1981 Kawasaki kz750

This question probably won't get answered but what the hell, here it goes:

My brother has a 1981 kawa kz750 which smokes excessively out of one of the pipes. There is also a general loss of power. I think the problem has got to be shot piston rings. I know that this is a definitely a big job and it is probably over our heads. My question is: is it possible to fix a problem like this on my own if I educate myself like crazy, or should I bring it to a shop and have them fix it, or is it not worth it to fix it at all? How much would a shop generally charge to fix something like this? Let me just say that everything else on the bike is mint. Absolutely no rust, custom pipes, no scraches anywhere, nice paint. It really is a beautiful bike, if only the engine worked! It would be such a shame if the bike would have to be parted or sold away. He bought it for a cheap price too. Let me know what you think.

Also, if you think I could fix it, what would be the first steps? I know that I would first have to check the compression on the cyclinders to see which piston is messed up. I understand the theory behind it (of at least I think I do) but I'm not sure how practical I'm being. I'm probably missing some steps along the way. Any info would be great.

Thanks.

 

Pull the plugs... if you find one soaked with crap then that's where you start

Compression test isn't hard to do... I'd suggest that.

When did it start smoking?

Piston rings could do this - and probably is the cause... however I suppose it could be a wildly out carb... or a pipe full of water ( hey it has happened)

 

It started smoking right at the end of Fall on a short trip around town- nothing out of the ordinary happened to seem to cause it. First my brother noticed a pretty big decrease in power. He parked the bike and sure enough the next time he started it, it was smoking like crazy. He thought that maybe there was just excess oil burning off so he rode it up and down the street waiting for it to stop, but of course it never did. Come to think of it, oil was even dripping out of the pipe- that's why it seemed so likely to be bad rings. He started the bike a number of times after that and it would start smoking immediatly, so he would just turn it off. He was so upset (it was his first bike and he hadn't had it more than a month) that he parked it and barely looked at it until I told him that all might not be lost.

I forgot to mention that the custom pipes the PO installed were Straight Pipes. The guy was apparently some engineer who was precise about everything he did, so I didn't question his mechanics (i'm in no place to question anyone's mechanics really) , but I remember reading somewhere that if you don't do certain things when you put on a different exhaust (jet carbs, etc.) that things can get really messed up. So maybe that's why everything got screwed up. It's just a theory though, I don't know.

I'll definitely check out the plugs and check compression. Thanks for the response woot.

 

I agree with snosheriff, compression test is an easy start. And definately get a good manual for the bike. Then dig right in and get your hands dirty. I believe that anyone, that can comprehend what they read, can rebuild a motorcycle engine. I rebuilt my first one, (CB175), at the tender age of 14, without a manual. With a good manual specific to the bike, and all of the information available on the internet, I know that you two can bring the bike back to life. I can almost guarantee that when you two get that engine rebuilt and running correctly, you'll never take it or any other bike into the "stealer" for repairs again. :wink: You'll be too proud of your work to let anyone else touch the inards of your baby. That's when the bike truely becomes yours. :mrgreen: :mrgreen: Plus you have all kinds of help right here on this forum. Most of the people here on this site own multiple makes of bikes. We're not XJ snobs here, any make or model of bike is in our brotherhood. Have fun, open her up, and bring her back to life.

 

Wait a minute - how far has he ridden it since he dropped it? If we're talking ~10 miles then maybe it's not what you think it is...

One common thing is for the bikes to flood themselves when dropped onto their side. In fact I bet it was a bitch to start right after it was dropped.

Are you sure it was oil that came out of the pipe, and not carbon soaked gas?

Still - the first place to start is with the plugs... pull the plugs - dry and clean the bad one(s) which I'd guess are on the side that it fell to...

anyhow come right back - we wanna know what's going on. This is like cluedo...

Woot

My undereducated guess as to why? It's a vacuum petcock... no gas flows without a vacuum. If you hit the kill switch you have no flame and the system stops very quickly - however - if you lay it on it's side it still has spark and it still wants to run... however it basically stalls itself by running lean or more likely running rich. The vacuum works particularily well on the lower carb as it's well below the gas flow now. It sucks in a lot of gas and stalls. To get it to start you've got to get the mixture back to normally in the cyclinder - so you have to wait for alot of it to evaporate. You can help that along by removing the spark plugs. This is in the ~10-20 minutes after a fall mind you.

Now what you've got likely is a fouled plug (from running rich for that short time) and possibly some fuel still in the pipe... the other thing that you do expect in the pipe is some moisture which will also steam.

 

One other comment - that pipe probably flows really well... checking the plugs will tell you if it was running lean... if the crash had nothing to do with it, then yes running lean ( which means HOT ) could have done damage to the pistons... like the two other members said burnt valves.

 

Wow, thanks for the advice guys! Woot, I'm afraid I misled you when I said "fall". I actually meant that it was during the season, autumn, not that the bike was dropped. That was definitely my fault (poor wording/sentence structure). If I read what I wrote I would have thought the same thing.

Checking compression today. I'll let you know how it works out.

Thanks again

I'm starting to feel good about things; that there is some hope afterall.

 

A hole in the piston or burnt valve would be clearly evident with a serious miss or "chuffing" out the exhaust. EG, Zero compression.

It all points towards oil rings or collapsed piston which may not show up in a compression test. I've had engines run fine without a miss or compression drop with the bottom skirt on the piston from the oil ring groove down totally disintegrated. Only noticable symptom was smoking, burning oil and a slight rattle from piston slap.

Does the bike run well and fire happily on all four??

Only reason I ask is the only reason I can think of that something like this can happen "suddenly" is piston failure or if it started running rough and losing power, has one of the clamps slipped on the carby main throttle shaft putting the sync out maybe allowing one carb to be way too rich.

Was he flogging it when the problem started. Needs an honest answer as if he was then piston failure is a high probability.

Hope I haven't scared you off yet.

Couple of more suggestions.

Has he simply overfilled with oil?(unlikely) Is the breather pipe blocked?

Fire up the bike and remove the oil filler plug. Is it blowing any pressure out?


Just some ideas to think about. Hope it's something simple.
Cheers
HG

 

If you can find a shop to work on it - you will PAY a LOT. These are 25 year old bikes. Most of the shops in my area WON'T work on them - They're too OLD.

So get ready to get to wrenchin'.

For info without a repair manual in front of you - KzRider DOT COM. But get a repair manual. Cyclemadness DOT COM is another place to check on the ole Kz's info.

Hope this helps - Sammowry

 

Sudden failure sounds like a dropped valve to me, as does the oil out of the pipes. You'll only know for sure once you pull the plugs (hopefully) or the head (definately).

 

yoyoming1001, sorry to say but the most equitable solution to your problem would probly be to buy a used engine off of epay, oops, ebay. The cost of rebuilding these older engines is just up there nowadays and most mechanics won't touch the older stuff. Did the bike sit for an extended time? I have had rings stick from sitting and a shot of MarvelMysery Oil down the plug holes is a good start to free up the rings. Turn the motor over a few times by hand with the plugs out and let the oil work it's way into the rings and cylinder. Let it set a day or two, then plug it up and fire that sucker up. It will smoke a LOT for a few minutes (great if you are experiencing a mosquito problem) and eventually clear up. Drive it or idle it for a while a couple of times to get the rings loosened up. Then try that compression test. Even a re-ring job can be an expensive proposition. My two cents. I pulled a low mileage XJ750 from a fella out in Utah for $75 (shipping was another $75) delivered to LAX. Not a bad deal at all! You should have as much luck as well but be careful, if your not willing to loose that much, don't buy off ebay. Hit a yard site and buy the warranty, it saves grief. I am not trying to disswade you from trying however. If your compression check shows lousy dry and good wet, your sunk, bad rings are likely and rebuild is necessary. If you have a bad dry and wet test, the valves may be the only issue and a head job can be done for a very reasonable price. A wonderful opportunity to get intimate with your machine and expand your knowledge is time well spent in my book (my wife has a different view, just ask her). Good luck and let us all know how the whole thing works out.

 

I still want to know what the plugs look like - I know that is my first question all the time but seriously it's a good starting point. Also that compression test would be good

Maybe I'm not quite so doom and gloom as the rest - but - we'll see after these tests I suppose.

Cheers,
woot.

 

I took the engine apart to see what was going on. Rings were completely worn out. Pistons were cruddy but still fine. Valves were seated ok, i would need to clean them up a lot. So now I have to decide if it's worth it or not to get new rings. Probably not.

 

BikeBandit DOT COM has ring sets for the 81 Kz750.

1 set for 1 piston = $51.99 Plus shipping.

How to the cylinders look? Is there any vertical (up & down) gouges on the cylinder walls?

Did you post or ask any questions to KzRider DOT COM or Cyclemadness?

And try a pm (personal message) to TAZZMANN here on this forum. He's currently working on a Kz650 - Maybe he can give some tips & support.

Sammowry