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

[Don R]

 I'm looking at a 79 QT50 Yamaha for a pit bike. Easier to haul in the race trailer than a cb750. An old friend has it, I'm considering a $240 offer. It needs the usual, tires, carb cleaned and tank de rusted it's nearly spotless otherwise.  Does have the title, manual and paperwork. No rust to speak of and nice paint. I'm assuming $50 for a used tank, $25 for a repro petcock, and $100 for tires. 
 My concerns are the engine seals leaking and it has an oil seep under it.

[grcamna2]

That sounds like a nice little pit bike/moped Don.I had one for a few years and enjoyed it,a very solid,strong little machine w/ shaft drive.I have a stock exhaust for the 79' model if you need it.

The check valve in the autolube pump will sometimes get a little dirt/sediment in it and allow 2-stroke oil to flood from the tank into the engine,that would be my first guess.

[Don R]

 Thanks. I went by to make an offer, he went for a walk or took a nap.

[Don R]

 I bought it, I paid $240, it's an older retired union brother and I didn't want to low ball him. Hope I didn't. I put in some gas and it fired right up. The petcock leaks and the tires are toasty, it needs a reflector on the rear plate and the tank is rusted a little. The battery has built in wires. Crazy.

[grcamna2]

I bought it, I paid $240, it's an older retired union brother and I didn't want to low ball him. Hope I didn't. I put in some gas and it fired right up. The petcock leaks and the tires are toasty, it needs a reflector on the rear plate and the tank is rusted a little. The battery has built in wires. Crazy.

Did it smoke like a crop duster? I would recommend using their Yamaha 2T standard 2-stroke oil,it's not that expensive and has the right consistency.I would think of completely cleaning out the Yamaha autolube tank,system,lines,check valve,etc. I hate when the pump slowly drips 2-stroke oil down into your engine and fills the crankcase over time.

[Don R]

 I added a little synthetic 2 stroke oil to the gas, cleaned the petcock, changed the o ring on it, added a shutoff and filter, the tank is rusty inside but I'm sure I can clean it. It did smoke a little but not as bad as expected.
 Thanks for the tips, this is my second 2 stroke experience, the other was a mini enduro, the most beat on little bike I ever saw. This one is pristine by comparison.
 I got a sweet manual with it so I have a fighting chance.

[grcamna2]

Those Yamahopper's are cool little bikes that will slowly carry you to where you're going,single speed so it needs to be cranking before you climb a hill. 

[Don R]

 I noticed that in the yard, got it going before turning uphill. I got enough lighting to go on the street, briefly made it up to 27 mph.  I already found a 60cc kit with the entire top end, studs and everything.
  An old high school buddy that races can port 2 strokes too. He helped me with the mini enduro. 2 sizes overbore and port job, it was awesome. lol.

[MoMo]

Don, you need a YZ80      coolest, fastest, little bikes

[grcamna2]

I noticed that in the yard, got it going before turning uphill. I got enough lighting to go on the street, briefly made it up to 27 mph.  I already found a 60cc kit with the entire top end, studs and everything.
  An old high school buddy that races can port 2 strokes too. He helped me with the mini enduro. 2 sizes overbore and port job, it was awesome. lol.

Does anyone offer different gearing for the final ratio ?

[Don R]

 I haven't seen anything custom yet, it's a single side swingarm and shaft drive. If it shows signs of being tired, I'd do the kit rather than bore it. $60 for a cylinder bore and piston is more than all the new parts.

[grcamna2]

Here's hoping you can just decarbonize it and she'll be good to go.

[Don R]

 I ordered a new battery, the low beam is out, I assume that's from the po riding it with the dead battery. I stopped running the engine when I noticed the battery won't take a charge. 

[Don R]

 We took it to the drags for pit bike use this weekend, it was a big hit, I saw two other ones there, another red and a yellow, all three are missing the right reflector. It must be harmonics in the bracket loosening them. When my new ones come I'll put on lock nuts.
 I put a fuel valve under the add on filter, the filter had enough gas in it to flood the engine when parked. It ran good as long as I turned the fuel off when parked. I'll clean the carb and gas tank this week, I got the petcock to stop leaking externally but the rust ate up  the 3 hole washer inside.

[jgger]

I noticed that in the yard, got it going before turning uphill. I got enough lighting to go on the street, briefly made it up to 27 mph.  I already found a 60cc kit with the entire top end, studs and everything.
  An old high school buddy that races can port 2 strokes too. He helped me with the mini enduro. 2 sizes overbore and port job, it was awesome. lol.

Don
2 strokes are easy as pie to get more out of. Rather than cutting on the barrel cut the piston, if you screw it up it's cheaper and easier to replace. On the intake side shave off on the bottom of the skirt, this increases the intake. I have cut as much as 3/8" off of a 175 enduro piston. Eyeball flow the transfer ports too and leave them a tad rough. On the exhaust file a ramp the width of the exhaust port, this is the same as raising the port. Eyeball grind the intake side of the barrel to get the humps out and leave it rough to atomize the mixture. Then do the same on the exhaust but polish it like glass. Last thing is match the gaskets to the port so they fit perfectly flush to the port they are sealing.

Did this to the old Montesa I used to Desert race way back when, and to my surprise I had to go Down in jet size. It ran like a strip-ed arse ape after that!

[MoMo]

I noticed that in the yard, got it going before turning uphill. I got enough lighting to go on the street, briefly made it up to 27 mph.  I already found a 60cc kit with the entire top end, studs and everything.
  An old high school buddy that races can port 2 strokes too. He helped me with the mini enduro. 2 sizes overbore and port job, it was awesome. lol.

Don
2 strokes are easy as pie to get more out of. Rather than cutting on the barrel cut the piston, if you screw it up it's cheaper and easier to replace. On the intake side shave off on the bottom of the skirt, this increases the intake. I have cut as much as 3/8" off of a 175 enduro piston. Eyeball flow the transfer ports too and leave them a tad rough. On the exhaust file a ramp the width of the exhaust port, this is the same as raising the port. Eyeball grind the intake side of the barrel to get the humps out and leave it rough to atomize the mixture. Then do the same on the exhaust but polish it like glass. Last thing is match the gaskets to the port so they fit perfectly flush to the port they are sealing.

Did this to the old Montesa I used to Desert race way back when, and to my surprise I had to go Down in jet size. It ran like a strip-ed arse ape after that!

You had a Montesa too?  Not many owners around, I had a LaCrosse-think it was a 1968. lt was a fun, fairly reliable bike....Larry

[jgger]

YUP! Owned 2 of them '69 250 Cappra-4 speed and a 72 Cappra. My Yamaha buddy used to call it the Spanish Whore.

Had to remember to never crash on the right side or it would flood out (Anal side float carb)! That ended when I set it up with a Mikuni, I was able to crash on either side then! Man I loved those bikes.

[grcamna2]

When you cut the piston on a 2-stroke,to port it, it does raise the rpm band to where the power starts to come-on and effectively focuses it within a small band,usually it gives it a more intense top-end power 'kick' and along with your choice of exhaust and intake can make it feel like a motocross type engine.The stock porting is best for getting low-midrange torque in a small 2-stroke engine.

[jgger]

Yes that is the other side of the equation, exhaust. Back pressure plays a part too as to where the power comes on. A large diameter short exit will give you low end power, where a small diameter long exit comes on at higher rpm's. An expansion chamber with a silencer on the "stinger" will give much better flow than a baffled type muffler.

Man if I had a nickel for every hour I have spent with a grinder or my finger stuffed in an exhaust port.........I could buy a cup of Starbuck's coffee!

[Don R]

 This is a case I'd refer to the expert. I'm just bench racing but there is a stainless expansion chamber with muffler made for a 50 cc Yamaha engine on ebay for $85. It's not made for this exact bike but I have a tig welder and some fab tools. lol.

[grcamna2]

This is a case I'd refer to the expert. I'm just bench racing but there is a stainless expansion chamber with muffler made for a 50 cc Yamaha engine on ebay for $85. It's not made for this exact bike but I have a tig welder and some fab tools. lol.

The expansion chamber is good for top-end accel. but w/ that one speed/auto clutch it would struggle w/ achieving any 'grunt' off the line and climbing a grade;oem muffler and airbox works nice for puttin' around.

[jgger]

I'm not edjumakated on that little engine.....does it have a reed valve? If it doesn't that would be a huge improvement, assuming it is a carb to intake manifold set up and not some kind of rotary valve.

Is the muffler/head pipe 2 piece? if  it is then just for s & g take the muffler off and ride it. It will absolutely kill the top end of the power band, but will give you an idea how back pressure and flow relate to where the power band is affected. Years ago my buddy was de-greasing his exhaust on his Mach 1 so we just HAD to try it with no mufflers......... very loud, sounded like a V-8, gobs of bottom end and no top end. The good thing is you don't have to worry about burning any valves! Haha!

[grcamna2]

I'm not edjumakated on that little engine.....does it have a reed valve? If it doesn't that would be a huge improvement, assuming it is a carb to intake manifold set up and not some kind of rotary valve.

Is the muffler/head pipe 2 piece? if  it is then just for s & g take the muffler off and ride it. It will absolutely kill the top end of the power band, but will give you an idea how back pressure and flow relate to where the power band is affected. Years ago my buddy was de-greasing his exhaust on his Mach 1 so we just HAD to try it with no mufflers......... very loud, sounded like a V-8, gobs of bottom end and no top end. The good thing is you don't have to worry about burning any valves! Haha!

The engine of that QT50 has a reed valve/crankcase mount carb. and the exhaust is one-piece;from memory,the auto clutch doesn't fully 'hook-up' until about 10 mph minimum

[jgger]

Well that makes it a lot harder to mess with. 

[scottly]

An expansion chamber with a silencer on the "stinger" will give much better flow than a baffled type muffler.

Biggest bang for the buck on old two strokes is a chamber.

[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