Swapping xj650 with xj700

Hello guys.

I have an xj650 and my piston rings gave up and its too expensive to fix unfortunately…
now im looking for a new engine but there is mo low milages 650 engine available, but i came across a really good looking xj700 engine.
I’ve looked on the forum and saw a threat about engine swaps, but i actually wanted to know if some one on here has actually swapped a xj650 engine for a xj700 engine?

is it an instant swap, or is it also necessary To change to xj700’s driveshaft, exhaust, ecu, carburator etc? The engine i saw doesnt come with a starter motor and alternator, are those also interchangeable from the xj650?

i’ve been building this bobber from scratch for three years along, and this would be the final touch!

 

As long as it’s an air cooled 700 it will fit, the carbs are the same, although jets differ, the engine is the same size. The tci advance is different I believe, but probably not much. So yes, slip it in there, check out the jets and change as required.

 

If you have a non-YICS 650 it has a mechanical tach drive off the exhaust camshaft.
If you have a mechanical tach it won't work with a 700 engine.

 

Carbs are different as well. 650s use the Hitachi HSC32, whereas (air cooled) 700s use the HSC33.

 

Isn't that odd, the 750s use the hsc32, wonder why?

 

I think the key is to remember the motivation for creating the 700cc version in the first place - the "Harley Tariff" here in the states. This was a protectionist tariff on bikes over 700cc coming into the US that went into effect in 1983.

So Yamaha did what any rational person would do and made a slightly-less-than-700cc engine that was better than previous 750cc engines. I suspect the "upgrade" to the HSC33 carbs was part of the "tariff buster" design to make the 700cc engine a good replacement for 750cc.

 

So the 700 with the 32 mm carbs should have at least the performance of the 650..
However, it does leave the question - what jets do they need in them? Maybe go with the ones fitted to the 750?

 

It has better than the performance of the 750. The jet sizes for the 700 are well documented. IMO there's no need to use 32mm carbs on one. The bore and stroke are different.

 

The 700 has same bore as 750 and same stroke as 650.
It has higher compression than the 650/750

 

Yees but, the OP doesn’t have a set of carbs for the 700 engine, hence using the 650 carbs. Which should at least give adequate performance, given they are big enough for a 750. Which just leaves the choice of suitable jets.
Personally I would go for the 750 jet set, safer to be rich...

 

Agreed.