Repair Diagnosis/Top-End Rebuild?

The bike has been running rough and louder than usual. It seemed to happen overnight. I'd recently changed fouled plugs, the next thing I knew the bike was running terrible. It ran like it still had fouled plugs-bogged down, unable to make clean rhythm. Deciding a good place to start was to actually "read" the plugs, I found 3 plugs with an acceptable light grey color but cylinder 2 had wet, gas residue?! My guess would be a worn intake valve causing a poor valve seal. Any ideas? I am guessing I will need a top-end rebuild and am looking at purchasing a Wiseco overbore kit and a valve job at the same time. Has anyone had this work done before? What is an estimated cost for parts/labor? Is it common to need a top-end rebuild on the KZ1000s after only 18XXX miles? My '86 Nissan 300ZX Turbo has 189XXX miles with an original top-end. I guess you never know.

I'm still not sure what happened here. I know there had been small pebbles caught in the sparkplug wells on a few of the cylinders-could've fallen into the combustion chamber when I changed plugs. Any ideas what is wrong w/cyl.2?

Also, does anyone know if the wiseco pistons have higher compression ratios than stock pistons, causing more wear and future re-builds more often? How does that change the feel of the ride for the mighty KZ? Thanks.

if it were me, i d pull plug two , put the wire on it and lay it close to the head and crank to test for spark first...next do a compression test.you should have some idea whats going on after
that.....good luck

check your compression and then check the resistance of your plug caps they do fail from time to time.

onanysunday wrote:

The bike has been running rough and louder than usual. It seemed to happen overnight. I'd recently changed fouled plugs, the next thing I knew the bike was running terrible. It ran like it still had fouled plugs-bogged down, unable to make clean rhythm. Deciding a good place to start was to actually "read" the plugs, I found 3 plugs with an acceptable light grey color but cylinder 2 had wet, gas residue?! My guess would be a worn intake valve causing a poor valve seal. Any ideas? I am guessing I will need a top-end rebuild and am looking at purchasing a Wiseco overbore kit and a valve job at the same time. Has anyone had this work done before? What is an estimated cost for parts/labor? Is it common to need a top-end rebuild on the KZ1000s after only 18XXX miles?

Yes. The machine shop that did my head way back when was contracted to do the maintenance on the local PD bikes which were KZ1000's. He told me they needed a valave job about every 20k miles with city riding.

Bikes eat up top ends way faster than cars. Bikes are air cooled so the valves and seals run way hotter and don't live as long.

Check compression, then check valve clearance. If you have low comp on only one hole, it's likely a valve.

before you go crazy and pull the top end apart please do some preliminary checks to verify the problem it might save you in the long run and like i said before check your spark plug caps you are 5kohm[5000] resistance.don't jump to any conclusions about your valve train get a leakdown test done. and do a compression test.

onanysunday wrote:

The bike has been running rough and louder than usual. It seemed to happen overnight. I'd recently changed fouled plugs, the next thing I knew the bike was running terrible. It ran like it still had fouled plugs-bogged down, unable to make clean rhythm. Deciding a good place to start was to actually "read" the plugs, I found 3 plugs with an acceptable light grey color but cylinder 2 had wet, gas residue?! My guess would be a worn intake valve causing a poor valve seal.

How do you go from fouled plug to bad intake valve seal in one jump?? Most people would blame the carb

Could be carb, could be ignition, you need to do some checks to determine that.

Any ideas? I am guessing I will need a top-end rebuild and am looking at purchasing a Wiseco overbore kit and a valve job at the same time. Has anyone had this work done before? What is an estimated cost for parts/labor? Is it common to need a top-end rebuild on the KZ1000s after only 18XXX miles? My '86 Nissan 300ZX Turbo has 189XXX miles with an original top-end. I guess you never know.

Does your Nissan 300ZX turn 9000 rpm?
Is your Nissan 300ZX air cooled?

Can't make a meaningful comparison between apples and oranges.

I'm still not sure what happened here. I know there had been small pebbles caught in the sparkplug wells on a few of the cylinders-could've fallen into the combustion chamber when I changed plugs. Any ideas what is wrong w/cyl.2?

Are you running this bike without air filter(s)??
If you have pebbles stuck in the spark plug cavity, I'd really hate to see what your pistons and combustion chambers look like.

Also, does anyone know if the wiseco pistons have higher compression ratios than stock pistons, causing more wear and future re-builds more often? How does that change the feel of the ride for the mighty KZ? Thanks.

Wiseco has at least 10.25:1 (maybe 10.5:1) and 12.5:1 piston kits available.

I don't see how increased compression alone is going to cause more wear and tear, but detonation due to lack of octane required by higher compression certainly will shorten the life of a motor.


The posts above this one offer some good advice.

Thanks for all the helpful advice. I never realized the top-ends on these bikes are so short-lived/fragile. Lame. I baby this bike, rarely taking RPM's above 5k. I will perform some of the easier diagnostic tests before removing the head/biting off more than I can chew (for now)

For years motorcycle enthusiasts have sung the praise of air-cooled engines and convinced me of their merit(simplicity), but if in fact, they contribute to premature top-end rebuilds because of their higher operating temperatures I have to make sure to go with a liquid-cooled machine next time. I never realized air-cooled was that much worse than liquid as far as longevity. Understandably, motorcycle engines will have a hard time lasting as long as a car engine because they rev higher (as a general rule) and are smaller, but there should be a reasonable compromise between one top-end lasting 18xxx and the other 186xxx. It appears this is simply the limitation of the older technology vs. the new. In any event, testing will show the cause to this problem.

I know that race machines are almost always running higher compression and many require a tear-down and rebuild after every race. I like the increased power and performance of higher compression engines and these 12.5-1 Wiseco pistons, but not if it has to come at the expense of reliability/burning through rings prematurely. Maybe original is best?! Thanks guys.

they are not fragile and they do last guite well i have over 30,000 hard miles on a 1075 kit that i built ten years ago, i normally ride [when i'm on a road trip] 90-100 miles an hour] i have drag raced it and i also go thru a minimium of 3 rear tires a year with not from milage[i love burn-outs] i have replaced a regulator/rectifier and that is it.it made 114 horse last time i dynoed it it im going to use it again this year as my main street ride and ill be going to texas and sturgis with it as well. do the checks that have been mentioned and you might suprise your self at how simple the problem may be.as far as using wiesco over stock the compression ratio of stock pistons is about 9-1 the wiseco street kit is 10.25-1 so it is not that much more. the pistons hold up well if the bore job is straight and true.

thanks t! Just curious-how did you know that you're regulator/rectifier needed to be replaced? What signs are the tell-tale signs that it needed to be replaced? I'm trying to learn more about mechanics..weak or sporadic voltage throughout the electrical system, poor spark?

timebomb33 wrote:

they are not fragile and they do last guite well

My KZ750 twin went about 58k on the original top end before needing work, and that is mostly city miles which are worse than highway on the engine. The guy at the machine shop figured at 58k I was coming in for the SECOND valve job, couldn't believe it was the first.

That said, his top end rebuild went about 17k before the valve seals failed and the oil started fouling the valve seats and spark plugs. he may have used bad parts, I didn't see them. That could be why he figures top ends only go 20k?

Anyway, I would check the valve clearance and re torque the head and check compression cold and hot before pulling the head. You may not need a valve job, but you might. Compression should tell you.

i had a charging problem it boiled off a battery on the way down to sturgis so i checked out the charging system and sure enough i narrowed it down to the rectifier i switched my harness over years before to a harness from an 80 mk11 so i had a solid state regulator and i found one in billings montana that fit. if i'm looking for a charging problem i first check the output at the battery then i would go down to the harness coming out of the stator and check for ac out-put then if it was alright i would do resistance checks at the regulator all the specs and tests are in the manual i always take a small pocket multi meter on the road with me just in case.if your looking for spark problems the first place i would check is battery voltage then i would check down at the points or pik-ups to see if you are getting at least 12 volts if you are, check at the coils for the same voltage,if you aren't getting that then check your connections and grounds. what kind of ignition do you have.also if you have stock plug caps or resistor type caps some times they will go bad check them with an ohm meter you should get 5kohm's resistance [5000 ohm's].

I am getting spark in cyl.2 I don't have a compression tester so I'll have to take it in somewhere to get checked..that is if there are any motorcycle shops open this time of year.