Yamaflop

[Lozza]

All depends on who leads development. Yamaha quietly abandoned 5 valves per cylinder after Furusawa-san built a 4 valve cross plane crank engine after Valentino Rossi said that was the engine he wanted to race with. Yamaha used 5 valves on everything since the mid 80's clung to that like a limpet when nobody else in the automotive world used 5 valves.

[rocketfrog]

1983 YZ125K - Rocketship. I cannot understand how Yamaha got it so wrong in such a short step. I cant say a bad word about the 83 YZ125, such a huge leap from the 82 model which was way too heavy for a 125 and all top heavy to boot.

The 84 model would have been a better bike if Yamaha simply improved the ergos on the K (slimmer tank like the 84/85) and better airbox access like the the L and N models.

[djr]

All depends on who leads development. Yamaha quietly abandoned 5 valves per cylinder after Furusawa-san built a 4 valve cross plane crank engine after Valentino Rossi said that was the engine he wanted to race with. Yamaha used 5 valves on everything since the mid 80's clung to that like a limpet when nobody else in the automotive world used 5 valves.

I had an Audi car with 5 valves per cylinder - it didn't seem to go any faster than the previous 4 valve model I had,
I believe Ferrari also built some 5 valve engines for a while, but I don't think either manufacturer builds any today,
an interesting idea , but probably not worth the extra cost

[Lozza]

I had an Audi car with 5 valves per cylinder - it didn't seem to go any faster than the previous 4 valve model I had,
I believe Ferrari also built some 5 valve engines for a while, but I don't think either manufacturer builds any today,
an interesting idea , but probably not worth the extra cost

Shit eh there you go never knew that someone else has tried it. A synopsis I read was about the angle on the rear valve and inlet ports were compromised also some sort of frictional losses from the extra springs

[Nathan S]

The big problem with 5V heads is that the flow around the intake valves interferes with each other, so they don't flow any/noticeably more than a good 4V head - a lot of extra complexity for no real gain (apart from in the sales brochure).

There was a version of  Toyota's venerable 4AGE that came with a 5V/cylinder head. It made more power than the older 4V version ... because the 5V one was delivered in a much higher state of tune (cams, individual throttle bodies, etc). For a serious race motor, the 16V version beats the 20V hands down.

[Lozza]

That head was done by Yamaha as there was some corporate tie up at the time

[djr]

That head was done by Yamaha as there was some corporate tie up at the time

Yamaha have been building some  engines for Toyota on and off since the late 1960s,
I read somewhere that they had built 4 stroke engines for Toyota before they had actually built there first 4 stroke bike engine, and supposedly there are some small parts in an early XS1 / XS650 engine which come from an engine they were making for Toyota.
Yamaha are now involved in some of the Ford V8s, cant remember if they make the whole engine or just the castings.

[skypig]

Apparently the head on the Toyota (2ZZ) motor in the Lotus Exige (before they went to a V6) and Elise is made by (designed by?) Yamaha.

Certainly the Yamaha 5v four strokes were powerful and long lived.
I've had an R1, and 2 250Fs, still have one of the 250fs.

As others have said: there doesn't seem to have been a performance advantage to 5 valves, and definitely extra complexity and cost.

[alexbrown64]

I love it when these VMX YZ125 posts come up.  I raced the F, G, J and K 125's and the K is still the best bike i have ever ridden.  And that includes years of MX on CRf450's, YZ250f's, SXF350's and the like. 
The K was very light weight, handled brilliantly, was reasonably fast, very reliable, and confidence inspiring.
In fact, i have two now.... and they have only just been finished off. 
These will be heading to the dunes, bush and tracks very soon...







Cheers,

[80-85 husky]

once Yamaha rejigged "monoshock" to a more conventional design, they really picked up their act

[skypig]

Those are some nice YZs Alex

They are '83 K models?
The article is slagging '84 L models. (As being much slower than the previous year)

Do I have that right?

[James Lee]

That yellow and white K looks so good I can't stop looking.Nice job on both,stunning bikes.

[alexbrown64]

Yes, the bikes went backwards for a few years after 1983..  The 84's looked good on paper with bigger forks, DLS brakes etc.. but ended up slower and less reliable.  Shame really.  Both my white and yellow K's are 83, but the U.S one has a lot of fruit on it including DLS brakes, larger carb, porting, PVL ignition, Jemco pipe, RaceTech emulators, gold valves, springs and valving, Takasago rims, aftermarket rads and silicone hoses etc..

[James Lee]

te author=alexbrown64 link=topic=40822.msg390384#msg390384 date=1471174786]
Yes, the bikes went backwards for a few years after 1983..  The 84's looked good on paper with bigger forks, DLS brakes etc.. but ended up slower and less reliable.  Shame really.  Both my white and yellow K's are 83, but the U.S one has a lot of fruit on it including DLS brakes, larger carb, porting, PVL ignition, Jemco pipe, RaceTech emulators, gold valves, springs and valving, Takasago rims, aftermarket rads and silicone hoses etc..
[/quote]
Are they DG rad shrouds and side covers?someone had some for sale on facebook but wouldn't list prices.The white looks really good with the yellow.
Is it just the 84 125 that was a flop?my brother bought a 250 new in 84 and still says it was the best 250 ever,he still talks up the double leading front brakes being better than disk,I just think he's stuck in the 80s.

[Nathan S]

The YZ250L is/was a great bike.

The same happened in the early 90s, where the YZ125 was a dog but the YZ250 was one of the best.