1979 kz750 twin swing arm swap to ninja 250

Hey guys I'm looking at picking up this bike tomorrow, it's a 1979 kz750 twin that has the front forks and rear swing arm from a 1990 ninja 250. Am I going to have any problems with the 250 chassis handling the extra weight? Is there an upgraded monkshock I could put in to stiffen it up if it starts to sag?

BUMP

it looks like beefier stuff than was original.
a call to
www.racetech.com/page/id/18
or some such should be able to answer any questions you should have about how to dial in the suspension.

leon

Nice looking bike. Id be sure to check over the mods very carefully before commiting yourself.

What should I be checking over? Welds and tightness of swing arm to chassis?

Head is also milled to make higher compression, anything to watch for?

I would look very closely at the welding for the new shock mount and would insure that there is no slop at all in any direction at the swingarm pivot. I would also be curious about the steering stem. Is it a full custom? Is it the original KZ stem? How was it fitted to the Ninja lower triple clamp?

The shock and the forks were barely adequate for a little 250, much less a bike the weight of that KZ. If they were not resprung and revalved for the heavier machine, they really need to be.

Milled the head??? I hope he knew what he was doing, so that you will not experience valve to piston contact at high rpm.

IMO - milling the head without other modifications is a waste of money. Even a large increase in CR results in relatively little gain in power without changes in cams, carbs, exhaust, et cetera.

I am going to take a look at it in person tomorrow and I'll let you know about everything. I know an ex500 rear monoshock will swap right in and that will have a much stiffer spring and will handle the weight much better.

He does have a dyno sheet for 94hp and has to shh premium in it. I don't know these motors well enough to know if that's dangerous or not for piston to valve contact.

TexasKZ wrote: I would look very closely at the welding for the new shock mount and would insure that there is no slop at all in any direction at the swingarm pivot. I would also be curious about the steering stem. Is it a full custom? Is it the original KZ stem? How was it fitted to the Ninja lower triple clamp?

The shock and the forks were barely adequate for a little 250, much less a bike the weight of that KZ. If they were not resprung and revalved for the heavier machine, they really need to be.

Milled the head??? I hope he knew what he was doing, so that you will not experience valve to piston contact at high rpm.

IMO - milling the head without other modifications is a waste of money. Even a large increase in CR results in relatively little gain in power without changes in cams, carbs, exhaust, et cetera.

Before buying it I would take the bike to a dyno and see the numbers for myself. Frankly, 94hp at the rear wheel seems mighty high for an engine that came from the factory with 55hp at the crank (see link). Getting that much power from it would take a heck of a lot more than just bumping up the compression ration by milling the head. It is physically impossible to get 94hp from that engine simply by increasing the compression ratio. Even pushing the compression ratio from the stock 8.5:1 to a whooping 12.0:1 would only bump up the crankshaft HP to 60 (see second link). His HP claim and dyno sheet sound very bogus to me, so I would have to put it on a dyno myself to see the facts. Ed

www.motorcyclespecs.co.za/model/kawasaki/kawasaki_75T%2076.htm

www.bgsoflex.com/crchange.html

94 hp at the wheel is pretty impressive. They were rated at 55 at the crank by the factory, so there are quite a few mods in there. I'd for sure ask about them out of curiosity if nothing else. If you have a friend who is knowledgeable about bikes and that you trust, take him or her along. You may have stumbled across a nice build. Ii hope so. A stripped down, updated, powerful 750 twin should be a blast to ride.

Dang 650Ed, you is too quick with the typing so early in the day :silly: