I shared this on another forum, but figured I may as well post it here too.
Enjoy!
So I ended up doing some repair work on an old Kymco Super 9. These older ones were liquid cooled, and had rear disc brakes. Today good condition LC Super 9's are pretty well sought after as they were a pretty tough and well-performing scooter for the 50cc market in the United States.
So, I got to work on the engine. The owner said that he had a shop look at it, and they said the piston and cylinder were trashed. I did a leak-down test on the engine. The leak down test showed an air leak in the engine. I couldn't find the leak immediately, so I had to take the CVT apart, so I could check the CVT side crank seal. When I took the variator out, I was a little bummed. The variator was trashed! Luckily I had a used spare that was in good condition:
Sure enough, the leak was at the crankshaft seal:
I tugged all around the crankshaft, and couldn't feel any play, but when I removed the seal, I got a pleasant (or unpleasant actually) surprise!
Yep! That's a piece of the main bearing cage...
So, at this point we know we're in for a nice big job... While I decide the best course of action in the back of my head, I figured I may as well tear down the rest of the engine, and see what else needs to be replaced. Perhaps the shop was wrong, and the piston/cylinder were okay?
Not likely... The previous shop was right about the piston and cylinder. Both were trashed. Below are photos of the piston. Guess I didn't have the heart to take photos of the cylinder...
Cool! Trashed top end. Trashed bottom end. Trashed variator... What now?
Well, a couple years back I bought a wrecked Super 9. It had a good engine with some 3000 miles on it, but the piston, cylinder, etc, had been robbed. The block was a good candidate though. It had a good crank, with good bearings and seals.
So, that said, out with the old uglydirtybrokenengine!
To go on the donor block, we needed a top end. We had a 47mm cylinder and piston that were usable, but no head... So I took a stock head to the machine shop, borrowed my friend Dan's lathe, and cut the head to fit the 47mm bore. The original was 39mm... More on the head work in this thread:
http://scooterinvasion.net/forum/viewtopic.php?t=3055 Anyway, no, it's not perfect but you know what? It works.
So, with the head done, it was time to begin assembling the "new" engine. I started by getting the cylinder mounted to the donor block. Then I assembled the piston stop and degree wheel.
I rotated the engine around until the engine stopped. Then I set the degree wheel to zero. Following that, I rotated the crank around in the other direction, and took note of the reading (47 degrees BTDC).
I then divided 47 in half, to get 23.2. I reset the degree wheel to 23.2 BTDC, and rotated it back around the opposite direction to verify the same measurement ATDC. Now the degree wheel is set up.
Now that I had the degree wheel set up, I could easily rotate the engine around to true top dead center, and measure the combustion chamber volume. This was important, since I had just machined a new head... I had to make sure this thing would run safely on pump gas.
Here, you can see just a bit of grease on the piston. This seals up nicely so that the fluid used for measuring doesn't leak past the rings, and cause an inaccurate reading.
The engine is upright, held in a vice with lead jaws (so it doesn't damage the engine). I can pour fluid from a graduated burette into the spark plug hole to measure the combustion chamber volume. I just needed a rough idea of the combustion chamber volume... I filled the fluid to the
bottom of the spark plug hole, and left the extra volume around the spark plug as a "safe margin". Normally, you would fill to the
top of the plug hole, then subtract the displacement of the spark plug for a more accurate measurement. Combustion chamber volume was (approximately) 5.5 cc.
So after a little bit of math, I found the compression ratio to be about 14.5:1. Because I didn't include the volume around the insulator of the plug, the actual compression ratio is a little lower than that... but still, I feel confident that this liquid cooled engine can safely run on 92 octane pump gas.
Next, was checking the squish clearance. I took the flywheel off the old block, then used a special tool to give me some leverage when crushing the solder that I had taped to the piston before torquing the head back down:
Before final assembly, I gave the cylinder a quick hone with a flex-hone (sometimes called a ball hone, or dingleberry hone):
Following that, I did a port map, for future reference. The exhaust is a bit low, but for a low RPM street scoot, it'll live.
Then, before assembling the transmission and such, I did a leak down pressure test. Here's the start...
I got sidetracked on the phone or something, and by the time I got back to the engine, a bit more time had passed:
Needless to say, the engine had no air leaks. I was ready to start really getting it back together now... The CVT cover was dirty, so I cleaned it up:
Assembled the transmission:
Got the new cover on:
And got the fresh engine into the scooter.
The coil mount needed to be tapped:
And the water pump needed to be rebuilt:
I ended up getting a new water pump to go on the scooter. The original exhaust was blocked so badly that the bike wouldn't run, so I found a donor for that too... I didn't get photos of that, but I'm sure you're tired of looking at my photos by now... The following is a photo just as it was put back together (less water pump and exhaust):
Since this rebuild, the owner has reported no issues, and feels that the bike runs pretty well. Off the line power is above average, and it cruises easily at 45mph. Many 70cc+ kits will do that, and better, but this is a lower end, low RPM deal... Plus, it was mostly built with odd used parts that I had laying around the shop.
In the future, the exhaust, carburetor, and transmission may be upgraded. Who knows? Perhaps there'll be a cylinder port job in the future.
Thanks for taking the time to read this far. I hope you've enjoyed it!
~Josh
