TOPIC:

I started posting my pictures in the newbie section as part of my introduction and show how I'm tearing down a 1981 KZ750 LTD that I picked up for $150.

In one of the pics, there is a hole in the top of the crankcase. Some members have suggested (and I agree) that a connecting rod has broken and done the damage.

Before I began tearing down the bike, I tried starting it. No-go. I could hear the starter click, but nothing happened. The fuel from the petcock to the carbs was good, the battery had enough charge, and I couldn't turn the engine by hand. I stopped there and surmised (based upon the hole) that a rod had broken, which is preventing the engine from turning.

One proposed fix was to buy a top from eBay. Although I've seen them being sold, I have read (and a KZRider member iterated) that the top and bottom are machined as one, therefore the two should not be mixed and matched. Another suggested fix was a JB weld. I know nothing about welding, but this seems like a reasonable alternative to simply scrapping the engine.

I am not too concerned about cost, because I began this as a lesson in engine mechanics for myself. However, if it seems hopeless, I'll go looking for a working engine.



Post edited by: vakzbikeguy, at: 2006/03/28 14:32

if a connecting rod broke i would replace the motor. it more than likely did damage to the cylinder and who knows if the piston that is no longer connected bent any valves. and the grenaded parts will scatter in the rest of the motor

Just my $0.02 worth but I would have thought a broken rod would have done much more damage and not right where you circle the hole?

Could it be the oil pump has self destructed and blown it's cookies through the top of the case?

In my experience, mixing and matching bottom and top end s on all motors (including my KZ650) is totally fine. Folks put 440 top ends on 400's and 750 top ends on 650's and those aren't even the same motor to begin with so I think you'd be fine.

But it sounds like your 750 may have problems in both the top and bottom end. While its possible to fix anything, you'd be suprised how cheap motors are (especially bottom-end's that tend to be more bullet-proof). I've paid 100-200 dollars for good used complete stock motors that where mechanically sound. You're mileage may vary, but I would try to find yourself a low mileage bottom end at least as a start if not a whole low mileage motor. They are out there, ebay, the classifieds, and junkers (especially junkers) are good spots to check. Worst case scenario I could pick up a reasonable motor from a Junker out in Kalamazoo, Mi, and send it to you (but between my traveling expenses and shipping that would be pricey.

You can also pick up a whole parts bike.
NBK

I'm hopefully going to have time tonight to perform an autopsy on the engine. I'll be sure to take pictures so that you guys can weigh in on the diagnosis. In the mean time, I'm starting to look for other motors just in case.

Dont rule out looking for another 750 that you can get the whole thing as a parts bike. I bought 3/4 of a 80 750LTD from a junkyard for $100 last year.

BTW did you get a title, or whatever paperwork is required in your state, with your bike?

You can mix and match top, bottom and inbetween ends. Many KZ parts are interchangeable. You may get lucky and have minimal damage, then again maybe not. Strip motor completely even down to taking out tranny. Get a shop manual for your bike (Clymer or factory). Your going to have to measure and inspect a lot of pieces. Pay attention to the crank, crank bearings, pistons, cylinder walls and topend ((for bent valves)(if rod broke with piston at top, piston to valve contact may have occured)). Put some thought into why rod broke (if that is case). You will have to dissassemble motor to make educated guess as to why motor grenaded. E-Bay and wreckers are good sources of cheap parts. Also look at classified section here.

DanOz7Five0 wrote:

Dont rule out looking for another 750 that you can get the whole thing as a parts bike. I bought 3/4 of a 80 750LTD from a junkyard for $100 last year.

BTW did you get a title, or whatever paperwork is required in your state, with your bike?

Yes, both my frame and engine are titled.

The discussion was about getting a new upper crankcase half and putting it on the original bottom half, not replacing the top end

Oil pump on the 750 is on the right side, not the left, nothing located there to make that hole except a broken rod.

The Factory Service manual will tell you that if one half of the case needs replaced, then both must be replaced as a set, and here's the reason why.

The crankshaft main bearing caps are machined directly into the cases, there are no separate caps, and they are machined in a crankcase set. So when the cases are clamped together, they set the main bearing clearance. Now given a large enough supply of cases, the proper measuring tools, and enough time, you may very well be able to find another bottom half to fit, but IMHO that's a serious waste of time when used motors are available for $250 - $350.

It's the same as telling someone they can mix and match main and rod caps, not a good idea.

And before someone mentions line boring, think about the other shafts that are located between the case halfs.

Post edited by: steell, at: 2006/03/28 18:45

VAkzbikeguy wrote:

DanOz7Five0 wrote:


BTW did you get a title, or whatever paperwork is required in your state, with your bike?

Yes, both my frame and engine are titled.[/quote]

Then you have something worth working on, not that no title means its useless, just another hoop to jump through. Many of these bikes dont have titles anymore, if that was the case Id start shopping to start over, but IMO a title on a frame (motor numbers may be listed in some states, in WI it doesnt show the motor on the title) is worth alot to begin with, and makes or breaks a project. Good luck and keep us posted.

I dunno about the direction of this thread... If I had an engine with a hole in the case, I would not even bother trying to start it but would have the cases split within a couple of hours. The hole doesn't appear to be in the location of a rod as it looks (from the small picture to be outside where #1 rod would be). Second, I would check the crank for staightness. I use a 17mm drill bit blank which is super straight and fit it through the rod ends. If it goes through, the crank is straight. Split the cases before proceeding and check things. You certainly can't mix and match case halves as Steve pointed out. You will chew up your bearings in no time. Replace the cam chain and if there is no damage to the crank and all is straight, fix the hole and rebuild the top end.

VA...can You post a bigger picture of that "Hole"
The one You have on here is pretty small...