XS650: Inbuilt Limitations   Leave a comment

When you talk with people who play with these engines, they will mostly tell you how solid they are. How destressed and strong they are built.

To a degree they’re correct.

This was the first 4 stroke motor built by Yamaha. So they were careful. Didn’t want expensive mistakes. The heritage was of course German. It was pretty solid to begin with. Add a little nervous over-engineering and you get the XS1. Built as a street engine. It didn’t have years of racing development behind it.

Many of the ideas were reworked into their second 4 stroke. The XT500. And look where the French took that. Bet Yamaha considered making Olivier an honoraray Japanese.

The fascination for Motocross and Enduro killed Yamahas’ interest in Flattracking while they were at the top. A real shame for Shell Thuet. Also for Bob Trigg.

The XS1 was immediately improved. Clutch. Forks. Crank. Brakes. Later, Percy Tait showed how to improve the frame geometry.

With some improvements the motor basically stayed the same for years.

XS1s make good restoration projects, although the heads breathe somewhat better. 447s make good café racer bases. And Specials lend themselves, with their layed forward rear geometry, to choppers and bobbers. Everyone’s happy.

Restorers will debate the interchangeability and logic of XS1 headlight bracketry or fork internals vs XS1B or XS1F and XS2. Corner carvers will relish the Standard frames improvements. Commuters the electric start. Easy riders the lower-slung specials. Sidecrossers and hillclimbers the simplicity and grunt.

They will all see strengths and weaknesses depending on the design problems they meet and the level of customising they do.

The further you go the closer to the limitations you come. The machines’. The machinists’. The mechanics’. The riders’. And your pockets’.

The problem is that, unlike global finance, the trickle down effect actually works in the real world of mechanics. Any changes you make here directly effect how that there works. Every action has an opposite and equal reaction.

In reality the reaction can be plural and the effects cummulativly overwhelming.

Means, you start playing with the motor, increasing the cubes, playing with the squish zone, allowing her to breathe, changing the timing and you begin experiencing the limitations.

Same with any system in balance.

Talk to the big cube and torque blokes and they tell you head design and the crank centre pin are the ultimate limiting factors. No matter what you do to the rest-it’s that pin that goes.  Some talk about converting the cases to take conventional bearings and running high pressure oiling. Or machining 1 piece auto style cranks and 2 piece conrods.

It’s not going to change the fact that they simply don’t like being shifted over 7500. The little end stretches. Any piston pin play and you have problems.

Fine. What gives next? Cylinder stud anchor points in the crankcase. Aluminium.

Liner thicknesses. Pistons.

Increase cubic capacity? RPM? You’re talking air. Has to come from somewhere. And go there too. Carburetion. Exhaust. Only as good as the head.

Also here the talk is porting. Cylinder centres and stud patterns. Machining existing DOHC heads to fit? Or making your own.

Got good flow? How to get it to the rear wheel? In a way you can control it?


1 .. 8 valve head ... wim mellendijk

8 valve head … wim mellendijk



And frame geometry.

None of this is new. Each pushes the envelope. Each tests the limits of it’s partners in crime. All is linked…ommmmmmm

Bud Aksland talked about..

‘…rephasing cranks, different firing orders, hundreds of valve, cam piston crown profiles, welded up ports,…the list was endless…the basic … was a stock set up with a weighted crank…the stock head casting was the limiting factor…is why the OU-72 was commissioned…purpose-designed casting with more favourable valve angles etc.’

‘…it is an engine with built-in performance limitations and if you are going to be successful in your attempt to extract more user-friendly power, it will have to be done with a systematic, integrated manner.’

Kenny Roberts talks about it too.

‘…Accepting the deal from Yamaha meant we had good roadracing bikes —real roadracers built by Yamaha.

But the dirttrack bikes weren’t so good. We had to make do with Yamaha’s XS650 twin, a steetbike, as the basis for a dirttrack racer…

…we struggled with the XS650 that first year. Still, I earned enough points that year to move from junior to expert.

In our struggles with the XS650, we too often resorted to using pistons from here and a cam from there and these valves trying to make the bike fast. We also managed to blow it up a lot!

…in ’72 Yamaha decided to get serious and added Shell Thuet to the team. Shell was an established motorcycle dealer in Los Angeles; he was very experienced, and he knew how to build a racebike. Like all good tuners, Shell knew how to make all the parts work together. He made sure to build reliability in before trying to build horsepower. Some things never change. This is still the best approach to building a competitive racebike’

Posted January 24, 2011 by xscafe in Frame - Handling, Motor - Breathing, Motor - Head

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