Jetset, number 68, March-April 1994 - Marks sprinter

COOL BEAST MB DEVELOPMENTS 250 LIQUID COOLED LAMBRETTA
Back in the early 80's when I was a young mod growing into a scooter boy, Scootering used to be fun, it was a hobby and most enjoyable, I would get to all the functions, rallies and Lambretta events for years, then one day the hobby turned into a business. For some people it stays a hobby but when bills and family take over it becomes something else!
The history of MB Developments is not new so I won't go into what makes me tick or the various work pieces and designs I have done, I'm sure most of you will be familiar with it all. I have always been involved and had an interest in racing of all types Scooters, Karts, Bikes, Quads whatever but have never taken part! Not my choice!
I got the bug for sprint racing in 1991 watching team members racing at Santa Pod. It seemed safe and before I got too old I had to have a go. A few scooter dealers were also taking part and records were being broken so it seemed the right thing to do. Plans for the engine had already started, a group 6 250 Special was needed for Graham Gee, which was to be a 250cc using 64mm stroke crankshaft. As these things take time rumours were flying that special crankshafts were not to be permitted in the next season, I was already committed as the crankshaft had been designed and undertaken by the engineers.
The basis of the engine was the crankshaft it was to look like a standard GP crank but the stroke was lengthened 6mm from 58mm too 64mm to achieve this the crank webbs had to be increased in diameter, a choice of connecting rods were available, I went for a Rotax and used Yamaha bearings, the crank width was increased to take the wider rod. The crank its self was machined from solid billet, case hardened and crank ground. On assembly it had no run out, not bad for a one off! This work took 18 months and £400.
The rest of the engine could not be started until the crank was assembled. So my sprinter took back place to the other dealers already having a go!
The crank was so much larger, to get it to fit 8mm of weld was required all the way around the crankcase a plate was welded to the mag gasket surface to allow for the wider crankshaft and at the same time strengthened the transfer feeds, always a weak spot on any engine! The crankcase face was built up to take larger transfer feeds for the cylinder and as a final last minute design I chose to use crankcase reed induction through a 6 petal assembly. This all sounds easy but the casing must have gone back and forth to the welders over 20 times, what a bore! My next one won't take as long I bought a TIG set just for that job so the new casing will cost £1500 before I start, who said sprinting is cheap!
Once welded the casing was ground down polised, bead blasted and machined to look like a factory job seeing no welding. All this welding distorted the casing so much that the drive side bearing housing sunk down on one side by 1mm not much but it could have been a mile, it was the down fall to all that engineering, by the time it was remachined it was a weak spot (more later).
Eventually the crank fitted in and turned free, all this and l hadn't started a cylinder, the crank had to be fitted to check ports timings and piston travel (very important)!).
I was still working within the Group 6 rules with no cheating, which is a waste of time it's just not cricket! The crank rules were not thrown out so it could still be used for road racing except the reed block fouled the frame and in the Group 6 rules modifying the frame was wrong but not for sprinting, how perfect!
Here was my time to race so I told the wife, BAD MOVE! but when a mans got to do what a mans got to do he does it. The cylinder was the next major step I chose a Aluminium AF TSI and started to modify it. The inlet was welded and two boost ports were put into its place and fed into the crankcase. The transfer feeds were welded and enlarged. For something special I added two extra exhaust ports running above the transfer ports, to do this fins were removed and extra welding took place around the exhaust port to give a larger diameter port. When all this was done there wasn't many fins left so I liquid cooled the cylinder, a job in itself! It ended up looking more like a motorcycle engine. A book wouldn't explain the amount of time and effort that went into the cylinder alone, the easiest way would be to say £1000 if you want one!
Slowly the engine took shape the rest of it was reasonably easy, a liquid cooled head was made, matched to the porting, steel liner, and Suzuki piston (All this development and a choice of piston that had been discontinued and ended up going to last over size due to lack of stock! What a cock up!)
I made a one off gear box modified from SX200 and LI150 items, I used an auto advance Japanese ignition system giving 20 advance at 10,000 rpm, finally the whole engine was assembled to a very high standard, finally making a one off fully blown exhaust system all fed through a 39mm Delorto. At last done, two years too late. Now for the frame!
The engine had to be complete to start the frame. Here I only put a little thought in to it as I didn't know what to expect! It only took a week! I did expect it to wheelie which I didn't want so I made it long and low with the riding position forward. To do this I extended the engine mounting rearward using three positions but the damn reed block fouled so I lost any experimental adjustment.
The frame became a tube with a small top section, Honda fuel tank, racing seat, brackets for the liquid cooled system, ignition and battery. At the front the steering was raked back, the steering column was lowered as were the forks, springs, ,rods etc. I used a drum brake and dropped bars. My feet were in a standard Lambretta riding position. The theory was, I ride a Lambretta daily and never miss a gear, how many times do racers miss in the heat of the moment with rearsets and I didn't want to wreck the motor that easily. My theory sounded OK to me, in practice I don't know! My engine caused too much wheel spin which is possibly worse that wheels lifting for a good time.
Once finished off it was sprayed white and green when fully rebuilt it was filled with racing C114 fuel because it was available locally in small quantities £15 per gallon expensive experimenting!
Initially the bike failed to start it took three hours of pushing to find a silly fault, once fired up we found that small holes in the alloy welding to the crankcase leaked as we were short of time I sealed them with an alloy paste which worked but spoiled my nice engineering a bit!
Once run and generally messed around with the motor sounded fast and powerful. Setting off was fun there was so much power, at certain revs it died and at the power band it shot off mostly side ways, it had a crazy power output - wheel spinning for at least 20 feet, either a sprint track was to have a more sticky surface or the frame was wrong or a different tyre was needed, we changed tyres to Dunlop TT60 eventually.
The next day it was Dynoed giving 40 horse power my eyes popped out! we did another run to make sure, I couldn't believe it, I expected just over 30 hp. The highest scooter tested to date was 30 hp and that was eating records! Was I hyped - up the next day going off the Avon Park for my first two wheel competition.
The excitement was only to be dashed when all prepared for scrutineering I noticed a crack appearing around the main engine mount, I'd come this far I had to race so out came some magic putty to clear scrutineering (silly me!) I was prepared to race to see what happens.
Again initially we had some teething problems ie fuel supply, parts coming loose, plug packing in, along with the crack and rain everytime I was prepared to race slowly brought me down to ground it was not going to be plain sailing.
For the first run I was surprisingly calm but it seemed like all eyes were on me as the bike was well turned out and more radical than any other and this 40 horse power thing had shot around like Chinese whispers.
As the lights changed my scooter shot off, it stayed straight and went like a train on tracks,but the jetting was rich and the clutch slipped even so a mid 14 seconds in quarter of a mile at 92mph, I was disappointed but with a few adjustments I had a 2nd run later in the day, the clutch didn't slip and pulled a 13.9 seconds at 97 mph things were improving, except a major teething problem:- The exhaust was mounted to the mag housing as the engine revved the mag housing and ignition system oscillated causing air leaks and my timing to move all over for both runs. As much as I tried I couldn't keep the thing to hold together so I was out!
13.9 seconds was the 4th fastest of the day I was disappointed when the main men said it was a bloody good time for my first attempt, don't forget they were 18 months in front with their developments. Any way to my knowledge less than 10 people have gone under 14 seconds in a standing 1 /4 mile on a Lambretta so my first go was reasonable!
Well two runs and a great days enjoyment for £15 thats what I call fun, gosh I was so excited, it was topped by going home with Wayne Bygraves's Babe Ruth G P with my engine, we did 100 miles on the motorway dropping to 75mph once because of a slow car, the rest honestly cruising at 90 mph and even 100 mph sat up! My poor car couldn't keep up he was just a spec in the distance most of the time, what a Day out!
To say we were buzzing when we got home was an understatement. By the way he won the fastest custom on the day and my Group 6 rider Shaun Hodgkin pulled 13.9 secs and won on the Sunday.
Back to my sprinter I had plans I ripped out the engine repaired its faults altered the frame to reduce weight, by altering the forks, seat section, engine position and resprayed it Red. The engine was retuned to meet Rob Millers Group 4 race and sprint record holder and Mike Davis's Group 6 race and sprint winner! The extra porting only made the motor more peaky on the dyno, so in my wisdom I changed to a GT200 Box and lengthened my exhaust system to spread power, what a fool I should have tested it, I didn't it knocked off 2,OOOrpm in every gear on my next run at Santa Pod.
What a waste of effort, I couldn't get below 14 seconds and my best speed was 92 mph at 8000 rpm. Still not bad, what if it still revved to l0,000 rpm as at Avon Park. But again still a good day my first run I tried a bucket fairing again untested it wobbled side to side causing something like a tank slapper on a motorbike, off came the fairing, the jetting was too rich and other runs seemed to improve only to get to a point where the bike was as good as I could get it on the day. Various methods were used to launch and never gave the same result, the bike went where it wanted making very interesting riding. There was a definite problem either the frame or tyre. 20 minutes from the end of the day I swopped tyres to a michelin it worked a whole different approach was needed and managed two more improved runs only to come to the 3rd then disaster smoke everywhere and a grating noise, a major breakage it was the housing holding in the drive bearing, the crank and bearing tried to swop positions with the drive sprocket, the grip of the tyre must have pulled the bearing out, well it is 40 horse power a GP200 is something like 10 horse so you must expect a major problem somewhere!
The next day with my bike out of it I rode the spare, my 150 special and competed all day eventually not bettering my time I think it came 2nd. I was quite pleased until Rob Miller had a go and knocked 0.5 of a second off my best, how does that man do it he's never even raced the bike before. Going to show there is something to setting off for a fast time. We later found out he smashed his own record in Group 4 by 0.6 of a second but alas was not his best weekend he only had the one good run never mind he did it, he even beat my 250 times and on a standard 200cc too!
And another good cheap most enjoyable weekend and I am definitely hooked, when you are involved its so much better, so take part and have fun you don't need 40hP. Scooters at around 26hp have beaten my times but just think if I get everything right and a good rider who knows. The record now stands in the 12 seconds well done you lads!
For the next time obviously a new casing I will junk the LC250 and put it in a road bike, a new aircooled version is next along with a modified frame and tyre set up! But will I ever get time! 1994 has been my busiest year yet! Forget the seriousness of racing I enjoy the fun, and it is for anyone.
Yours still riding a Lambretta,
MARK BROADHURST

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