This is the story of how getting an old dirtbike on the track can be quite a saga.
A few years ago, I bought a 1980 RM250 for a grand. It was complete but wearing crappy old plastics, the shocks were original, tyres worn out, chain and sprockets rubbish and so on. On the positive side it bore little sign of any major butchery and it went. the seller had a nice collection of superbly restored RMs and PEs and he assured me he'd gone through the engine and it was good.
I replaced all the obvious bits and took it off to Classic Dirt 9 at Wyaralang, but while I had fun on it, it went like crap. It wouldn't take any throttle in neutral and while it seemed to rev OK on the track, it was a very flat linear power. The clutch was very grabby as well.
So, I had the cylinder cleaned up, threw in a new piston, some Boyesen reeds and went right through the carby, plus a nice new Pro Form pipe. I play rode it a few times but it ran no better no better and the clutrch was rubbish. I got to and replaced the entire clutch, every last bit of it. That fixed that problem, but it still ran strange. Still it was a lot better with the clutch functional. I figured it'd be OK for CD10 but then that was canned, so back into the shed it went. A few months later I took it to Canowindra for the HEAVEN weekend in October 2013 where it went OK-ish but the stuffed rear shocks and weird power didn't help my cause, and I didn't gell with it.
I couldn't figure it out at all, everything seemed OK so why the strange power? I rode it at an end of year dirt track in Canberra, and it was a nice handler but I couldn't get it off the line as it seemed to bog on full throttle, yet it went OK on the track. It also got hot and there was some deto, something I'd not noticed earlier on MX tracks.
Over Christmas I pulled it apart and did some measurements. The cylinder was heavily ported with the inlet much much bigger than stock. But the really nasty bit was that the top deck had been shaved about 1.5mm. The piston actually sat proud of the deck. I got a special head gasket made up to restore the squish to something usable, installed some nice new YSS shocks and took it off to the Canberra club's Kings and Queens dirt track race in Feb this year.
Naturally things couldn't go smoothly and before racing started the tank split and fuel went everywhere. A bit of tape and so on and it kept enough fuel in it to ride it, but it still went slow. I also was way out on the gearing.
OK, back to the drawing board. Obviously I had to come up with a standard barrel. I managed to track down a cylinder and head and bought a new piston and got it bored to suit. Sat the cylinder on the bench in the shed, then on the weekend started to install it.. when... What the? A crack in the bottom edge of the sleeve? Why oh why? There was a small hemisphere of pitted and discoloured metal right there and I can only assume it had weakened over the years and the stresses of boring it led to it splitting. It was also pretty cold in that shed...
Damn.
Luckily though, I scored another barrel locally. It was only on the first oversize and I had a second oversize piston in the spares cupboard, so a quick rebore and it was good to go. That all went on just fine, as did a nice new MX71 Dunlop front. And now to install the new throttle cable. This was going so smoothly!
But of course, it was too good to be true wasn't it. The throttle housing was not original. Sigh. On to the forum and I tracked down an original assembly plus cable and it was in my sweaty hands a few days later.
So here we are, last Sunday, out at the local dirt track for running in. EVERYTHING is now right. At last. After a good few laps I started to open it up when it went off song. My heart sank... what now??? Back in the pits and the problem was obvious - the top of the carby had vibrated off. Fearing it had run lean or sucked in some dirt, I took it home right away. After pulling the top end off, it was apparent that all was fine. I cleaned everything up, checked for dirt in the carb and inlet etc, and proceeded to bolt it back together. Which went fine, except for the bit where a head stud finally called it quits and just sat there spinning...
Fork me!
OK, Fred at Tuggeranong Motorcycle centre rolled up his sleeves and helicoiled it, checked the rest of them, and now all i gotta do is nail it all together and we are go for Sunday's club day.
The big question is, what are the chances it will all go smoothly???
