30 BHP Blog

Next was to take the barrel to be re-bored, I used PJ engineering because I had heard good things about them. When I took the barrel to them they were very reluctant to bore it out to 71mm even though it was already bored to 70.6mm, they were worried that because the spigot was already very thin that they would end up with a pile of fins on the floor if they bored it to what I had asked for. In the end they did it for me because I promised to take full responsibility if the barrel did fall to pieces, A few days later I picked the barrel up which was still in one piece.
The piston I used was the Wiseco aftermarket piston made to replace the OEM H2 Kawasaki piston, the reason I chose not to use the OEM piston is because the ring peg positions are not in a suitable position for use in a Lambretta barrel and would have to be re pegged, I didn’t want to re peg the rings because I felt it would be another thing to affect reliability. The Wiseco ring pegs are both opposite the exhaust port and as such are perfect for use in a Lambretta barrel.Which ever piston is used though there are some other mods that are needed to the skirts, one skirt needs cutting because it blocks the one transfer both in the window approaching TDC and the bottom of the same transfer at BDC, also I cut the inlet skirt to change the inlet timing. More about that later though..

The piston has a 30mm deck which means when used with a 116 rod you don’t need to pack the barrel up as high as if you would if you used a standard Lambretta type piston which have a higher deck. I had to use a 1.5mm base packer with 3 base gaskets to get the piston to sit where I wanted it to at TDC.
When I had the head machined I asked them for a 12-1 compression ratio, this meant a combustion chamber of around 22cc, I had thought going for a high compression would be good to start with and if it turned out it was too much I could take some more out of the combustion chamber to reduce it a little. Also because the piston crown is nearly flat I had to use a spark plug spacer to lift the plug a little to prevent the piston hitting it at TDC.

The carburetor I decided on was a Mikuni TMX 35mm  with a UNI filter  fitted via a hose under the panels, With 6 slide, 300 main jet, 56 needle top clip, 20 pilot jet.
The exhaust was a JL 3

Once it was up and running I started doing some miles to run it in, while I was doing this I was doing many plug checks. While doing one plug check at around 1/4 throttle, I turned the engine off with the key, rolled to a stop then pushed the scooter up the kerb to take the plug out with out being hit by a passing car. The plug was a nice colour, so I put it back in and tried to kick the engine over only to find that it locked. I turned the flywheel back and the engine turned with ease but then stopped again when it had done nearly one revolution. very confused I managed to get recovered. When I stripped the engine down I found that what was jamming the engine was one of the crank web weights had dislodged and was stopping the con-rod passing between the webs.