I hope you enjoy my story as much as I've had bringing it to you,It all started on October 18, 1992. I ran wide on the exit of the right hander onto the Amaroo Park straight during the 250-production class race. The rear tyre flicked the kerb sending me off into the armco barrier. I got sandwiched between bike and barrier and must've cushioned somehow the blow as the bike sustained minimal damage however I wasn't so fortunate.
The bike I was riding was a Suzuki RGV250 that features aluminium beam frame and a 90 degree two stroke v-twin engine putting out some 60bhp at the crank. It weighed in at 130kg. It was a real pocket rocket!
The 250cc-production road racing class in Australia is very competitive and is full of young testosterone charged teenagers riding current models with the best tyres and equipment, the total opposite of myself. I soon realised there wasn't much future in this and set about thinking of the how I could convince my wife in letting me continue racing whilst maintaining a growing family and a roof over our heads. It was about this time that the singles class was beginning to take off in Europe and the formula immediately appealed to me on the grounds of modification freedom, low cost (wellrelatively low) and sound. Plus, my brother who is also a bike nut and a first class CNC machinist needed somewhere to show off his handicraft!
Well it didn't take long before the verniers and rulers came out and the two stroke sold off to make way for a Honda XL600R engine to go in. It turned out to be a pretty good match. The motor fits in perfectly. The rear case mounts are exactly half an inch narrower than the Suzuki's lugs thereby allowing quarter inch steel fabricated mounting plates on each side to rigidly mount the engine where we wanted, and yes the sprockets lined up! The front engine mount is also a steel fabrication that is attached to a set of hangers that are attached to the frame beams. Lugs welded onto the aluminium frame fixes the hanger at the rear point with the forward end fixed to original frame mounts. A pair of lugs were welded onto the inboard sides of the frame beams whereby a transverse tube with lugs attaches the cylinder head to the frame.
It may sound like it was a smooth operation but as you could appreciate the current version is far from the original concept which started out with rubber bushes and aluminium mounts, which being a supermono virgin, I thought would be adequate. Wrong! There is nothing else like a hammering big single to quickly teach you the pros and cons of various mounting techniques and materials applications.
Other chassis mods include home made triple clamps with less offset for 96mm of trail, sliding block chain adjusters mainly for to allow fitment of a 5.5 x 17" Marvic rim. The front is a 3.5 x 17" Marvic. Wheelbase is 1390mm with a rake of 24 degrees. The forks are standard RGV 41mm fitted with Progressive Suspension springs to take my svelte 95kgs. The rear shock is also standard and seems to do a good job at my pace. The brake calipers are standard 4 pot Tokicos clamping on home made nodular cast iron rotors floating on aluminium centres. They're powerful but I'm looking for more feel and am still evaluating different pads at the moment.
As for the engine I'll start at the top. Camshaft is a White Bros track spec grind. The published specifications for this cam are as follows:
INTAKE
EXHAUST
Valve Clearance
0.004"
0.004"
Valve Lift
0.380"
0.365"
Duration @ 0.020"
280 Deg.
274 Deg.
Lobe Centre
104 Deg.
106 Deg.
The main rockers have been deburred of sharp edges and shotpeened. The sub rockers have had their wear faces hard chromed for durability. The springs are R&D with home made titanium retainers. White Bros. supplied the shortened bronze valve guides. The valves are standard Honda.
This model engine is a 1985 vintage with the twin port/carb arrangement. I decide to use the standard RGV carbs which are 34mm semi-flat slide Mikunis (I've disabled the power jet circuit to not complicate jetting). I settled on this combo mainly because of cost as I already had the carbs and also because they feature approximately 25 degrees of downdraught that played right into my idea of how I wanted the cylinder head ported.
The port roof was raised at the entry by about half an inch and blended into the valve throat and home made manifold. This was achievable by weld build up of the cylinder head above the ports as there was ample room. The port floor was raised accordingly by weld build up also. This decrease in valve to port angle along with the cylinder inclination of 15 degrees matches the downdraught of the carbs. The port necks down progressively from 34mm at the carbs to 28mm approaching the valve throat. The stock port diameter was 28mm. I've resisted increasing port size in favour of gas speed. The whole intake is about 8" long from valve to carb bellmouth. I'm told this dimension is important for pulse tuning as is the exhaust but whether it's right or not I don't know, it's just how it all come together for the right fit. I've got the flow figures somewhere but they don't mean much as I never got the before figures recorded, doh! Exhausts were ported to flow to 68% of the inlets. The volume of the chamber was reduced to 50cc and quench area increased slightly. Compression ratio is 12.3:1.
The cylinder is fitted with 10mm high tensile steel studs necked down to 8mm and the head is screwed down with home made 8mm increased grip length tool steel nuts. Gasket is standard and torque increased 10% over standard. This area has never given any problems in this configuration and is much more reliable durable at assembly and disassembly.
The exhaust is consists of a pair of 1.5" headers approximately 23" long dumping into a pair of home made carbon fibre skinned mufflers. This is an area I'd like to do more work on although it seems to work fine. Ignition is standard CDI.
One way of increasing acceleration for such a big cylinder was to reduce reciprocating and rotating masses. The piston I use is a 102.4mm Wiseco forged unit which weighed in around the 400 something gram mark. The con-rod is a replica of stock but slightly thicker and machined from billet titanium and shotpeened. It weighs in at around two thirds of stock. It has a nickel aluminium bronze little end bush and a hardened ground steel sleeve in the big end for the roller bearing. The flywheel lost about an inch off the diameter. The counter balancer was ditched and the crank
out-of-balance factor adjusted to 68%. The trade off here is that the bike vibrates (throbs) at idle and lower rpm but is quite smooth at racing speeds. I originally ran the bike with the standard 75mm stroke crank with a 100mm piston but it just didn't seem fast enough so while the engine was apart I slipped in the 80mm crank and 102.4mm piston (660cc). I know it's moving away from the ideal stroke/rod ratio but I rationalised it by virtue of the lower weight of the reciprocating components and the fact that these engines aren't about high rpm anyway although having said that it revs past 8,500 with 15/39 gearing no problem.
The clutch is fitted with Barnett kevlar friction plates with standard steel drive plates along with a set of heavy-duty springs sourced from White Bros. The gearbox is standard and is an area I'd like to improve on but I'm not sure where to start. First gear is useless and second is rarely used on our local tracks. I've read that Oscar Rumi in Italy has a six speed in his Honda single racer but I haven't been able to find out much about it.
That's pretty much it for now. It has taken roughly five years to get it to this point. It hasn't been on the track as much as I'd like but the future looks brighter for more outings. One unfortunate aspect is that over these years the class has waxed and waned in popularity. Maybe it's been too hard for most guys to come up with a good reliable combination and they've dropped out! However there is a few of us hanging in there.
Future plans? Well testing and dyno time is expensive (it's never been on one yet) but I'd like to try a longer duration cam, different exhaust configurations as well as bigger carbs, say 37-38mm perhaps. Also I'd like to try an airbox but space is tight. If not at least some cold air baffling. Hey that should keep me going into retirement!
Don Ricciardiello (dricciardiello@qantas.com.au)
Sydney Australia
10/07/98