YAMAHA RD350
Scoop has a fettle and a fiddle.
The RD350 has proved to be a hugely enjoyable bike for which I am eternally thankful. I state what many may think is blindingly obvious simply because there are some bikes that you fettle, repair and nurture for ages only to find on that very first ride it just doesn’t do it for you. It simply doesn’t float your boat, flick your switches, move to your groove or rock your world. Fortunately in this instance at least nostalgia is exactly what it used to be. The Yamaha has proven to be flickable around country lanes, more than adequate on fast A-roads and a pleasure to ride in urban areas… and not many older two-strokes are quite so all-round complaint in my experience. The bike will happily hurtle up to and beyond the legal UK maximum and stop pretty sharpish as well, despite its Jurassic series brakes. With the almost 40 horses packed into a 250 chassis it still makes for a fairly potent piece of tackle and occasionally catches its ageing owner out. Hurtling around the back roads of rural Bedfordshire we recently crested a bridge at what’s best described as indecent haste only to find we had somehow landed in the middle of a veteran and vintage ride out. The good old boys on their good old bikes seemed a little less than chuffed to find a ring-ding within their midst and a Japanese one to boot. We parted company and for once it wasn’t the RD that made the most smoke! It’s a grand bike but I’m not blind to its shortcomings as evidenced by the upgraded ignition from Power Dynamo and the decent shocks from TEC Bike Parts. One area where there’s a need for some creative input is within the cylinder head shape and this is a subject I’ll cover in more detail at another time. However, a potential weakness of the bike is the use of fire rings as head gaskets. This was a period Yamaha foible (and possibly others) whereby the head bites down solely on a thin band of angled copper which is retained within a cast lip on the top of the barrel. Back in the day the early air-cooled RDS acquired an unnecessary reputation for cracking cylinder heads. The prime cause was simply operator error; over enthusiastic tightening saw the heads pulled down too tight and deformed beyond the horizontal. If full head gaskets had been used instead of fire rings it would have been impossible to distort the alloy head casting. The recognised cure is to fit RD400 full gaskets but this requires some serious machining of the barrel’s head joint. Fortunately those lovely guys at Spa Motorcycles of Cheltenham can help out. Using the milling machine of their neighbours (Race Techniques) the fire ring retainer can be machined off to allow a proper head gasket to be used. A look at the two Yamaha options (see pic overleaf) shows the differences. While we’re fettling and with the top-end off, let’s have a quick look-see at the bottom-end of the motor.
Thankfully it all looks rather good and unmolested (see pic, right). With no discernible slop or free movement, people much smarter than me conclude the bike’s mileage looks to be genuine. A set of fresh little-ends and new circlips are installed for good measure while everything is apart; it’s almost rude not to and it’s good insurance as well. One of the areas where Yamaha really compromised was the level finish applied to the pistons. While they’re dimensionally accurate and made of decent, seizure resistant alloy they generally have very sharp inner edges to port windows, cut outs and the side skirts. You might not think this is a big deal until you grasp such surfaces as stress promoters and potentially engine wreckers. Back in the 70s more than a few production racers found their engine letting go in a big way but struggled to find the cause. The key symptom, often totally overlooked, was lumps of piston skirt everywhere. Looking at this crack on a Yamaha 200 piston (below) you can see just how bad this effect can be. Know that similar fissures can work their way to the gudgeon pin area and guess what happens next boys and girls? To negate any likelihood in my motor, chief techno chap Rob Pemberton fettles every single sharp edge off the RD’S pistons with a selection of needle file (below right). Against expectation and form, yours truly rolls up his sleeves in a real bike workshop and cleans up the piston, which bears some 7000 miles’ worth of carbon. The original Dykes piston rings are still within specification and are reinstalled. The entire RD twin series remained almost to the end very much “work in progress” with Yamaha introducing hard won technology from both the track and R&D each successive model year. The later coffin tanked RDS benefited from a link or balance pipe which ran on beyond the RDS through the LCS and on to the Power Valves. Depending on who you listen to and what you believe the pipe between the
two inlets smooths out power pulses, equalises inlet vacuums, enhances power or amplifies torque. Personally I subscribe to the torque theory and if the link pipe was okay for Yamaha it’s fine by me so I’ll have one please. Obviously it’s not a standard fitting on a ’73 model but I’m all for making the best of what I have. Tour-max markets a set of inlet rubbers (pictured above) that take a larger bore balance pipe for the RD400 and fortuitously their kit also fits RD350S. It’s a simple and convenient swap that also delivers supple inlet rubbers… result. Final slice of techno is the installation of a KOSO head temperature sensor system (pictured below) which sounds horrendously technical but, apparently, even I can fit one! The idea is that two-strokes can run hot under the wrong circumstances, crap fuel, blocked jets etc. which may lead to a potentially expensive blow-up. By keeping an eye on the head temperatures you should be able to stop the damage before it happens. Better still, you can instantly see what effect changes to jetting, ignition timing air filters and the like make to a motor. The pickup is a ring that takes the place of the washer on the spark plug with the unit itself mounted up on the bar clamp via a convenient bracket. A small amount of wiring is needed in the headlamp bowl to supply a power feed and hey presto you have a read out of maximum temperature of either head plus lots of other data, pre-sets, warnings and other useful info. You can tell what an engine is doing without an infinite number of plug chops, know if it’s the main jet, nozzle or needle that needs changing and lord knows what else. You might think it’s technology for its own sake until you’ve used one; trust me it’s a sensible investment. So quite a lot has been going on with the RD350 and it’s not finished yet. There’s a few more considered upgrades I want to investigate and although I’ve no desire to chase performance for its own sake I’d like to maximise what’s there along with making the bike as modern day relevant as possible. It won’t ever be a show bike for sure but it will be a proper rider’s RD350 by the time I’ve finished.