Wednesday, July 14, 2010

kitting puch magnum

so, i've always been a purist when it comes to mopeds. i like to keep everything stock, or as close as i can. that changed today, when i did a cylinder/carburetor/exhaust upgrade to my 1978 puch magnum xk. i decided to do this because, while the stock magnum is great for getting around boulder, i can't make it easily to any of the surrounding cities (speed limit is at least 45 mph, even on the back roads). the stock moped tops out at around 33 mph. so, to get some more speed out of it i ordered a bunch of parts from treats and 1977. what i got:

- k-star 70cc cylinder kit w/ high compression head. i got the k-star because it was cheap and i didn't come across many people who have had issues with it. for those of you not familiar with mopeds, they all come stock with ~50 cubic centimetres of displacement (the engine size). this is because larger engines are considered motorcycles and therefore require a special license, higher cost of registration, and higher cost to insure (plus they use more gas). in order to get around this, you can buy a "kit" that replaces the cylinder, head, and piston and increases the displacement.
- 19mm dellorto phbg carburetor. a bigger engine needs more air and fuel, which means it needs a bigger carburetor (the part that mixes the air and fuel before it goes into the engine).
- 19mm intake. a bigger carb requires a bigger intake.
- tecno estoril exhaust. more air/fuel means more exhaust and therefore a larger exhaust pipe. i chose the estoril because it has a large powerband (range of RPMs where the engine is creating good torque) and the powerband is located in the middle of the RPM range. this means that it allows relatively good low end torque, which is important for starting, while still giving a good top speed.
- uni pod air filter. clean air means a cleaner carburetor and a cleaner cylinder, which makes for a happier carb and cylinder.
- ignition micrometer. i was tired of my little stick with lines drawn on it. this makes it easier to accurately set the timing (when the spark plug fires).
- cylinder studs. these hold the cylinder and the head to the crank case. i could have used the stock studs, but the threads on one of them was stripped. pow! new studs.

so, let the fun begin. first step, take the old head, cylinder, and piston off:

here is the k-star kit that i'm going to put on with the hi compression head:

first i have to put the new piston on. here's the new piston on the original connecting rod as well as a photo of the new and old piston showing the difference in size. woo! 20 more cubic centimetres:

now i just slide the cylinder over the piston, which is surprisingly easy since there is only one piston ring:

uh oh! first problem. my cheap-o k-star kit has a poorly tapped spark plug thread. this means the spark plug will not thread onto the head:

solution: buy a $30 tap and chase the threads (or send it back, but i am not patient). the proper tap for this spark plug is a 14mm x 1.25:

it was tricky getting the tap started in the existing threads, but i mustered enough patience and managed not to wreck the head. just be sure if you do this to clean off the head thoroughly. i used some isopropyl and a brush. pistons and cylinders don't take kindly to little metal shavings:

the head now attached with the spark plug in place:

next comes the carburetor and the intake. one of the float bowl vents was interfering with the moped's frame, so i just pulled it off. it still leans a little to one side, but it should be fine for now:

finally comes the exhaust:

i didn't expect the exhaust to mount properly since it was designed for a puch maxi and not a magnum. the mounting points don't line up, so i have to fabricate something. for now i'm just using a $4 exhaust mount that should keep it from breaking in half for the meantime:

the results: to my surprise it started right away. it was loud as hell because they sent me the wrong exhaust gasket, but i'm working on fixing that. so, i took it out on the street and it ran pretty well. it was bogging at wide open throttle, which i thought was because it was running lean, so i raised the throttle needle up a notch. it ran a lot better, so i did a plug chop and found it was still running lean, so i up-jetted to a 92 (from the stock 85). after another plug chop it looked better although i think i'm going to try the 95 i have to see how that runs. i'd rather be running rich, especially while i'm breaking in the new cylinder. so, i haven't had anyone pace me in a car to figure out the top speed (the speedometer only goes to 30 mph), but my best guess is she's topping out at 45 mph right now and i should be able to get a little more once i dial in the tuning. i also need a bit of tubing to attach the air filter to the carb, since there is no room for the larger filter. in conclusion: sweet!

next step:
- fabricate a mount for the exhaust. i really wish i had welding equipment.
- tune to perfection.
- install low rise handlebars
- get a voltage regulator so i stop burning out break and tail lights.
- paint the frame.