Eureka! WooHoo!

david f martin /

I found it!

After a week of bumming rides to work and back and feeling very depressed, I finally got my toolbox home, got off from work at 1pm today, and got to work on my bike, out in the yard, in the sunshine...

This is what I'd done so far, in case you hadn't followed my previous threads...

Cleaned carb. Drilled holes in the aircleaner. Bought a new jet, stock sized. Drilled jet. Replaced suspect muffler with straight pipe (VW tailpipe). Covered holes in aircleaner, restricted the exhaust. Bought a new battery (suspect CDI). Priced a new top end from the Yamaha dealer ($200, ouch). Whined to the MopedArmy list and felt sorry for myself.

Today, when I got home from work, I pulled the whole damned head and cylinder.

Immediately I found that the exhaust port (which is supposed to be about the size of my thumb) was clogged up, with a very small hole slightly smaller than a pencil for the exhaust to exit. I scraped it out with a pick and my pocketknife, and cleaned up the other ports while I was at it.

Piston has some pretty deep grooves in it. At first inspection, the cylinder seemed to have some deep scoring, too, but upon closer examination, what I thought were grooves turned out to be carbon deposits left from the gaps in the rings. I cleaned those up with my pocketknife, too.

Put the top end back on, cranked the bike and revved it. She revved up quickly, sounded like a dirtbike. Called my sister with the news...

Took it for a test drive. Holy shit, now it moves! Better than it ever did since I've owned it! It was like riding a small motorcycle, rather than a bike with a hamstermill on the back wheel... The clutch engages when it should. Now it climbs hills, accelerates, and makes a lot of noise (damned VW tailpipe). This has been a very good day for my bike and me.

Issues I still need to address: (1) I better break down and buy a muffler, before the neighbors call the cops. (2) The ratchet mechanism for the kickstart is getting iffy. (3) I'll have the cylinder honed, get 2nd oversize rings and a new piston. (4) Maybe a bumper sticker, "Don't laugh, mine's paid for"...

Thanks for all the help and sympathy. I don't know what I'd've done without this forum and its Gurus. I'll try to get a fresh pic up in the next day or so.


Re: Eureka! WooHoo!

That's great to hear. has got a lot of QT50 parts, probably has a muffler. Probably a lot cheaper than getting it at the yamaha dealer.

Re: Eureka! WooHoo!

Reeperette /

Toldja to pull that cylinder, didn't I ?

>>(1) I better break down and buy a muffler, before the neighbors call the cops.<<

Bi-Turbo is the way to go, get the two piece.

>>(3) I'll have the cylinder honed, get 2nd oversize rings and a new piston.<<

The only dude I know of that bores Moped Cylinders is this place.

And even with Wysco, I have not been able to find a piston that small, but ask him, cause he just might know....


Re: Eureka! WooHoo!

Ron Brown /


Congratulations! It is "only an engine", after all.

Just to enlighten the rest of us, do you have any idea how many miles it took for your exhaust port to plug up so bad?

On your scored piston, if the rings are still a good fit in the grooves, the cylinder bore is still good and the piston not too loose, just ride it. From the performance you describe, it does not sound like you need a major top end rebuild.

Many 2 cycles have scored pistons from siezing up but all you need do is file off the high (shiny) spots and clear any burrs from the ring grooves. Use a file as abrasive paper leaves grains of abrasive embedded in the aluminum piston, doing bad things to the bore.

Don't get too many bugs in your teeth,


Re: Eureka! WooHoo!

david f martin /

I figure I put about 600 miles on it since I got it. I bought it from a guy who works on motorcycles. He'd tuned it up, got the lights working, etc., but I don't know if he'd pulled the cylinder and cleaned the ports. I do know that the bike is now running faster than it did when I got it, so the ports may have already been blocked up somewhat...

The piston is cheap compared to the cylinder ($20 vs. $180). As far as priorities go, I'll check the plug to keep the mixture right, work on the ratchet for the kickstart, buy a muffler, then replace the piston and rings, since the bike is running well right now.

This has most definitely been a learning experience for me. Thanks for the mantra, Ron.


Re: Eureka! WooHoo!

Ron Brown /


I would guess that you had a mostly plugged exhaust port when you bought it, or you would have to be burning way too much oil.

Anyway, take alook in the port when you install the new muffler and see how it looks.

And, your welcome.


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