Dave Smith's XL250 road racer

2 July 2000

Rick

I have hopefully attached four zipped JPEG pictures to this e mail. Two are taken at Aintree Liverpool, which is the same place that the famous horse steeplechase, the "Grand National" is run, on the green bits, not the black bits! The one on the concrete is at East Fortune, complete with emergency back brake cable repair, and the other at Tye Croes, with my brother in the background. I have appended some ramblings, and could distill these into something more concise if you wish.

Best Regards

Dave Smith

Note: Dave was one of the two mates (along with Briam Gomm) who contacted me originally, prompting me to ride the World Twinshock Champiohship. Brian and his wife Jean are delightful people, both visited Hawkstone, cheering me on, and providing Susie and I with a large dose of that fine British hospitality.

Details regarding the bike, and experiences with it.

The engine is an XL250 motorsport sideport, from 1972/1973 which was found in a Suzuki field bike, having been ridden on the beach by a friends son, without air cleaner of course, until it burned more oil than petrol! Having settled on a suitable number of negotiable beer tokens, the engine was mine!

Having a rough but useable CB250 G5 rolling chassis in the garage, it was decided to build a single cylinder classic road racer, to replace the AJS 250 four stroke single, constant speed, variable noise device, that I was riding at the time in the single cylinder class. Having bent the swinging arm one day when ratchet strapping it into the van, I came to the conclusion that the handling was not interesting anymore, but dangerous, and it was doomed for a dark corner of the garage! 

The XL engine picked up on the original G5 bottom rear engine mounts, and a reversed removable top rear engine mount, which made fabricating and fitting the rest of the mounts straightforward, the only area that caused concern being the asymmetric bends of the lower cradle. A set of re-chromedDucati forks and a Ceriani 7" double sided sls brake was fitted and the hand lever converted to a twin cable balance tree arrangement. The complete front end mated up with aftermarket Honda G5 taper rollers without alteration. The frame was also seam MIG welded along its pressed steel spine.

On stripping the engine it was found, rather unsurprisingly, that the bore was worn, oil filter was solid, and the clutch pushrod bore in the outer case was broken off. So a rebore, and an outrigger bush for the clutch pushrod, screwed and dowelled to the case sorted the mechanical problems out. The original cam was ground to the same profile as the race cam from my CB77, which has fairly conservative timing, but lots of lift. The lobes were still hard after this, but the end cam bearing surface was also badly worn. As a temporary fix, this was turned down, and a correctly sized phosphor bronze bush pressed on, the journal in the head being left alone. This temporary fix is still in place and working fine! The fly wheel assembly was removed to improve acceleration.

Carburetion was by a 32mm Amal Mk2 with a short bell mouth, fitted on a Suzuki inlet rubber. Fiberglass was supplied by a local racer, Ken Helliwell, and tyres were KR124 Dunlops that were lurking in the garage. Exhaust assembly was a silenced Megaphone with expansion box, taken from my brothers large stock of experimental Aermacchi systems made up over the years in attempts to civilise his short stroke 250, mated to a Triumph front pipe, again from the garage. Ignition was by battery and standard points. An old Girling hydraulic steering damper was also fitted running to the front fairing bracket.

The bikes first meeting at the extremely bumpy East Fortune circuit in Scotland revealed a couple of problems, seized rear shock absorber, and a broken piston circlip. As a new oversized piston was not immediately available, and the circlip groove was damaged, the bike was run with gudgeon pin buttons for the rest of the season, mainly at the Aintree and Tye Croes circuits. The only difficulty encountered in the rest of the season being a strange misfire on left hand corners, eventually traced to my knee blocking the extended carburetor bell mouth that we had fitted to cure what appeared to be excessive blowback from the carb at medium revs.

Over the winter the engine was stripped, and a CB250 RS (XL250S) piston fitted as it has a higher dome. The lower deck height of this required a shortened barrel. At this stage, whilst opening out the ports for larger CB250RS inlet valves, disaster struck, and the gasket face of the head was gouged. This resulted in a lot of metal having to be removed to clean it up, the face now running across the edges of the valve inserts. The piston was machined to give .060" squish clearance, and 11.5:1 compression, reducing the higher dome to about std spec in the process! At this stage the exhaust 
primary length was shortened as well, but probably not by enough as it turned out. All of this created a further problem as the cam chain tensioner would not bow enough to tension the cam chain. A two link smaller chain would not quite fit, so the end of the tensioner that is bent into the circular form, was heated and unrolled, and a solid metal dowel silver soldered in place. The tensioner was quenched from red heat in oil, and tensions everything up fine.

A swan neck induction manifold was also made from some old exhaust pipe, (Royal Enfield this time, also from the garage!) which allowed the carburetor to remain inside the frame. A set of long Mk 1 Girling gas shocks, with check springs were fitted.

The bike is now considerably faster, still retains good low and mid range power, and handles and stops superbly well, a major advantage being its rapid acceleration away from the line, which is a bonus in the short scratches over here. No doubt a lighter rider would find substantially better acceleration and speed! Reliability to date over some 7 meetings this year has been total, apart from a snapped rear brake cable. 

Future intentions are to fit a megacycle roller bearing cam, and a calculated exhaust system, as after some correspondence with Swiss it appears that shouting down the open end and pronouncing it resonant is no longer good enough! I may also get my brother and the third member of our race team, oily tom, aka Ellis Moore, BSA speed demon, to make a slightly higher fourth gear, or lower fifth gear pinion set, to close the gap between fourth and top a shade. They are practicing precision engineers, whereas I am just practicing.

I might also get round to setting up my old Heenan and Froude water brake dyno, which is currently sat in the garage, and doing some real engine development. 

Dave Smith, North Wales, United Kingdom JSmith2859@aol.com