30 BHP Blog


The first thing I did when I stripped the engine down was to make a trace of the barrel before I made any changes.
this is that trace:


The port timings were,:
Exhaust 172, Inlet 158, Transfers 117 .
Conservative considering how well it went!

I made the exhaust port 4mm wider, the inlet 5mm wider making them both 48mm wide, raised the exhaust to 34.5mm from the top of the barrel, left the transfers 47mm from the top and lowered the inlet so its bottom edge was 97mm from the top of the barrel.
The crank I decided to buy was a Tino 62 x 116 so together with these ports I would end up with 184 degrees exhaust, 132 degrees transfers and because i went a little wild shortening the inlet piston skirt I would have 170 degrees inlet timing. When I did the ports I decided to go for very oval shapes in the hope it would give a little reliability. An oval exhaust port eases the rings back into their groove to prevent snagging and an oval bottom edge to the inlet port reduces the pressure on the pistons inlet skirt which in turn reduces the risk of cracking occurring along the skirt.


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.

This put a stop to getting it run in. The crank was sent back to Tino to be repaired as it had only done about 200 miles, while it was away I was off the road but as it was around November 2007 I didn’t mind as I wouldn’t be doing any miles through the winter. Then the crank got lost in Milan airport, Tino didn’t have it and the courier service had only tracked it to the airport. about 3 months later I did finally get it back. When it did come back I rebuilt the engine and got it back on the road hopping to get it run in. A few months later I noticed a rubbing noise coming from the flywheel, on inspection it was obvious that the crank had twisted as the flywheel was oscillating, so I had to strip it down yet again. I also found that one of the gudgeon pin clips had come out resulting in a badly scored bore and a pebble dashed head. The crank this time went to a place in Birmingham to be trued and welded. The next day I was fitting the crank back in and rebuilding the top end using the same piston with new clips, I also took the high spots off the squish area of the head corsed by the free clip.

This is where its gets silly…..

When I kicked it over to start it, it fired once and then stopped, it then sounded horrible when I kicked it over again. So I started taking it apart yet again, when I took the mag housing off half of the crank came out with it.


To say I was getting sick of this crank would be an understatement!
I then decided to buy another crank, at the time there were no other 62mm stroke cranks available so I had to buy a 60mm stroke crank and opted for an Alpha crank from MB. The problem I had now was that I had ported the barrel for a 62mm stroke so the port timings would be different with a 60mm crank. The main problem was going to be the inlet timing because the shorter stroke would mean longer inlet timing, the solution to this was to buy another Wiseco piston and cut nothing off the inlet skirt. I still had to do the mod to the other skirt as previously mentioned.
With the 60mm stroke and 115 rod the port timings ended up at:
160 degrees Inlet, 180 degrees Exhaust and 125 degrees Transfers.

The 60mm stroke was installed in July 2008, It ran ok but it didn’t have as much pull as when it had the 62mm stroke.
The next month was Isle of Wight weekend, On the way there we had a rear wheel blow out which was probably due to under inflated tyres for the weight I was carrying at the speeds we were doing, two up with loads of luggage.

It just happened that I was wearing a cheap helmet cam at the time and managed to catch all the action.


Not having much luck am I so far? read on, it does get better……………. eventually.