30 BHP Blog

We did make it to the rally and thank god we didn’t have any serious injuries.

Later that year I sprinted badly at the NSA 50th anniversary but did manage to do 1 run under 17 seconds.

then in May 2009 I took the scooter for its first dyno run at Diablos. It managed 22.7 BHP and 19lbs torque.

Disappointed with this result I sent my broken 62mm stroke crank to Chalky at Replay scooters to be repair properly and on the way home from the LCGB Euro at Lincoln I went to Mallory Park where the BSSO were racing to pick it up from him. As soon as I got it I stripped the engine down again to rebuild it as it was always meant to be. This time the crank had a 115mm rod, I Also put the old piston back in which had the inlet side skirt cut, the port timings were:
184 degrees Exhaust, 132 degrees Transfer and 170 degrees Inlet. and less than a month later I took it back to Diablo’s for another set of runs.
This time it was better, making 26.3 BHP and 20 lbs Torque.

For the rest of that year I had no problems at all, the GP got me to were I was going and got there fast enough, including my first Coast to Coast but I knew it had a little more to give, with the 62mm stroke the piston crown was sitting 2mm below the bottom of the transfer windows, I hadn’t lowered them because I didn’t have the 90 degree angled tool to be able to reach them, in the autumn of 2009 I met scouse Mark (he will love me for saying that lol) who had the tool needed and was willing to lower them for me. Lowering them would give a larger transfer area and could only make it better.

When I took the engine apart to give Mark the barrel, I noticed that there were marks on the con-rod and after taking the crank out again it was obvious that one of the weights had come loose again and had been rubbing the side of the rod. I had seen that John at JB Tuning was selling 62mm x 115mm cranks so I decided to buy one. I also bought a new piston which I only cut 2mm from the inlet skirt to give slightly lower inlet timing and re-chamfered the exhaust port.
When I put the engine back together the port timing were now, 186 degrees Exhaust, 134 degrees Transfer and 164 degrees inlet.
In March 2010 I took it back to Diablo’s for another dyno run. This time it produced 27.2 BHP and 21 lbs torque.


The new port map looked like this:


As stated earlier, the ports were done very oval. Later that year the GP made it to the Lambretta Euro in Spain with out any issues, at the start of that rally I started using a cheaper 2 stroke oil, Rock oil groundsman30 BHP Blog. Its about a quarter of the price of Rock oil Synthesis which I was using and the spec of the oil looked good enough for an old Italian shopping bike.
The oil spec meets API TC, TD JASO FA, FB, FC GLOBAL ISO-E-GB, GC,GD

However on the way home from Cleethropes later that year, the scooter started running badly, no mater how I jetted the Mikuni it just ran very rich to the point of four stroking. When I got home I tried a fiends Mikuni to see if the problem was with the carb, it turned out that it was, with the newer Mikuni with the same set up the engine ran well but with mine it ran rich, I came to the conclusion that the fixed atomiser had worn so I looked around the Internet for another carb. I kept reading about bikers who were changing their Mikuni’s for Keihin Airstriker’s so I decided to get one to see how it would run.
Just before the Run to the Hills it arrived with the various jets and needles I had ordered, As no one was using these carbs on Lambretta’s to my knowledge I had no idea where to start with the jetting. With all jetting you start with the pilot, get a good tick over that drops down quick, however with this carb, no mater what I did with the pilot jet it just wouldn’t tick over. Instead it would just cut out as though the pilot was too rich, the air-screw had no effect at all with any of the jets I had, when I phoned up to get some smaller pilots I was told that I had the smallest one already in my arsenal. This left me a bit stumped. I decided to make the cut away in the slide larger to see if this would help, it did but I had to cut it very steep so I ordered the weakest slide which is a number 7. I ended up with a set up that I could ride with but was far from right.
The scooter made it to the Run to the Hills and ran quite well on the way home.

its next outing was IoW in August, we set out from Birmingham late on Saturday afternoon because of work commitments and while ridding on very little throttle due to a slipping clutch it heat seized solid so we got recovered home. next day I took it apart and saw it had been running very weak at that throttle position with the new carb. Obviously the small pilot I had used to get it to tick over wasn’t supplying enough fuel. The piston was ruined so I had to buy a new one.

This time I decided to buy the Wossner version of the H2 piston, it’s a lot lighter than the Wiseco, its Teflon coated and it has cut outs either side above the gudgeon pin. I had to do the same mods to the skirts but the ring pegs are in the same place as the Wiseco so there is no need to re-peg.

Once the engine was rebuilt with the new piston I decided to take it to the petrol station close by to fill the tank before I started re jetting the carburetor. I ended up taking the carb apart and drilling the pilot circuit because the hole behind the slide was very small, much smaller than any other carburetor I have seen.