Engine differences through the years?

[Ilovemountains]

I just got home from a road trip, I went to pick up an engine, carbs and a few boxes of miscellaneous parts from a guy that I won on Ebay. The bike he was parting was a 1978. Does anyone know why Yamaha changed the cylinder and head shape from the 1977 model? My 77’s top end is square, where as the 78 that I picked up today is much more rounded from the fins to the shape of the head. Any ideas? Any known interchangeability issues?

 

[Ilovemountains]

No one on here knows why they changed out the top end? Was it for the "look" of the cruiser version or was there some real science behind it? I only ask because I now have one of each engine, my frame and swing arm are at the powder coaters so I’m about a week away from bolting one or the other into the frame, if the early engine "77" stock is better I’m going with it, if the so called "79" is better I might throw it in the frame. Anyone with any real history knowledge of this??? anyone?

 

[bcware]

There should not be any difference (macroscopically) between a '77 and a '78. Later models have electronic ignition, but even then you'll only see a different "points" cover on the top end; other than that they look the same from the exterior.

Furthermore, you can google '77 and '78 xs400s. I do not see any difference in the photos I found of '77s (I have a '78). Make sure you're getting what you think you are. The best bet would be to get us a photo.

 

[drewpy]

the shape was mainly cosmetic, I liked the square shed ones on the roadsters as it reminded me of the laverda's. The rounded shape in the UK were used in "custom" styled bikes.

Yam changed to partial rubber mounts in '79ish, but that was a frame mod not and engine one.

 

[Ilovemountains]

I bet the spare engine and carbs etc. that I bought that was supposed to be a 78 was probably a 79 with a frame date in 78. I dont have any pictures of it but here is a link to one that matches my 77's engine:

http://www.ebay.com/itm/77-XS400D-X...Parts_Accessories&hash=item33680e1abe&vxp=mtr

This one looks just like the one I picked up Tuesday, the seller says its a 1979 to that makes sense.

http://www.ebay.com/itm/Yamaha-XS40...ies&fits=Model:XS&hash=item5d365ba991&vxp=mtr

I wonder if the 77-78 engines cool better with the larger, more squared off top end. I got the engine, carbs, 2 large boxes of parts including the bars with all of the controls, the complete wiring harness etc all for $180 cash so I think I did OK! I dont need the other engine but to have it for spares like the starter and starter clutch etc its good to have! I just picked up a new old stock wiring harness and my frame it at the powder coaters, Ill get some pictures up next week when I get it back! Matt

 

[Scorpio1963]

I wasnt realy sure what the jug shape on my bike looked like till today.So thinking of this thread I went and looked and they are roundish.I was expecting them to more square than round.