Kz 400 Compression Test

Hey,
I have a 1978 Kz400 w/ 10,000 miles on it and it has been burning a good amount of oil. I ran a compression test with the carbs wide open and the engine warm, here's what I got.

Dry:
Left: 160psi
Right 180psi

Wet:
Left 225psi
Right:255 psi

Not sure exactly how to interpret the results. I haven't adjusted the valves yet since there a real pain. With that being said what conclusions if any can be made from these results?

Thanks,
Nate

Bad rings are indicated by the large increase in compression, but it seems way too high, actually. Is it possible you added too much oil? Too much and the combustion chamber volume will decrease causing artificially high compression.

I think it was 3 cc's of oil in each cylinder. Wouldn't bad rings also cause a low reading in the dry test. The manual gives 142-156psi as "new" values with a minimum of 107psi.

Post edited by: Shoey949, at: 2007/11/23 16:44

Your compression rings might be ok but the oil rings may not be. Sure it's burning oil and not leaking? Could have some bad valve stem seals also, does it seem to burn more when started after sitting?, especially over night?

defintely burning oil, I can't say I've noticed any real patterns since it's actually my dad's bike.

Shoey949 wrote:

I think it was 3 cc's of oil in each cylinder. Wouldn't bad rings also cause a low reading in the dry test. The manual gives 142-156psi as "new" values with a minimum of 107psi

Yeah 3 cc's may be too much. Yes bad rings would give low readings dry. I should have mentioned the dry readings seemed ok. I was just blinded by the 255! It should only need a drop or two to wet the rings.

180 is excellent. 160 is only roughly 10% lower than the 180. Perhaps a valve-clearance check should be done, and an adjusment may bring up the 160 cylinder.

The other source of burning oil is through the valve-guides. New valve guides are a big job, but you might get away with just replacing the seals.

Post edited by: loudhvx, at: 2007/11/23 22:02

if its 180psi you'll be needing some octane boost if its a pent head.

looks like your comp tester is fubar,if it has the possibility of making 255 you could run it on diesle
:silly:

Post edited by: themachine, at: 2007/11/24 01:57

Lou,
Have already got the head pulled so where part way there. As far as guides go would just involve replacing them?

-Nate

I've never had to replace the guides. Someone here would probably know how... or better yet post a question on Ivar's KZ400 forum:
www.armbell.com/kz400/index.php?sid=c35c...fcc2365&mforum=kz400

That would be the best place for 400 twin specifics.


Also, the gpz 550's have 180 to 190 psi compression. They run on pump gas. I have one that has all four cylinders around 200 psi. That one needs octane boost. (But, I believe smaller cylinders typically can handle more compression without knock problems.)

My 2 cents worth .
Borrow or buy a different gauge .
Something looks and sounds funny with your #'s .
I have to wonder how that engine can even spin over with #'s like that .

Since the head is off, did the 180 cylinder have a lot of carbon buildup? (Yeah I just re-read your post that 156 is a "new" reading.) That could explain the high reading. And there is the possibility that the gauge is off, as was mentioned.

yeah the 180 cylinder had a lot more carbon build-up then the 160 one.