Rebuilding an 84/79 QT50

So I’ve never had a noped/moped but have had some big bikes. I was give a 1984 QT50 that would move me around at about 27mph. It was in rough rough shape though.

I ended up with 1979 frame, forks and another lower end motor that spins freely, not sure of the year on the extra(could be the primary motor real soon)engine.

So here I am now with 3 Mikuni VM12 QT carbs and one Minky clone. I’ve got the frame I’m using, handle bars, upper forks and both racks at the powder coat folks shop right now.

Thanks to this site I decided to undertake this project, although so far it’s seemed pretty straight forward. If anyone is aware of anyone, anywhere in the North West Indiana/Chicago area that rebuilds these motors on a routine basis and is capable of doing a heck of a job, please point me in that direction. If not I’m not at all opposed to slowing doing it myself.

For the frame and colored parts I’m doing a more modern Yamaha blue. Motor I want resprayed in factory silver.

The bike below I guess for all points and purposes is my donor bike not the actual bike even though it’s the one I’ve got to run and ride around the neighborhood. All the other stuffs off being refinished.

Thoughts? Ideas? Concerns? Free offers of help? (edited)

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Re: Rebuilding an 84/79 QT50

Below is my freshened up seat. Well not my seat, this things going to be used by a younger rider, I’m just having my fun with it now. But seriously any tips, pointers, advise or even “hey call... they can rebuild your engine affordably” would be great.

The other image is the new color. (edited)

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Re: Rebuilding an 84/79 QT50

Jack Rutherford /

They are not too bad to rebuild. I’ve always got the case apart with a rubber mallet and you just need a bearing puller for the bearings. And a flywheel puller as well. Just keep everything organized and you should be fine.

Re: Rebuilding an 84/79 QT50

Took the head off tonight for a good look at the top of the piston and to check out my cylinder. I thought it looked really good.

New ring, new studs and nuts and I’d say this ones solid.

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Re: Rebuilding an 84/79 QT50

Upon further inspection my piston seems to have a “U” stamped in the top, any ideas of what this means? I was hoping for STD, but it may be. About to mic it.

And it get 39.97-39.98 or so

It seems smooth as glass in that bore. (edited)

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Re: Rebuilding an 84/79 QT50

Sure enough, there’s that “U”... I’m not going crazy after all.

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Re: Rebuilding an 84/79 QT50

I took some more measurements... see photos

I took these measurements from both sides with identical results. I’m not sure that head gasket specs like this help but it sure doesn’t hurt to know what your working with.

I’ve got a collection of precision measuring instruments. One is for extremely thin thickness measurements. May have fun with that one here.

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Re: Rebuilding an 84/79 QT50

You're measuring the very top edge of the bore .

That's the part that rarely wears any at all .

Much better to take measurements incrementally down through the complete bore .

Like at the 1/4 , then 1/2 and 3/4 down .

Normally the bore will be a touch larger at the bottom .

You can do the same on the piston .

By the time you're done , you'll know the piston to bore clearance ( important information ) .

If you install new ring(s) , be sure to hone in a decent crosshatch or the new will not seat .

Re: Rebuilding an 84/79 QT50

Got the head cleaned up today, may blast it with the carb bodies(soda). Either way it’s got about 2 lbs less carbon on it now.

The pistons up next and I may just paint that head with some high heat or powder coat and be done. Anyone ever powder a head?

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Re: Rebuilding an 84/79 QT50

Alright, so what you just said is what I have always known to do. I've never personally done the honing, but I have been told to run up to hardbor and grab this little device that doesnt so much hone as it does clean everything propperly.

This brings me to yesterday, I call a place that's done a bunch of PW engines and run by them what's going on and he tells me, "If your cylinder looks good, buff the carbon out of the the head and the top of the piston, DO NOT TOUCH the cylinder walls with anything, throw in a new set of rings and bolt her back up."

So you are of the opinion that running the device through the cylinder for 3-4 minutes is a good idea? I hate conflicting advice when the one I've always practiced seems right, but the person I hear the other from is experienced....

Maybe you're him, but do we have a master QT50 Engine builder around here???

> P D Wrote:

> -------------------------------------------------------

> You're measuring the very top edge of the bore .

>

> That's the part that rarely wears any at all .

>

> Much better to take measurements incrementally down through the complete

> bore .

>

> Like at the 1/4 , then 1/2 and 3/4 down .

>

> Normally the bore will be a touch larger at the bottom .

>

> You can do the same on the piston .

>

> By the time you're done , you'll know the piston to bore clearance (

> important information ) .

>

> If you install new ring(s) , be sure to hone in a decent crosshatch or

> the new will not seat .

Re: Rebuilding an 84/79 QT50

Think about exactly what is happening in the cylinder .

The old rings have conformed / formed to the cylinder and in the process have worn the wall clean/smooth . That cylinder is likely no longer the cross hatched round as it was at manufacture . New rings are perfectly round . Without a decent crosshatch , how will those new rings ever form to the cylinder and provide good compression ?

New rings in an old untouched bore is a fantastic recipe for massive blow by .

Re: Rebuilding an 84/79 QT50

I concur with you're line of thinking. stick device down the cylinder on the drill it is. Any better recommendations than the hardbor freight on? Do they make one of those brushes with balls on the tip that'd be better?

Re: Rebuilding an 84/79 QT50

I prefer a bar hone , as they tend to hold straight rather than following any deformity .

Check with your local auto parts store . They might have one to 'loan' you .

Check You tube for a quick tutorial on usage .

Re: Rebuilding an 84/79 QT50

Piston and cylinder... not good... but how bad?

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Re: Rebuilding an 84/79 QT50

Mind you it was riding this way.

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Re: Rebuilding an 84/79 QT50

Ha . That looks almost as bad as the piston I'm running in my FA50 right now .

You could measure what you have and order an appropriate sized piston and rings .

Or simply clean what you have and run it .

Sand the high spots off the piston wall . Clean the ring landings and dome .

Run a quick hone through the cylinder to clean that .

Be sure that every thing is free of any debris and put it back together .

Re: Rebuilding an 84/79 QT50

Jack Rutherford /

I use this flex hone on qt50 and other cylinders. Is it the best thing to use? I don't know but it has worked fine for me in putting a decent crosshatch on the cylinder walls. It just takes 20 seconds maybe and run the drill in both directions. A new piston and rings are so cheap, I would just go that route. Of course, measure your ring gap and adjust as necessary or run the ring expander under the 2nd ring if your gap is too far from the minimum spec.

And of course, you need to thoroughly clean the cylinder and ports after honing it. New rings require cylinder honing. You've got a bit of wear inside your cylinder and it looks like that piston has soft seized several times - both indications would strongly suggest that a cylinder honing is mandatory. Not sure why some pw50 guy with experience would say otherwise. I guess I could speculate - he was probably assuming that you didn't have easy access to a hone but who knows what he was assuming, right?

I've seen qt's running with similar and worse piston/rings/cylinders but since you've got it apart, it only makes sense to replace what you can.

Re: Rebuilding an 84/79 QT50

I like this advise and I am in. So save the jug, scrap the piston and ring and clean up. Thank you guys, I will be back for more.

Re: Rebuilding an 84/79 QT50

Alright so I’m all set up to hone, set my mind on an inexpensive 40mm replacement piston. I do have two rotating lowered engines.

The one that I’ve really set my mind to using has a bit of a clunk to it at the low part of its stroke, but that could be due to having a shaft attached, the other doesn’t.

I think I may get one full top end and see how that does. I am short a top end set up on one motor.

I’ll be ready for next week to get here. Should have something running

Re: Rebuilding an 84/79 QT50

I'd be making absolutely sure exactly what any 'clunk' is before trying to put a unit in service .

Re: Rebuilding an 84/79 QT50

I really wanted this to be my (split the cases and totally redo it, from the ground up" motor. but I may be forced to use it. I think its just fine but i am going to explore more.

Re: Rebuilding an 84/79 QT50

Honed the jug, old piston drops right through now(no rings) glaze is gone! One score remains!

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Re: Rebuilding an 84/79 QT50

$300 plus with Treats today.... MLM stock plus pipe, gaskets, seals, other stuff I can’t think of.

FZ/FT60 top end kit from EBay. New foot peg bracket, some other exciting stuff. Roysenen reeds.

Bigger jets.

Stock piston just in case.

Let’s see what this thing can do!!!

Powder coated says Monday he’s done!!

I can’t wait to start putting things together.

I’ve got a second frame waiting to go! (edited)

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Re: Rebuilding an 84/79 QT50

Let me know how that pipe works for you, been dying to pull the trigger on one for mine, stock with Boysen's. Quick as is, wondering how much "help" it is. Thanks!

Re: Rebuilding an 84/79 QT50

You got it. There will be videos.

Re: Rebuilding an 84/79 QT50

Thanks! :)

Re: Rebuilding an 84/79 QT50

Jack Rutherford /

Return the yt60 knockoff top end. That will only fail miserably. Get the malossi kit or get Yamaha oem yt60 top end.

Re: Rebuilding an 84/79 QT50

Jack Rutherford /

One reason why that knockoff kit won’t work.

Heads

Re: Rebuilding an 84/79 QT50

Dang.... I was hoping to hone, chamfer and get by with it. Bummer

Re: Rebuilding an 84/79 QT50

So I ordered one more OEM Yamaha QT50 cyl and head(cyl will need a fresh hone. When I have a running bike I will order a Malossi kit. I will ship the EBay kit on back.

Even with a factory OEM yz60 head the China kits are still crap heh? I’m bummed about it but I had that feeling until someone told me they ran that kit and it’s great luck.

That said, I was afraid I’d chosen badly and cheaply. Nothing good comes cheap and easy. (edited)

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