KV6 loss of compression on No4 cylinder

[Geoff130]

I recently bought a ZTT 190 with a misfire amongst other things. I was told it started after a plug change by the previous owner. A quick check with TOAF said the misfire was on cylinder 4. I tried swapping the plug and coil pack with no change. A compression check shows no compression at all on 4, all the rest are ok. The water level is fine and no signs of a head gasket fail and the oil is fine too. Ideally I'd like someone to tell me it's a blown fuse but I suspect it's going to be obvious when the head comes off. Is there a common failure on the kv6 that causes this?

[bl52krz]

Sounds like a valve/S not seating properly. Check for broken valve spring’s. What does the engine run like, if it runs?

[Yorkshire GOC]

Zero compression could be a holed piston, stuck/knackered valve, bent stem or a broken spring Head off to have a look .

[stocktake]

But to give you a clue.
Compressed air line into the cyl. listen for where air comes out. Inlet/throttle assy, exhaust, rocker cover

[dave lincs]

As Dave said compressed air into the cylinder will give you a good idea where to start, So a cylinder leak test it is to start with

[Ducati750cc]

To do a cylinder leak test buy or borrow one of these :


https://tinyurl.com/yjwcjoa8



Easy to use, make sure the cyl you are testing is at TDC on the compression stroke then:


Listen at the exhaust outlet, any sound of air leaking will be down to valve/seat/spring problems.


Same with the air inlet, i.e. throttle body/ manifold will point to the same as above, with the inlet valve.





Remove the oil filler cap, any sound of air escaping will be due to holed piston/ring/bore/liner problems.


Any sound of air escaping and bubbles appearing in the expansion bottle is a sign of a head gasket problem.



These are the main tests whilst doing a leak down test, but there are others that can determine such as head gasket gone between bores etc.


If no compression at all with a compression tester, presuming the O ring on the end of the tester is OK and seated correctly on the plug seat, then you can be pretty sure that the head will be coming off, but a leak down test will give much more info.


When you say that it started after a plug change, are there any signs of the previous owner cross threading the plug hole resulting in both the plug and tester not seating correctly, very remote though and if this had happened it would not result in 0 Psi and air loss would be heard from the area with the engine running.


Was the plug change just a normal service plug change or was the previous owner trying to cure a misfire ?

[SD1too]

Originally Posted by Geoff130

... I suspect it's going to be obvious when the head comes off. Is there a common failure on the kv6 that causes this?

No Geoff, there isn't a common failure causing this and I wouldn't rely on what the seller told you either!

With reference to b152krz's reply, I would begin by removing the cam cover which is a lot less work than immediately removing the head. This will expose the camshaft and might reveal what's happened, particularly if you turn over the engine by hand. If not, then you can proceed with the other tests described.

Simon

[Geoff130]

I have a receipt for the service from a garage to back up what the previous owner told me, however none of the coil pack connectors where locked and number 4 was barely on. I suspect along the line someone knew a bit more than they're letting on. The plug threads felt fine when removing the plug and inserting the compression tester. I'll try taking cam cover off and see if anything else is apparent.

[dave lincs]

Originally Posted by Geoff130

I have a receipt for the service from a garage to back up what the previous owner told me, however none of the coil pack connectors where locked and number 4 was barely on. I suspect along the line someone knew a bit more than they're letting on. The plug threads felt fine when removing the plug and inserting the compression tester. I'll try taking cam cover off and see if anything else is apparent.

Geoff do a leak test first simple to do and will point you in in the right direction

[Ducati750cc]

Originally Posted by dave lincs

Geoff do a leak test first simple to do and will point you in in the right direction

What Dave says ^.


All a compression test will tell you is either, compression not within spec, low compression, or no compression and that's all !


A leak test will let you know in what area to expect problems, to evaluate what is needed for further investigations ( and potential costs ) and as SD 1 says, whipping off the cam cover is quick and easy, it will allow you to see if all the valves and springs and followers are doing what they should from upstairs, though this is not a 100% confirmation of the valves being OK.


There are rare and unusual happenings with valves in the combustion chamber from burnt valves / seats, dropped seats, dropped valve heads, cracks between inlet and exhaust ports / seats and so on.


With symptoms that you have my son with the garage would do the above, plus checking in the combustion chamber with his camera.


He hates disturbing / stripping anything down until he is pretty certain that that's the only way forward.



None of these are particular to your engine, but given to let you know there are sometimes some weird happenings with engines.


Keep us informed, the investigations and results adds to the knowledge on here and can help others .