Topic: Yamaha QT50 pit bike

[Don R]

 I'm not a yamaha guy but got this pit bike from an older friend a few years ago, the tires were dry rotted so today I popped on a new set. I burned out the muffler with a propane torch to remove old caked oil residue and now it runs slower than before.
 I saw a video that stated more often than not it's too much fuel that seems like too little so I may try a pod filter, (I just threw 400F pods away) The carb adjuster screw is on the back side and not easy to access so I may need to remove it or rotate it to adjust and rotate it back after.   

[grcamna2]

What's the plug look like Don ?

[Don R]

  That plug is probably pretty old, lol. I'll take some time to check the air filter and plug today. It's still running the oil injection too. It seems to be adjusted and working properly. (I probably should clean it out and re-fill with fresh synthetic)
   Someone, on a QT facebook page mentioned 4 stroking which I'm not too sure what that means. Sounded like a sort of rev control. I have unplugged the rev limiter which is prone to failure and deleted on newer bikes. It stopped dying every 100 feet after that. My carb doesn't look like the ones I see in the articles on adjusting. Another early difference I guess. The PO didn't change it as far as he remembered.

[jgger]

I had an old beater 2 stroke yamaha that the po couldn't make run. I pulled the cylinder and piston and put new rings, no bore. Cleaned everything up and it ran like a top. The piston had an old oil residue that looked like a rubber coating, once that was gone it smoothed things up alot.

2 strokes need good compression on both the up stroke and down stroke because that is how the fuel is transferred to the combustion chamber........but you probably knew that.

2 things that jack with them the most are bad rings/bore and bad crank seals.

[grcamna2]

Yes,the rings may be carboned-up Don.

[Don R]

 Crank seals are up for this winter. We have a two day race left this year and then I can give it a go. I have a cheap 60cc kit but hear the piston/bore clearance is already too loose on them. I had planed to shim the cylinder up a bit and trim the top for the aftermarket head.
  I had a friend (ex flat track racer) bore out an 80cc mini enduro that I did a "get it running restoration" on. He later asked if it ran OK, I told him he made it so fast I almost rode it through a fence.  He just smiled. I'm sure he ported it. I rebuilt the brakes next.

[grcamna2]

Crank seals are up for this winter. We have a two day race left this year and then I can give it a go. I have a cheap 60cc kit but hear the piston/bore clearance is already too loose on them. I had planed to shim the cylinder up a bit and trim the top for the aftermarket head.
  I had a friend (ex flat track racer) bore out an 80cc mini enduro that I did a "get it running restoration" on. He later asked if it ran OK, I told him he made it so fast I almost rode it through a fence.  He just smiled. I'm sure he ported it. I rebuilt the brakes next.

The 80cc mini enduro,is that yours ?

The Yam. QT50/Honda Express are both shaft drive but it would be nice if you could change the final drive ratios like on a standard moped or Suzuki FA50  for a faster top speed.

[Don R]

 The 80cc mini enduro was a drag racer's pit bike. They rode it to pieces then let his dog pee on it. I had to cut off the chain and the front sprocket had 3 teeth left. It was the single most abused piece of equipment I ever saw. I spent over a thousand on parts and didn't do any cosmetic resto. Even so, the first time he took it to the track someone tried to buy it.
 Some guys swap the pw50 and QT50 rear drive back and forth.
   Treatland seems to have the QT50 seal set in stock. One side is common and one isn't.
    I got a battery and a couple spark plugs ordered and 12 cb750 Denso plugs while I was at it.

[scottly]

I burned out the muffler with a propane torch to remove old caked oil residue and now it runs slower than before.

Perhaps chunks of carbon residue plugged up the exhaust? "4-stroking" means a two stroke is firing every other stroke, and is usually caused when the motor is "loaded up", or flooded by an accumulation of fuel in the crankcase. My CR480 would do it after lugging the motor, and sometimes when starting up cold with the choke on. In either case, a handful of throttle while short-shifting it to put a load on the motor to heat it up would clean it out in short order.

[Don R]

 No parts yet, I love it when they sell things they don't have in stock.  The package will be here before 8pm yesterday.

[grcamna2]

I burned out the muffler with a propane torch to remove old caked oil residue and now it runs slower than before.

Perhaps chunks of carbon residue plugged up the exhaust? "4-stroking" means a two stroke is firing every other stroke, and is usually caused when the motor is "loaded up", or flooded by an accumulation of fuel in the crankcase. My CR480 would do it after lugging the motor, and sometimes when starting up cold with the choke on. In either case, a handful of throttle while short-shifting it to put a load on the motor to heat it up would clean it out in short order.

Possibly hold the muffler in one hand and get a large rubber hammer to hit the case of it to loosen-up some hardened crispy,caked carbon inside from when you torch-cooked it? Try blowing high volume compressed air through it to see how it breathes.

I remember when we cooked 2-stroke mufflers:they needed to sit in a red-hot fire until they completely stopped smoking;if we removed them from the fire too early it seem to plug-up more.The mufflers we did needed to be completely cooked-out and them when we were finished and they cooled down,we whacked them with a rubber mallet and all those crispy 2-stroke carbon flakes would release from inside the baffles and we would blow them out from both ends and be 'good to go' with very good flow 

[Don R]

 I cut up the 6 volt headlight bulb to insert a socket and an odd two wire single 12 volt element bulb in it. It has a low beam now. Not real bright at low rpm but it works good enough for a pit bike. About $10 at the parts store with a spare bulb.

[grcamna2]

How does it run ?

[RAFster122s]

6V VW bulb for taillight can be used as well...
DrATV has 6V bulbs and they are Stanley headlamp bulbs...

[Don R]

 It's running good except 25mph is about top end, I've been waiting for a float valve and spark plug for two weeks. I made the mistake of ordering them along with 3 sets of cb spark plugs. 
 David, thanks for the tip, this will make the light work if I need to make a bathroom run during the drag race association cookout this weekend. I may upgrade later. The stock bulb was 20W/20W

 I moved a couple bikes to storage, sold one, drug another home and will leave the Yamahopper and a 750 in the race car trailer this winter. This will free up room to work on my Firebird.

[grcamna2]

It's running good except 25mph is about top end, I've been waiting for a float valve and spark plug for two weeks. I made the mistake of ordering them along with 3 sets of cb spark plugs. 
 RAF, thanks for the tip, this will make the light work if I need to make a bathroom run during the drag race association cookout this weekend. I may upgrade later. The stock bulb was 20W/20W

Don'I was always wondering if the QT50 CDi box had some type of governor built-in to it..

[Don R]

 It does and is unplugged. That made it die and a few seconds later it would re-start. Just like a gas cap issue. It was such a problem they left it off of the newer bikes.

[grcamna2]

It does and is unplugged. That made it die and a few seconds later it would re-start. Just like a gas cap issue. It was such a problem they left it off of the newer bikes.

Yes,Those QT50's need more flow on the top-end because they're always running flat-out 

[Don R]

 I've been using the QT for a pit bike so the Grandma can ride the Grandkids around on the golf cart keeping them occupied while their Mom drives the dragster. It died at the last race so last week I burned off the spark plug and with a propane torch and then used a turkey baster to empty the gas tank. I had read that the oil injectors aren't reliable and haven't removed it yet so I added a little 50/1 fuel. That was a bad idea since it fouled the plug.
 Anyway, it hasn't ran this well since I've had it, it got up to a new personal best of 29 mph on a hill. If it could go 30+ I'd probably put a plate on it and street ride it. It seemed to run better the more I ride it, I did buy a chinese big bore kit but I hear the cylinders are way too loose for the piston. The answer is, to get a bigger piston or bore out the old cylinder to fit the new one.
 I think I'll just keep riding it at the track like it is, I can unload it through the side door easy enough and it can go over the pit speed limit, so why fool with perfection right? The headlight and taillight are both working again. There's something to be said for a happy machine.

[grcamna2]

Don,the oil injection works well with good quality injection-specific oil.
Do you still have it set-up with injection ?

[grcamna2]

I used to ride my 79' QT50 down some steep hills and the little speedo needle was pointing back toward me;the injection never gave out.

[Don R]

 On the QT50 facebook page there seems to be movement towards pre-mix but I think I'll leave it as is with the injector. It has amzoil synthetic in it now, but I never emptied it to see the oil in the bottom. I might use the turkey baster and empty it to be sure the bottom of the tank isn't gunked up.

[Medyo Bastos]

I capped off the injector on my jr50 and been running 50:1 premix since


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[grcamna2]

I ran the OEM Yamaha Yamalube 2-stroke oil in the tanks of my 70's Yam. 2-strokes(RD350's and the QT50)and that was a light oil,like around 10WT compared to a heavier viscosity oil.
The little oil lines/passages are small dia. and it likes the light stuff.

[Don R]

 I like the amzoil synthetic, less smoke.

[grcamna2]

I like the amzoil synthetic, less smoke.

Is the Amsoil an injection-specific oil?   if it's heavy,it will not have the speed to fill the oil lines  & passages fast:it'll be a bit slow,more in the cold.