1985 QT50 build/project

1985 Yamaha QT50

Here’s the latest project. This is a 1985 QT50 that I picked up in Covington, Ohio from Tike’s Cycle Shop (Thanks Tike). As I went through it, I kept saying to myself, “What the hell was this guy thinking?” “This guy” being the previous owner and not Tike as Tike was selling it on consignment. I often wonder why people trying to MacGyver simple fixes with whatever they find laying around the garage. More on that later. Anyhow, I had a mostly rebuilt engine laying around so I took the old one off and put the “new” one on. I’m still breaking it in. I put on new rear brakes as well and for some reason, they are really catching the back hub. It’s doing this to the point where the rear wheel won’t spin freely at idle and you’ll hear a rubbing noise when you give it throttle. I’m going to have to take it apart and check it again. I also need a new rear brake cable. If you are looking for front or rear brake cable, visit my eBay store or message me through eBay or email me directly (my email is at right side of page).

qt50 rear brake cable

You can see that this one is shot. The previous owner had put a really small spring over the cable and then doubled it up with the stock spring. The small spring helped the cable keep its form. Why not just get a new cable? I’m looking for a replacement that isn’t the stock rear brake cable.

qt50 wiring mess

Here’s the wiring mess he left me with. The bike does run and all the lights work. It will start up with or without the “key”.

qt50 not-oem key

He must have lost the original key which resulted in the wiring mess.


I added a K&N filter and deleted the oil injection. Most of the time the oil hoses are so hard and brittle that it’s not worth messing with. If you do get rid of the oil injection pump, remember to fill tank with premix gas/oil. I almost forgot that and almost forgot to fill transmission oil Remember it only takes about 11 ounces. You should not fill it to the top. I’m trying 0W-20 synthetic oil in it this time.

Other issues:

Slight front fork bend; needs battery hose; needs front brakes; footpegs are broken. We’ll get to the finish line soon enough though.


Well I’m wondering if it is the new super stiff clutch springs and not the rear brakes which are making things interesting. Perhaps that’s why the rear wheel isn’t spinning easily at idle.

Latest video of breaking in engine:

Update 1/28/16 – So I wasn’t having a fuel delivery issue at all. If you watched the video, I mentioned that I had trouble starting this qt50 especially after I had just ridden it. Turns out my needle jet setting was a bit too rich and I was fouling spark plugs. Right now, I’ve got the needle clip set on the 4th position (just one shy of the richest position) and it seems to be running fine. I was also probably using too cold a plug so I switched to a hotter plug as well. If you have starting issues, it’s always a good idea to check your spark plug. You can do it in about a minute and it’s often the culprit.

Update 2/3/16 – I just put on a better rear brake cable that I purchased off ebay. It arrived in great shape. I just duct taped it to the old cable and as I pulled the old cable out, I pulled the new one through (do this with any cable you replace including the throttle cable – it makes life much easier). While I was at it, I added a battery drain hose as well as a new fuel line and fuel filter. Turns out my petcock is leaking so I’ll replace that soon as well. I just need to replace the brakes, pop on a different set of forks, clean up the wiring, add some wheel reflectors, put in some thicker tranny oil, and maybe recover the seat. I’ll go for a ride later and post a top speed video in its current configuration. I’d love to get a temp gauge on the puch head to see how it’s cooling.

Tip of the day – A big sheet of cardboard is your best friend when doing any work related to fuel or oil. Just stick it under the center stand and make sure it covers the area below the carburetor and the oil drain. It makes clean up a snap and you avoid staining the driveway, garage floor or elsewhere.

Just took her out for a ride. Running great! My GPS program says I was doing a top speed of 36.7 which seems a bit optimistic. The speedometer said 30. I’ll have to take it out for another run shortly.

Ok, 36.7 mph was a bit optimistic. A second run rendered a top speed of 29.9 mph. Not bad. I’m going to set a goal of 33 mph in basically stock form. Let’s see if I can get there.

2/4/16 update – Even though the brakes were squeaking, they didn’t need to be changed. Still plenty of pad on them. With use, the brakes and the inside of the hub get smooth over time and this causes the squeaking along with dirt and dust on the brakes. You can rough up the hub and the pads with some sand paper and that will probably take care of it. Using a little brake cleaner on the inside of the hub will help too. I tried some 10W-40 oil in the transmission and that just delayed the shift into gear even longer. The clutch has to fight through heavier oil and it takes more rpms to engage. Guess I’ll go back to the thinnest stuff, I have 0W-20 or try some synthetic transmission fluid.

QT50 takes on the NC50 in the video above. Switched to synthetic transmission fluid to fight the stiff clutch springs. Helped a little.

2/11/16 – How about some minty fresh engine pics?

QT50 top end engine rebuild

QT50 top end engine rebuild side view

Put a new 60cc top end on this engine. Replaced the engine hardware and cleaned it up real nice! Next – put it on the QT50 and break it in. Yeah, I know, that cylinder head is probably upside down, I guess I’ll switch it. That’s a Puch hi-compression head by the way. We’ll see how it works out.

2/14 update – I must have been in a hurry. I just noticed I had the ignition coil bracket on wrong as well. I’ll fix that and mount it up soon. I want to get it sufficiently broken in to ride in about a week. The weather is changing here in Ohio in mid-February. High 50s on Friday.

I follow the general break-in procedures listed on mopedarmy.com. I’ll heat cycle the engine five times. I have another temperature gauge with mount that I’m going to put on. Heat cycling involves running the engine until it heats up to operating temperature (about 325 degrees F) and then shutting it down. Let it cool back down to outside temperature and repeat. I generally put a little load on the engine by blipping the throttle at intervals throughout this process. Putting a load on the engine helps seat the rings.

After five heat cycles, I take it out on four or five rides of about 5-8 minutes each. On the first two rides, I try to stay around 1/4 throttle. I go up to about half throttle on the third. 3/4 throttle on the fourth and a little bit of full throttle on the last ride.

Since I’m going to 60cc from 50cc, I’ll need to up-jet (increase the main jet size). I have the stock main jet in now which is either a 70 or 75. I’ll probably go up to an 85 and see how that works. With a 60cc kit, temperature is always a concern so I’ll keep an eye on the temp gauge. I’ve got the Puch hi-compression head on. It’s cooling fins are significantly bigger than the standard cylinder head (which is basically what is included with these eBay kits except it’s made for a 44mm piston). If temps get too hot, I can throw on a second head gasket to reduce compression and thus reduce temps or I could switch to a different cylinder head. The Puch Gigantor head requires cutting the footpegs though. I’ve got that on my other Yamahopper and I finally got some new jets to try to get that thing tuned right.

2/19 – Ok, I’ve got a goal of getting this thing to 33mph in stock form before I put the new engine on. I removed the high tension clutch springs and put the originals back on. High tension (very stiff) clutch springs are not good for original stock engine. It has to rev almost all the way up to get going and then don’t even slow down before going up a big hill. It won’t make it. I’ll video today’s ride and do a speed test to see where I am. Then I’m going to sell this re-built engine and put the new re-built engine on. I decided to make another modification to the 2nd engine. I’m going to put a 3.17mm copper spacer where the base gasket goes and see if that fully opens up the exhaust port. I’ve got the high compression head on it so it should all work out. The main issue with these Yamahoppers seems to be that the engine gets heat-soaked because the piston blocks part of the exhaust port at bottom dead center.

2/22/16 – I installed a Trail Tech TTO Temperature Gauge. It replaces the spark plug washer and reads the cylinder head temperature. The Puch E50 head is perhaps a little thinner than the stock QT50 head. I think the Trail Tech Temperature sensor is also a little thinner than the spark plug washer. These differences were just enough to allow the piston to hit the spark plug. I heard a ping yesterday and it stalled out on me. I heard another ping at the end of my ride today and it would start up (after quite a bit of effort) and then it wouldn’t idle. I took out the plug and saw that the gap had been reduced to nothing. I’ll either add the spark plug washer and use the trail tech or just use the spark plug washer. Hopefully, that cures it.

In good news, I bumped up the main jet to an 80 and it ran a lot better. Throttle response was much peppier. I’ve got a leaky petcock that I need to replace as well.

2/23/16 – Just when you thought you were done making big mistakes, you’re not. It turns out the Puch e50 cylinder head was made to fit a 38mm piston. The QT50 piston is 40mm or 40.5mm. After the piston bangs on that too small cylinder head long enough, it starts to break apart around the edges – basically where it won’t fit in the cylinder head. After wondering why it was running so lousy and so hard to start, I took off the cylinder head and inspected the piston. I’ll add a picture later. The damage was minimal but any damage on a piston is serious no matter how small. I’ll have to replace the piston, probably the rings as well and hone the cylinder. Pieces of the piston scratched up the cylinder. Hopefully, I can save it. If not, I’ve got others.

I’m going to check my other QT50 and see if there is any piston/ring damage. I keep down-jetting and it still runs rich. My first thought was the Boyesen Reeds. Now, I’m wondering if there is a piston or ring problem.

8/8/16 – UPDATE – I ended up selling this one to a friend and received the yellow one on the main page back in partial trade. I installed a ~4mm rev plate and had the cylinder decked an equal amount. After sorting out some wiring issues that were causing the bike to ground out over bumps, this one runs like a champ with top speed of 33mph for a stock bike.