Gp 643

[Gorkon]

Does anyone have any information on Yamaha's first fan cooled big bore two stroke? Were these just bigger jugs on the 443 case, or were they a completely different animal?

The reason that I'm asking is, I may have a bead on picking up one of these engines and am curious as to what I can do with it. I'm probably going to grab it regardless just because of its rarity, I don't think they sold a whole lot of GP 643's.

I have a few ideas but know so little about these engines, i.e. Differences between 72 and 73? What kind of power will they make? Will one bolt in, in place of a 433?

How about it, anybody have extensive knowledge about these fan cooled 643's? G.

 

[Gorkon]

No info?

Wow, no one has any info on these 643’s? I’ve been poking around the net and info on them is pretty scarce. Almost nothing on the fans, lots more on SR’s.

Something about those first 643’s really has my attention lately for some reason. Considering it being the 72 world champion and the fans only being in the lineup for two seasons!

Anyhow, digging through things lately, and found my 72, 73, 74 and 97 brochures. We were looking at the GP’s and started discussing the sled. Before you knew it, somebody said, ‘Wouldn’t it be cool if….” And the bench racing was on.

Next thing you know, this thing’s got a modern aluminum chassis, with custom fiberglass belly pan, so that big twin can sit down low and take advantage of all the newest clutching. A trailing arm front suspension with Ohlins with modern aluminum ski’s, a 30 lb, Wahl brothers racing suspension with billet wheels, able to take advantage of the best modern tracks, CDI ignition, custom made, ceramic coated, twin pipes with aluminum silencers, titanium trailing arms, (Did I mention we were drinking beer too?) some modern porting and racked flat-slides, center mounted tank with an SR seat, and some V-Max-4 scoops for good measure and cooling too, whew!

Then we got the goofy idea of building this thing as a tribute sled to the 72 Championship, even made a logo, sort of. We got pretty carried away, and being in a room with some of them crazy “vintage guys”, I think I even heard some heavy breathing!?

Anyway, after we got into the “fat chance of that happening”, for awhile, coming up with all the excuses why it couldn’t/wouldn’t happen, I thought, it’s not really a bad idea, and not hard to do really. After all, we have the technology!

So we did some crude drawings which I’m posting here to maybe generate some interest and have some fun with it. Pretty cool fantasy sled. In reality, it may be easier just to use a 97 up trailing arm chassis and fabricate the belly pan and use an original hood to fab up a mold to incorporate some hidden hinges etc. It would be fun. Any other ideas?

Still hoping someone will come forward with some 643 info. Enjoy. G.

 

[RICKYAMAHA]

Hey Gordon I found that the EW643B had 42 hp @5500 rpm and the GP had 50 hp @6000. Also the carb on the EW is the same as on the SR witch has 2.
Interresting!! All the 643s were rolling 18 inches tracks in 72 and 73 except the 73 SR 643B witch had a 15 inches.
Richard

 

[Gorkon]

Yes, thanks for that, however, that much I knew except for the carb size. Also, the 72 GP 643 made 43 HP at 5500 rpm as opposed the 73's 50 @ 6000.

I am curious though as to how much power a guy could pull out of one these 643 fans and if one would bolt in place of a 433? I have found that the free airs are capable of some awesome numbers for an engine that old, but finding info on the fans is like searching for hens teeth!

I know they didn't sell a lot of them compared to the 433's, but surely someone has to have one and have played with it? G.