XJ900

I’ve been thinking of all the things Yamaha did to enlarge the XJ650 into the 900.
The stroke is 8mm longer so most think the block would be 4 mm taller. It’s not actually just 1 mm taller. The difference is in the conn rods. 900 rods are shorter so this is how they keep the same basic block height.
Shorter rods really add stress to the piston crank assembly at high RPMs.
The Yamaha engineers then put the big end of the connecting rods together with the bolts upside down so it wouldn’t foul inside the 650 cases. Does anyone know if this is still done today?
It’s like the engineers were challenged to make the biggest motor they could from the XJ650. They gave them one concession though as the valve cover is longer to accommodate 4 supports for the camshafts.
This was 1983 and Yamaha was still a year from releasing the FJ1100.
Funny that it was targeted at the market for 1 year but continued in Europe as the 891 for a decade longer.
In Canada and I think Australia 1984 the xJ750RL came out. Frame mounted fairing but looked to be the same bike just with the smaller motor. Maybe with a 750 Yamaha made it obvious it wasn’t supposed to be the fastest, just a good all-around sport tourer.

 

The 900 f became the 900s, with a revised if not totally new "sloping" engine, still with the old hat 2 valve asthmatic head. I would guess the rods would have got the bolts reversed by then?
But, ultimately, the 650 type engine served well - it was reliable, adaptable and easy (ish) to maintain.
Its a shame they never made a 16V head for it...

 

I rented an XJ900 Diversion for a ride through Norway, 5,000 kms total. The bike did everything well, somehow the marketing guys didn’t think it would sell in N.America.
Everyone wanted either a sport bike or a cruiser. Too bad.

 

I think that in Norte Americano there was the "protect Harley Davidson tariff" that came into being around that time, so the 900 was dropped in the USA (and Canada was probably not a large enough market to support that model by itself), and then it never made it back after the tariff was (rather quickly) lifted, and the FJ's, etc. took over. In the rest of the world, the marketing and distribution and service etc. was already rolling along, and they must have sold fairly well, as the 900's hung around for a long time everywhere else.

Yamaha should have brought it back to North America, the 900 is probably the best all-around model of the series (followed by the 650RJ and maybe the 750RL, but we never got the "mini-900" here in the USA, so I've never ridden one to say for sure, but a girl can dream.........). But I guess they were too excited/pressured by the changing styles, and we know the rest of the story.

Given Yamaha's proclivity to try and mix-and-match frames and engines (750A, D, E, etc.) in those years, I think the marketing guys were actually pushed into trying to hawk whatever the engineering people wet-dreamed about and tinkered with, seeing what they could make "stick". Yamaha, in those days, always seemed to me to be an engineering-driven company, and the marketers were just along for the ride and trying to keep up (and make up) reasons to try and peddle the latest "R&D/parts bin" bikes that engineering/design came up with. They were also obsessed as unseating Honda as the #1 producer of motorcycles, and got their tits squeezed in the wringer for it, over-extending themselves wildly.....and it was at that time that I think the marketing guys (and probably the green-eyeshade folks) "took over" the direction of the company, for better or for worse.

Don't get me wrong, I think that overall they are a great company, but all long-lived organizations goes thru evolutions over time as to who (internally) is really calling the shots.

 

So theroetically uf the rods are shorter on the 900 they could be used in combination with shortening the 900 cylinder on a 650 engine in order to make vertical clearance?

 

I'm not sure I understand your comment.
If you install 900 pistons & block on the 650 crank & rod assembly you still need to shorten the cylinder block to gain compression back.
If you install 900 pistons, rods and block on a 650 crank you will need to shorten the block even more.
Note however much you shorten the cylinder block, let's say it is 4 mm, you will have 8 mm excess in your cam chain.
That would seem to be a lot of slack to take up with the tensioner.
Also note that a longer rod makes more torque with the same piston force, and since it's less angular than a shorter rod, reduces sidewall loading and decreases friction.
All of this adds up to more power.
So if you're contemplating a big bore 650, best to use 650 rods.