Started out with a midrace zip, a failed gudgeon pin circlip wrote the top end off and damaged one of the main bearings.As I currently am broke I split the engine and fitted new mains and seals. It now has the following parts: Stage6 HPC crank leo vince tt exhaust stage6 reed block stage6 sport pro variator stage6 r/t clutch and bell Athena CDI. Allegedly this CDI has an additional 3 degrees of advance up to 7k or so, matches the standard cdi for a bit and then retards timing by 3 degrees at 10k or so. The reed block looks impressive but the standard intake manifold opening is slightly smaller so the flow "steps" up into a larger opening. Using a stuffer made the reed block opening smaller than the intake manifold opening so decided it was better to run without. Went with a 64 main jet in the carb as a starting point.Left snorkel in airbox and brass plate in mouth of carb. this jet was always on the rich side on a derestricted zip with a leo vince tt so should be a safe enough size to start working with. I wanted to use the original variator but I just tried 3 of them and none of them want to slide over the splines on the HPC crank. I could have sworn they fitted fine before so put the sport pro variator back on. No idea what weights to run, it was supplied with 6.25g and a medium contra spring. This made it rev at around 11k. A soft contra spring brought it down to 10k. Went with the original spring and 7g to start off with. I will have to experiment a bit to get it right. Thats why I had wanted to use the original variator as i know it sits at 8500rpm on 5.5g rollers. The clutch may as well stay in because its great. Changed the red springs to white and wound the preload all the way up as a starting point. Went to fit the ignition system and realised I was going to have to play a game of flywheel roulette. I had 2 flywheels in front of me. One was definately from a piaggio zip sp. The other was either from an aprilia sr50 or my other zip. One was darker coloured than the other and the pickup trigger on the outside was positioned 90 degrees further round relative to the keyway. Looked at the sr but the flywheel was so grubby I couldnt tell which one it matched. Picked one at random and hoped for the best., Went for my trusty leo vince tt as it looks nice, sounds well, is easy to set up and runs very nicely on a 50cc. I looked at my original cylinder and decided I couldnt be bothered trying to squeeze a dremel in there. Then I realized my digital caliper had a flat battery. So eyeballed the piston and took roughly 1mm off the crown where it meets the transfers as the factory durations looked to be very short. The exhaust port was already close to 70% of bore width so left it untouched and didnt alter the height as didnt want to end up with it mismatched to the exhaust. The next thing I did was look at the head, with the base gasket in place I had what looked like a huge piston to head clearance. So I chose to fit the barrel without a base gasket, just using some anaerobic sealant instead. This brought the piston to head clearance down to 1mm or so at its tightest point which should pep things up a bit. After tht I struggled mightily for a few hours getting the engine back in with nobody to help, plumbed and wired everything up, then realised my throttle cable was on its way out. Liberated a fresh one from my other bike and got everything fitted. Threw some coolant in and fired it up briefly. Its in my living room in a terraced house and its 2am so cut the engine after a few secs and gave myself a pat at least it runs. Tomorrow I have to guesstimate the correct roller weight, adjust clutch, adjust carb and then bugger off for the day for a scooter rally. Hopefully it will be okay. It feels more responsive on the throttle compared to a standard 50 with the tt. We shall see tomorrow...................
Took it a quick test today. With the modded transfers and increased compression it was noticeably improved over a standard cylinder. However it didnt want to pull past 50mph, would get there fairly quick and refuse to increase from there. Completely standard with a derestricted standard exhaust it would have scraped its way to about 53mph indicated so I knew something was up. Then an awful rattling sound started.It was much louder than the engine noise. Took it home and checked the variator it seemed okay so fired it up with the drive belt unconnected. Still rattling like imminent death. Pulled the top end, again it all looked good but put a completely unmodified standard 50 top end on anyway. Conrod seemed fine too. Still rattled. Took off flywheel cover and checked behind it, fired engine up again and noticed when I blipped the throttle the flywheel would slide out towards me a few mm. Took the flywheel off and checked keyway was intact, refitted it carefully, fired it back up and all was fine. I had first fitted it with an impact driver and hadnt realised it wasnt driven all the way up the taper. Oops, really stupid mistake to make but thankfully no harm done. Tested it again and the standard cylinder is definately down on power compared to the modded one. Top speed remains at 50mph. Bugger Tomorrow I will fit a standard set of reeds, the ones in it may be too thick to respond properly at the lowish rpm this engine is turning. However I really think the power deficit comes from the HPC crank. When I fitted one to an engine running a stage6 sport pro before I noticed a big drop in bottom end power but this was more than made up for on the top end. Will try reeds first anyway....................
