Saturday, July 10, 2010

Piston flaws, intake restrictions and math

I got a wonderful package of goodies from 1977mopeds yesterday. Most notably, it included a new cylinder and piston for my wife's bike. 50cc TCCD 7 port "Racey" cylinder. I like these cylinders, even though I've had a (2)stroke of bad luck recently. They don't make loads of torque, but put a reasonable stock gearing and up the compression and you do just fine, and they really wind out. They love small carbs, and if you put a 6p on them or some other such high RPM pipe, you can really rev the heck out of these things. So I got home from work at about 5:30, opened the package, and began assembling my wife's engine so she could ride it last night. So I pressed the bearings on the new crank (the Stuffy Alu crank), new seals, yamahbond the engine cases, assemble clutch, make a new thick base gasket to generate proper port timing and head clearance (as the stock gasket doesn't fully reveal the exhaust port) chamfer the cylinder and ring, set the ring gap, file this casting leftover out of the piston window (which remind me of the bushes from Super Mario Brothers), bolt on the head, install on the bike, and get it ready to ride in less than 2 hours. A pretty good clip, at any rate. Hopefully it all continues to work well. I've got a 12mm bing on it at the moment because I forgot my nice 14 intake and carb down at the shop. Here is a 14mm intake that I discovered a wonderful restrictor in. 10mm. That thing is tiiiiiiiny. It was on a freespirit, with a 14mm carb no less.

I also was working on my Hobbit race motor yesterday and was doing math to try and calculate very exactly the exhaust duration of my Athena cylinder. I'm aiming for around 190 deg, stock is 180, and with Travis' nifty little Wizard spacer, the timing ends up just over 200. I think I'll just end up raising the ports a little and not running a spacer, rather than space it up and deck the cylinder, but who knows. If you are doing the math, don't forget than it isn't really cosine but inverted cosine to use the formula correctly.

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