QT 50 top end overhaul

I've read some interesting threads about QT 50 mods but the single most effective thing I believe you can do for a QT 50 with any kind of mileage is to overhaul the top end.

After tinkering with these great scoots for the last couple of years I've learned through trial and error that once you put a new piston and ring set in (and wrist pin bearing if you need it) that most of the idle demons and poor performance stuff goes away. Great surprises in mid throttle and top speeds of 30 to 35 are not uncommon on the flats (even carrying me at 185 lbs and the wind in my favor).

If you are going to do any mods, this is where I would start fooling with the air box drillouts and exhaust port work.

The one thing that I like the most is how quiet these things run once you've replaced the piston and rings. I've got 2 runners right now and both have stock mufflers. Since the rebuilds (#1, new stock piston and ring set, with only a good hone job, #2, new 1 over piston and ring set with a bore and hone job) I've lost the piston slap or clatter, totally. The only thing I hear is the wind in my ears and the whine of the transmission and pinion gear. THESE QT's are tight.

Anybody working on these scoots should really consider this. It's fairly easy. It gets a little pricey if you have to bore the jug but well worth it.



Re: QT 50 top end overhaul

Yeah .. top end jobs always improve the owners thoughts about the machine .. quieter , faster, you name it.

So ... how many miles were on the ones you did Joe ?

How much did the parts cost you ?

Did you ask how many oversizes the pistons come in ?

(4 maybe?)

Re: QT 50 top end overhaul

I forgot this ... I almost rode a QT a week ago.

I was at a flea market and a guy had one.. but he wasn't at his display at that moment.

They said he wanted about $370 for it... way too much IMO.

They sure look different than what I'm used to ... looks like they have high handlebars and low footpegs .. like they would be roomy for big people.

I like the shaft ... clean, quiet, no maintenance or wear like a belt or chain.

I will own one someday ... I like Yamahas.

Re: QT 50 top end overhaul

The oversizes that I remember went to 2x. The walls on these scoots are thin so I don't believe that they could survive 3 or 4 bores and take the heat. It's easy enough to find jugs that are still the original size to bore 1x. In some cases the walls are still clean and a simple hone and ring job will do the trick as long as the cylinders are still within spec.

The mileage on the first scoot was over 7k and the other was

2500. In the case of the 7k scoot the lower crank and tranny are in great shape and there was plenty of lining left on the clutch. The drive shaft and pinion gear are in excellent shape.

The 2500 scoot crank is perfect as well.

There is another scoot I have restored and it is the best runner to date hasn't been rebuilt yet as the compression is 125 lbs and regularly runs over 30 mph on the flats. This scoot was a frankenscooter built over 3 weekends from 3 different QT's that were parted out. It has quite a bit of piston slap and is fairly noisy. Until it starts using oil I'm not going to tear it apart. With this much noise it will have to be bored out as the cylinder is too far out of spec.

In each of the 4 motors I've redone so far the scoring has been awful and one of them had a broken piston and rings, it's compression was down to 70 lbs. The funny thing was it had good speed once you got into the RPM's around 25 mph. Out of the hole it was really slow.


Re: QT 50 top end overhaul


I've ridden all types of 2 wheel contraptions since I was 10 years old. In my stable of rides that are in the garage at the moment are 3, 1980's QT's, 1 1970 Honda Trail 70, 1, 1987 Honda 700 Super Magna.

I rebuilt the Trail 70 with my son as a project we could do together. My wife liked the 70 but said it was too heavy at 150 lbs. We found a QT 50 in Montana and had it shipped down to Florida in 3 boxes. I had it back together again over a weekend and started fine tuning it right away.

What intrigued me with the QT wad the shaft drive and the light feel of the 95 lb scoot. My wife likes it becuase it doesn't feel any that much different than her bike and weighs as much as she does.

When you get a chance to ride one I think you'll be hooked.

As for the price, I'll put it this way...if you go back to the flea market and it's still there, let me know what's missing on it (Take a look at my red one on my profile) and if you can start it, I'll buy it. ; ^ )


Re: QT 50 top end overhaul

david f martin /

Here's mine (stolen last year)...

Not that fast, but I loved it.


(forgot the picture)

david f martin /

forgot the etiquette

Dave ... please put a < in front of addresses like that

.. and a > behind the address.

2 seconds 'work' by you .. saves 15 seconds work each by 30 people.

... 2 vs 450 ... not a hard choice

Its simply the polite thing to do.

Re: forgot the etiquette

david f martin /

Hmm.. Lemme try that...



Re: forgot the etiquette

david f martin /


Thanks, Fred!

Learn something new every day...


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