KZ900 brand new rebuild, low compression

ok now we can start, what was done to the cylinder head what kind of pistons and also what kind of cams and valve train, is this a stocker rebuild it sounds like it with only 120psi on 3and4 did you use new pistons and rings, have you tried to get it going since the compression test, have you tried putting a tablespoon or so of oil down the holes to see if the compression will come up also what kind of gauge are you using is it a screw in type or a rubber seal type somtimes if you screw the compression gauge in part way or sort of crossthreaded the o-ring on the adaptor wont seal and compression gases will leak past the tester one of my students did that last week as a matter of fact did you have the throttle open all the way.

Patton wrote:

Am thinking the engine would hardly run at all on just two cylinders.

As known, compression may escape through either valve, or the spark plug hole, or past the piston and compression rings, or around the head gasket.

Besides bent valve or holed piston, other possibilites might include:
Weak or broken valve springs;
Warped cyl head;
Loose spark plugs;
Damaged head gasket or installed upside down;
Broken or damaged o-ring;
Excessive ring gap or failure to stagger gaps;
Broken piston rings during reassembly.

Some auto-parts stores such as Auto-Zone loan tools to customers, and perhaps a leak-down tester would be available. Could ask around. And maybe also consider having a local motorcycle repair shop do a leak-down test.

Good Luck!

I was also thinking warped cyl head, or possible blown head gasket- doubtful tho. Could be any of those listed above, which I have thought about all of them. I do not want to tear into it again > Ha Ha

I guess there are only two things left. Bad valve job or the rings aren't installed correctly. That is all it can be, if the bore clearance is correct, head gasket is correct, cam timing is correct, valve clearance is correct?
The motor will tell the story, when you take it apart....

there are many things you can do before you pull the engine down to double check get a vacuam gauge and hook it up to 1&2 and check what your getting for vacuam if it is a lot lower than 3&4 then you have a problem try redoing the the test again hold the throttle wide open and make sure the gauge is seated properly retourque the head and see if it has loosened up that can happen with 30 year old cylinder studs

timebomb33 wrote:

ok now we can start, what was done to the cylinder head what kind of pistons and also what kind of cams and valve train, is this a stocker rebuild it sounds like it with only 120psi on 3and4 did you use new pistons and rings, have you tried to get it going since the compression test, have you tried putting a tablespoon or so of oil down the holes to see if the compression will come up also what kind of gauge are you using is it a screw in type or a rubber seal type somtimes if you screw the compression gauge in part way or sort of crossthreaded the o-ring on the adaptor wont seal and compression gases will leak past the tester one of my students did that last week as a matter of fact did you have the throttle open all the way.

Cylinder head was rebuilt, used stock valves after a good cleanup- none appeared bent cracked warped or misconfigured. Everything else is new, seals and springs are new, too. Stock pistons and stock set of rings. Stock cams and stock chain. Yes, the bike will start up and run, revvs decent but definately sounds like a misfire...or two. Ha Ha. Has hard troubles starting after warm up, seems like it floods out. Oil down the cylinder did nothing for compression. I am using a decent quality compression tester, it is a screw in type but also has rubber orings to seal the holes.

ok then.do you notice any blow by out the crankcase vent when you crack the throttle?does it smoke?stock rings,do you mean stock replacements or origional rings?was bore measured/honed?were valves and seats ground?

PLUMMEN wrote:

ok then.do you notice any blow by out the crankcase vent when you crack the throttle?does it smoke?stock rings,do you mean stock replacements or origional rings?was bore measured/honed?were valves and seats ground?

I notice no blow by. The bike doesnt smoke whatsoever. By stock, meaning OEM replacements when being rebuilt. I Used parts that were ordered from Z1. Bore was measure and honed to 45 degree hatch markes, and was very sound. Remember, the bike had only 6k miles. Valves were placed on a valve cutting machine and the seats were never touched.

if there was a problem with the pistons or rings the motor would smoke and it probably would run with it being so low on compression i would defintly try the compression check again if the valves were a problem bent or not sealing it would cause blowby not just if the rings weren't sealing. how does it run in the higher rev ranges.

You can spend weeks trying to figure this out through all the possible reasons this extremely low compression can be occurring... All of which has been provided from the others is an accurate assessment....

OR, you can go to/order/purchase one of these from Harbor Freight Tools... for $30, how the hell can you beat that price...

In a quick and simple leak down test, you'll be able to hear the escaping air and follow it accurately for where it's (leakage) occurring...



On another note, cylinders 3 & 4 are also on the low side @ 120psi, should be around 140psi (OEM spec) or higher on a new build in using new bore/hone, pistons and rings, gaskets and valve/head operating ideally...

OMR

My money is on rings improperly installed.did you stagger gaps as per manual?

timebomb33 wrote:

if there was a problem with the pistons or rings the motor would smoke and it probably would run with it being so low on compression i would defintly try the compression check again if the valves were a problem bent or not sealing it would cause blowby not just if the rings weren't sealing. how does it run in the higher rev ranges.

Hhigher revs it seems to run more crisp, although it still does not sound perfect. It won't run if it's below 2k rpm. It sounds like some of the cylinders cut in and out- meaning firing. It just doesnt have the smooth purr the motor should have, it sounds more lopey which to me sounds like a misfire or no fire at all. I did the compression test at least 5 times and in every reading I was geting 0-5psi on 1 and no more than 30 on 2, and 3 & 4 still seem low at 120.

One, I wouldn't be starting this motor again until you isolate this extremely low compression readings... Remove valve cover for review...

Not trying to insult here but are you 100% positive your cam shafts are installed correctly....

This image is from the right side of the motor, reviewed from cylinder 4 in, cylinder 1&4 TDC.... Note the lobe orientation and intake cam dimple mark... Exhaust cam sprocket mark should be horizontally aligned with head decking. Timing advancer should be on "T" 1 II 4...

Match yours?

If using adjustable cam sprockets, positive they are degree'd correctly?

If above mentioned holds true then remove the head and cylinder and verify your ring orientations....

No joke here, you just spent a considerable amount of $$$ in this re-build and a measurement of 5-30psi is whacked.... Sure it's a PITA but why take the chance, don't be lazy, open her up and confirm...

Not work a bent valve!

OMR