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.