In the last year I gave zuma 50 to my wife. She immediately got to work and bought an athena70 kit.
Laid the bike over and took off the old cylinder.

Removed the circlip.

Inspected the piston.

Put on piston ring on new piston.

Slid new cylinder on the engine.

Torqued head down.

The exhaust port matched the expansion chamber. Tuning stayed spot on.

Other tuning notes. The clutch springs had to be softened. stock is pretty close to perfect.  Running a stock variator with about 24 grams of weight and a heavy contra spring. It runs pretty good.

Zen is Zen

My new expansion chamber seems to be working better than my old one. It has a very smooth diffusion cone that expands to about a 90mm ID. my last one was 76mm and my first one was  70mm.

The stinger is 18mm rather than the calculated 14.5mm I may try sleeving that eventually.

Here are some photos:
Acceleration is the best that it has been.

I have decided to use a prebuilt  ZEN pipe for my zuma. from treatland.tv It has a detachable head pipe nozzle that has a 24mm ID. My current exhaust flange is has a 23.5mm port. I should be able to match it perfectly. The cones and body are formed seamlessly. It was hydroformed.

I worked on tuning my Zuma. I have the head pipe a slip fit and it allows about 2" of movement. I first slid the pipe out. It really made the acceleration better. The top speed was one mph less. I moved the pipe in an inch from where I started with it. The machine really too a hit on acceleration and the top speed went up .6 mph to 48.6.

Perhaps I could lighten the rollers a bit and get the acceleration back. I had calculated that one inch is about a 4oo rpm peak change.

I am going to reassess my pipe design. My convergent cone is an average of 28 degrees. To my understanding a steep convergent cone will produce a strong reflection but over a narrow rpm range particularly in the rpm above the peak power rpm. It may also lead to increased temperature of the piston. I have not had problems with that issue.

I have decided to make a new convergent cone. I am designing for 18 degrees. Most formulas suggest between 14 and 20 degrees with the higher value for higher peaks.

Hopefully the engine will have a the ability stay in the powerband during the variator phase of running and then pull past the peak for top speed.

August 5, 2010

Got a chance to work on my Zuma today.

I put im a new contra spring. a blue polini spring. I have two alternative choices, a white spring was 3.74 mm x6 turns and the blue was 3.84x6 turns. I miked the stock spring. It was 3.99 x 4 turns.
The ID of the springs are 44.5 mm and the free length is 68mm for stock and 110mm for the aftermarket springs. I didn't measure the compressed length but would estimate it at 55 mm.

I am not versed on spring constants. I think it would be safe to say that the stock spring has more force but less preload.

I put in the blue spring as it was thicker than the white.

The bike runs better, I am still running the 4gram x6 weights and the bike picks up faster at low end and doesnt flatten out at 30+ mph (before pulling to 50) like it did before. I would say that at low rpm it had more force keeping the rpm higher. I didn't get nearly as much rpm drop off at higher rpm at around 30 mph.

It is irritating to see the speedometer go to the peg and hear the rpm continue to grow.

My compression is pretty low as my cylinder was raised and the head was not machined equally yet. My squish is entirely too fat.

July 12, 2010
Too hot to ride today so I decided to take my zuma motor apart. I remeasured my ports and decided to open them up a bit. I looked at the port tunnels on the cylinder and realized that I would not meet my goals with carving. The exhaust port is already at 60% width and I have not done much to make it wider. When I got done with my files the exhaust port is now about 24.6 mm from the top of the cylinder. It is about a mm above what I was running prior. I used my files and worked for several hours on smoothing the ports. I could reach all the way into the transfers. My transfers went up half a mm to 32.5. The left side was .5 mm lower than the right. I did my best to equalize.

The problem that I am encountering is that the tops of the port tunnels are very close to the stock port figures. I decided that I would add a second base gasket to increase mean port timing and remove material from the top. My stock squish was 1.4 mm. I removed some material from the cylinder head. I didn't get enough removed and my squish is now 1.7 but better too much as I can cut off more. I am going to take it to about .8mm.

I have been running 24 grams on the stock variator. Too much right now, the bike is a dog until 33-35 mph then rockets past the speedometer stop. I didn't use my GPS as I figure I need more static compression. I may need to go back to my 19.5 (total) gram set of weights. I am hoping that getting the compression back up will do a lot for mid range. That is my next step.

MacGyvering my Zuma

July 10, 2010
I measured my squish band yesterday it was 1.35 mm. I used the rosin core solder method. I checked several times and only once did I get that low of an amount. Several other readings were in 1.7mm range. The stock head has a rather strange shape of the squish area.

Next time engine is apart I am going to check the jetting by the oil line method and perhaps reduce the squish along with some more porting.