies every few blocks then it will start right back up. The PO said it has a new air filter he may have exaggerated. I really don't want to brave the 95* garage heat. I'm retired not supposed to sweat

[grcamna2]

Don, is the tank clean? fuel cap venting ?

[Don R]

 Yes, the PO had rust in the tank, I've flushed it clean and added a little pre mix, but haven't acid washed it yet. I put in a new petcock with the screen and an after filter but have not cleaned the carb yet. I'm assuming the float bowl is running dry due to rust in the float valve.

 I've been busy working on the race car and trailer. Driver got a new helmet and I put on the HANS device. Bolted down the 7/8 plywood floor, the screws are backing out.
 My drag bike is on the lift, I don't have the title on the 72 frame but have a chopped up late K0 I could use. I'm going to pre-assemble it for a test ride then blow it apart over winter for paint and detailing.

 Yamahopper carb may need to wait.

[Don R]

 I've been riding it at the track, recently it began acting up again so I'm assuming I'll need to read the manual and give the old hopper some love. I read Yamaha is again using this engine on a kids bike and the parts fit. There is a factory 60cc cylinder and piston and an aftermarket 70cc set with a matched head. There's also a 4mm cylinder shim that raises the port for performance but you need to cut the 4mm off the top.
 I've had it over 30mph indicated in the pits. When I get it right it'll be time for tires. 

[Don R]

 I'm assuming it needs crankcase seals. This would be a good time for a total overhaul, the 60cc kit is cheaper than a stock rebuild. I've found most of the parts on ebay. The question is use the oil injection or plug it and use pre-mix. It will need a clean-out for sure.

[MoMo]

FWIW, Don, I usually plug off the old injection units and run pre-mix when I redo an old 2-smoker.  More for the feeling of security knowing it cannot fail.. Larry

[seanbarney41]

If you are gonna do that, make sure the pump doesn't also lube the crank.  Thinking that is mostly a Suzuki thing though.

[Don R]

 It's a sweet old smoker. An older friend gave me a deal on it to be sure it got a good home.
  I hooked him and his buddy up on the Veterans honor flight (they aren't computer literate) and our car club donated enough money to pay for it. They both served in Korea.
  I know a guy that has raced on dirt his whole life and has a shop. He tweaked a mini enduro that I restored for a mutual friend. I may let him check it out so I don't seize it up.

[Don R]

 Late summer last year it began to run 100 feet and die. I assumed it had incorrect oil or vacuum leaks in the crank seals. I posted on the qt50 facebook page and one of the guys said unplug the ign. control box near the battery. It's apparently a starting rpm control that goes bad. Many of them didn't even have it. I pulled off a wire and it runs awesome again. Now what to do with the 60cc big bore kit?

[Tracksnblades1]

A Yamaha YSR 50 had a factory expansion chamber with a stinger and silencer.
I dont know the exhaust port area or port timing on the ysr or the QT 50s.

The Ysr50 engine was similar to or was in the older GT80 DT100 family.
Some use GT80 or DT100 for direct swap big bores.
I remember those having the spanner wrench exhaust nut.

We have a YSR 50 if you need some measurements.

[Don R]

 There's an MT50 mini moto for kids that uses the same engine, in production at least until recently. Guys swap engine parts and the rear axles to get the gear ratio.  I bought a 60cc Chinese big bore kit but most guys say they aren't worth the effort. Apparently the sleeve is inferior and the piston/wall clearance too high.  There is a better quality one but if I can burn out the exhaust and get the carb right this one should top out just over 30mph. Also an expansion chamber or two available, most guys don't jet them up. Sorta like many of the pod guys, they don't do the work to get it right.

https://www.treatland.tv/MLM-yamaha-QT50-techno-EDM-circuit-pipe-p/mlm-qt50-pipe-401106.htm