Compression - is there an easy answer?

So my 1976 Kz900 had it's head swapped out. It runs, but I'm losing gas out of the second carb. I tested compression on this cylinder and it was WAY low (like 65 psi roughly). Adding some oil put it up to almost 150psi (going from memory here). The other cylinders are all in the 110ish area. What's the easy answer? I've checked the valve clearances and they are all good or slightly higher than spec'd out. Can I just swap the rings on the pistons and be good to go or will I possibly need to do more? The engine has (I believe) about 45,000 miles if the numbers are accurate.

Any help would be great.
Thanks!

Post edited by: DoubleDub, at: 2006/08/07 18:52

If you take it all apart you may as well do all 4.110 psi sounds low.check the manual.at 100psi it will start but lack power. I think you whant 130 to 160psi.

compression loss can be several possible problems.

Rings are worn/not seated.
Valves are not seated or a valve is bent.
Valve clearances are not set correctly.
Head Gasket is leaking.

With a 45,000 mile engine, any (or all) of the first three are possible.

You need to check the valve clearances and is they are too tight change shioms and retest.

If the valves are not seated or a valve is bent, you don't have much choice but to pull the head and fix the problem.

If the rings are a problem and the bike has been sitting for a while, it's possible the rings are gummed into the piston on the bad cylinder. Running a does of Ringfree or Seafoam through the engine may loosen a stuck ring.

If the problem is worn rings / bore - the only solution is to rebore and replace pistons / rings.

I have already had the cylinder block off and reassembled everything - the bike has been sitting about 6 years prior to all the work I have done recently. I had to swap the head with a used one as I had stripped one of the spark plug chambers. I have checked the valve clearances, and it has a new head gasket and cylinder block gasket on it. I'm just trying to find where the low compression would be coming from.

Thanks, Jeff. I was worried I'd have to rebore and replace the pistons. That's further than I was hoping to have to go. May have to sell the bike at this point.

Make a leakdown tester and see where the leaks are coming from. I made one out of an old spark plug. Drilled out the middle and attached a hose. I then introduced air through the spark plug hole with my fancy setup. All you have to do now is listen to where the air is coming out of. Good luck

Since compression popped up on the cylinder where it was down after checking with oil, this indicates the seal of the rings on that cylinder are not good. It wouldn't be prudent to just expect that a new set of rings will fix the problem. The obvious answer is to take the block off and MEASURE the diameter of the bores and MEASURE the diameter of the pistons. This is done in a very precise way so that the bores are measured in at least three different depths and in three different angles to check for out of round bores (called ovaling). The pistons are measured on the skirts below the piston pins in a precise location. GET A SERVICE MANUAL for the exact method. Anyway, once you know what the bore diameter is and the piston diameter, the difference is the CLEARANCE. This clearance figure is specified in the same SERVICE MANUAL. If it is within spec, hone and rering. If it is outside spec, bore and new pistons/rings OR buy another used block/pistons that is in spec and hone/re-ring. It is a waste of time to rebuild a top end and not do this measuring. You can use a digital caliper from Harbor Freight (under $20) and a set of TEE BORE GAUGES (eBay - under $20).

easy answer??

yep... MACHINE WORK and NEW PARTS
switching used worn out parts is no way to go

this is the mighty KAWASAKI 900 you are talking about
take pride and spend some money to get it tip top

hell if you want a crapper get a cb 750

Yeah - this is what frustrates me. I don't have the cash to make this bike run right - I was hoping to get it working on a cheapo budget and do the work correctly this winter, but it's not looking like that can happen. I think I'll probably have to sell the bike as it is right now as I won't be able to put the money I should into doing what's needed. Thanks for the guidance all.

You did make sure the ring gaps were staggered prior to installing the cylinder block, right??

With 45000 of hard riding and lots of heat the bores are oversize and the rings are not sealing anymore. Any time gas leaks into a cylinder it washes out the rings, too much of this and the cylinder wont seal. Readings of 90 and 110 that pop to 150 indicate worn rings. The valves are sealing though. Yp your gonna have to take the head off and measure the bore. If its ok then go for new rings. Id check the guides and lap the valves, and check the cam chain rollers for wear. This is important. To seat new rings start the motor, rev,idle,rev higher,idle,rev,rev, shut it off, let it cool some, do it again. Rings needed to be worn in immediately upon start up and they will always seal. Make sure the proper grit honing stones are used. Once u get these engines running they have a habit of staying running.... good luck. Ive seen oversize pistons for sale on ebay quite often. That would be a good place to start. Any shop with a small enough bar can bore for around 100$. Some machinists do side work also... maybe u can get by with honing and the oversize pistons.....

Post edited by: nads.com, at: 2006/08/09 00:30