The final countdown
Some point soon, there’s a chance Gareth’s 1971 Yamaha DT360 RT1 might emerge from lockdown, blinking into the sunlight like the rest of us...
THERE’S NO DOUBT Brexit has put me back this month, but finally there’s an end in sight to this project. As anyone who has tried to get anything in and out through Dover recently will know, parts that used to take up to 48 hours are now taking up to three weeks. And, due to all the formfilling, anything I order through CMS in the Netherlands is subject to an 11 euro ‘admin charge’. Worse still, anything that was made outside the EU which has to come into the UK through mainland Europe is now subject to a tariff. This varies by category, but a textile jacket will now be 12% more than it would have been before we jumped ship.
While waiting for bits and bobs to arrive, I decided to completely remove the head and barrel, partly so I could have a good look at the condition of the bore and piston, but also to drill out the damaged exhaust stud. These are notoriously fragile and suffer a lot of damage from heating up and cooling down their entire working lives. The bore was in surprisingly good condition and the marking on top of the piston shows that it has already been rebored to two sizes over standard. The dual piston rings also made me suspicious that my supposed 1971 RT1 MX was actually an RT1B (the MX usually only runs one piston ring). A quick online check with the chassis number confirmed this, although other tell-tale signs like the 360 exhaust should have given this away had I been paying more attention. For racing, this actually might be better in terms of spread of power. The MX engine is known for its peaky delivery – not what you want when leant right over on wet shale. That said, the enduro pipes are very restrictive, so I may have to look at a custom exhaust in the long-term.
While in bits, I took the opportunity to take the head
‘BREXIT HAS PUT ME BACK, BUT THERE’S AN END IN SIGHT’
and barrel to JJ Vapour Blasting in Spalding for a proper clean – Jim recently moved up from the west country and builds engines for racing cars as his day job. A quick blast in his chamber brought them back to ‘like new’ condition, and without any damage, something you can only really achieve with vapour blasting. Now you can even see the original casting marks on the alloy.
Back at my garage, I was now ready to set the ignition timing. The piston was fairly easy to get back into the barrel without bending or damaging the fragile rings. A mark on the new flywheel lines things up nicely against the backing plate and before setting the timing just before top dead centre, I fabricated a wooden bracket for my gauge to screw into and bolted it onto the rods that hold down the cylinder head. By carefully winding back the flywheel nut with a socket, I was able to set it at exactly 2.6mm before ignition. I was very precise here, as any deviation either way could cause poor running, increased fuel consumption and even detonation of the piston.
Using new gaskets (fibre on the bottom, copper on the top) I refitted the cylinder and head. After greasing up the sleeves, I torqued down the head studs. This is very important for decent compression; I’ll re-check all them after the bike has been run up to temperature for the first time.
A few days later, a beautiful pair of shocks arrived. Tony at Alf Hagon – a wealth of advice and useful information – had correctly predicted the original units would be too
short with the profile of the 19in rear wheel. He advised me to create and fit wooden template ‘shock’ to work out the correct length, and it turned out that I needed shocks 20mm longer than standard. He supplied a pair of custom overdamped items with soft springs especially for flat-track use, fully adjustable with 20 damping positions and a three-way collar to control preload. These 2810 series shocks look the part in black, are available for many different applications, and cost just £199 (plus £20 for the custom elements).
The next part of the jigsaw puzzle was get the new wheels to fit centrally in the forks and swingarm. The 140-section rear tyre was just too wide, so I had to change it for a 130, the same width as the front. Using some small stainless steel cylinders and large washers sourced from the internet, I was able to experiment and get the wheels bang in the centre. The only problem is the guide rod for the rear drum brake is very close to the tyre. For now, I’ve bent it around a bit but plan to have a thinner customer item fabricated.
Time for the moment of truth – trying to fire the thing up. I filled the tank with a 40:1 two-stroke pre-mix, after managing to spill plenty of it over the newly-painted tank. No change there then. I have read that, with good quality oil, you can run these engines at less of an oil ratio – but for now I’m going to stick with this ratio.
The RT1 has a decompression lever fitted as standard, but on my bike it had been disconnected long ago. As it happens, this isn’t a big problem and the engine kicks over quite freely. It’s easy to damage the frame and sidestand when starting, so I was careful to kick it over while safely sitting up on a scissor jack stand.
Amazingly, after only five attempts it jumped into life, filling my garage with two-stroke fumes and a loud, highpitched buzz that only an air-cooled ’70s two-stroke can deliver. After all the hard work, I couldn’t quite believe it was finally running – I was beaming from ear to ear.
Initially, and after a bit of fiddling about with the carb, it was ticking over nicely... but then the revs rose sharply and I couldn’t get it to stop. I had to kill the engine, and a friend correctly diagnosed over the phone that there was an air leak on the intake side. These engines have to be completely gas-tight – I’d forgotten to fit the new gaskets on the intake manifold block, so this one certainly wasn’t.
Having got that sorted and taken it for a run, it rides surprisingly well, with no sign of the evil, narrow powerband I expected from an old air-cooled two-stroke. It’s also pretty torquey, but the power runs out quite quickly. I change the front sprocket from a 13-tooth to a 15 to try and smooth this out, but it’s still not revving up properly. I suspected the carb needed a bigger main jet after the removal of the original air box, so I took it to Mark Brewin at BSD to set it up properly on his rolling road. There was good and bad news: he got it running perfectly... which revealed a slipping clutch. The nylon in the original actuator rod had become brittle over the years and completely cracked apart. Mark explained that this was causing the clutch to not completely disengage, meaning the plates would quickly start to slip.
I sourced a new unit and plates from the US online and am currently waiting for them to arrive.
As I write this, I’m booked to ride at the newly re-opened track at Mildenhall in a few days’ time, so I’m keeping everything crossed that they’ll arrive in time!
I may even get to race the thing this summer...
Contacts
• Feners at B&C Express for help with Renthal bars, chain, gearing and sprockets: bandcexpress.co.uk
• Tony at Hagon for expert advice on shocks and how to set up the rear suspension properly: hagon-shocks.co.uk
• JJ Vapour Blasting (Spalding) – find them on Facebook • Howard Carter for 3D printing the replacement parts needed