72 or 78 xs650 motor option?

Flash1977

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Hey folks, I'm new here and have a question. I am building a xs650 hardtail and have a choice between a 72 and a 78 motor. The bike is the 78, but I believe the non titled xs2 I have has a better motor. My question is, which is a better motor.? Or what are the pro and cons . The 72 has the pressure release. I don't know if that's a pro. There the nostalgia of the 72. Keep in mind that both motor may need work and in the mean time I'm still looking for a know good engine I can buy cheep. I'm just sick of waiting for the right bike . I have a titled 78 which I believe is a good platform from what I've read so I'm gonna start there . Let me know your thoughts. And or any info I could add to aid your opinion. Thanks
 

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Jan_P

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Some tests is possible Boroscope into cylinders
Test to kick it get a feel for compression
Trying to shift gears
The older looks better exterior can save time polishing
Have Carburetors but perhaps can be moved

And the older looks as a good starting point for a restore / rebuild I would hesitate to split it.
We are talking 50 years old
Is it the right frame ?

But otherwise I agree on the later being better choice.
 

WideAWAKE

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Based solely on the looks of those motors I wouldn’t run either without pulling them apart.

I’d start by pulling the 78 apart. If it was solid, use it.

If it’s tore up, pull the 72 apart and see if it’s worth the money it needs to rebuild.

Parts are easier to come by on the 78 so even if it needs more to get it going, quite possibly cheaper and faster to build in the long run.

The upside to the 72 is that they are a more aggressive motor. I love my 72 (thanks mailman haha) and would take its ride over my later bikes any day of the week. A little under gunned for highway riding at any real distance in my opinion. Likes to sit in the pocket about 60.
 
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Flash1977

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Some tests is possible Boroscope into cylinders
Test to kick it get a feel for compression
Trying to shift gears
The older looks better exterior can save time polishing
Have Carburetors but perhaps can be moved

And the older looks as a good starting point for a restore / rebuild I would hesitate to split it.
We are talking 50 years old
Is it the right frame ?

But otherwise I agree on the later being better choice.
Yes but the xs2 doesn't have a title . So I have been parting it out. I would trade the frame and engine for a known good engine.
 

Flash1977

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Based solely on the looks of those motors I wouldn’t run either without pulling them apart.

I’d start by pulling the 78 apart. If it was solid, use it.

If it’s tore up, pull the 72 apart and see if it’s worth the money it needs to rebuild.

Parts are easier to come by on the 78 so even if it needs more to get it going, quite possibly cheaper and faster to build in the long run.

The upside to the 72 is that they are a more aggressive motor. I love my 72 (thanks mailman haha) and would take its ride over my later bikes any day of the week. A little under gunned for highway riding at any real distance in my opinion. Likes to sit in the pocket about 60.
Thanks for the info . There will definitely have to be some work done to use either which is why I'm trying to which one to put the time and money into, though I'm still Holden out for the possibility of finding an affordable known good engine somewhere up by western NewYork
 

Flash1977

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The 78 engine is the better choice. Mostly for parts availability. I believe the later engine has more durable connecting rods.
You can sell or trade for more with the earlier engine. Someone needs it.
That's what I was thinking and still kinda hopen I find that right individual that's looking to trade or purchase the 72
 

5twins

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In New York, I don't think you need a title until '73? On earlier bikes, you just use the old registration (sign it over to new owner).
 

Flash1977

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It said I can't use the hhb top motor mount with the 72 , but I think as long as I use the 78 frame it should be fine.
 
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