low compression on pot no. 4....

[ndevans]

Knew I'd regret borrowing a friend's compression tester. Pots 1, 2 & 3 are all 170-180psi. 4 is 60.

I suspect a damaged/burnt valve or broken piston ring. But first I am going to re-check the valve rocker clearances, cos it cost's nothing and I really don't want to get the head off if I can avoid it.

Could maybe explain why I didn't get more than 37mpg on the Orkney run??

[MoggyTech]

I'd say you are right to suspect a sticking or burnt exhaust valve. The head gasket normaly goes between 2 and 3, but you really need to pull the head off soon if the valve clearances are ok.

Other things to check. Any oil in the rad water, or water in the oil (Head gasket or cracked head) Any blue exhaust smoke? (Valve stem oil seals or piston ring)

Try testing cyl 4 on a wet test with a few squirts of engine oil into the cylinder. Little change in reading means valve(s) big improvement means piston/ring/bore damage. Your other readings are top notch, so get #4 fixed ASAP, you don't want to wreck and otherwise healthy engine.

[bmcecosse]

The readings on 1,2 and 3 are astonishingly high for a standard engine - but maybe that's the gauge. Yes #4 looks bad - and if it's been running with a closed up valve I'm afraid the valve and seat will be in a state. But - worth checking/adjusting before pulling the head. I would get a head gasket in stock anyway!

[ndevans]

if the valve clearances are ok.

Checked them last night, all ok.

Any oil in the rad water, or water in the oil

No, and not aware of blue exhaust smoke either.

Try testing cyl 4 on a wet test with a few squirts of engine oil into the cylinder

Tried that last night-the reading on no 4 climbed slowly to 100 after cranking for about 15secs, then 150 with a bit of oil squirted in.

The readings on 1,2 and 3 are astonishingly high for a standard engine

It's a standard 1098, but the head has been skimmed at least once possibly twice. I don't know by how much.

Looks like new piston rings. I guess it's worth doing all 4, plus renewing the valve guides and stem seals as well.
cheers all.

[bmcecosse]

All heads start off at 2.750" - so when it's off you can check how much has been removed. Climbing to 150 like that it indeed looks like piston/ring problems - but the others do look ok and if it's not been burning oil i would tend to leave well alone and just pop this #4 piston up the bore for a look see what's happened. With that very low initial figure i fear it may be a bit more than just new rings required!

[ndevans]

OK. This weeks star prize for being a prize idiot goes to........me.

I have now taken the time to read and thoroughly digest all the various posts on the messageboard, the tech tips manual, haynes etc, and now I know that I did the compression test wrongly.

I did it with the engine COLD, and without wedging the throttle open.

I have now re-done it with the engine HOT, and with the throttle wedged open. I now get 160-170psi on all pots

So, given that #4 was at 60psi when cold and at 160 when hot, do I have a healthy engine?

cheers Neil
*slopes off sheepishly in embarassment*

[bmcecosse]

Hmm - well - it certainly looks to be ok now - but really it must have been wrong before - which suggests a sticking valve. Perhaps when you squirted in that oil for the wet test - it helped to unstick the valve! I suggest you run it for a few days - then test again and see what result you get. Only other possibility for #4 being down is that maybe - with engine cold - if you tested 1,2,3 in that order, by the time it got to #4 turn the engine had started to generate oil pressure which slows the starter quite considerably with cold thick oil - and the battery would also be starting to feel the strain. This is the main reason for hot test - oil will be thin and battery fully charged !

[ndevans]

Ok. Thanks for the advice. I will do another check with the engine cold this weekend, then do it again with it got and see what i get. The engine had been stood for over a week when I did the first test-maybe it takes a while for the bore and valves to get fully lubricated. Cheers neil

[bmcecosse]

I wouldn't bother with cold test - just adds confusion.

[Packedup]

I generally do the tests with the engine cold - Messing around with burning hot spark plugs isn't my idea of fun! And as I've currently only got one of those nasty "press fit" testers, I struggle to crank with the throttle open too (lack of hands). I'd still expect all the cylinders to be fairly similar simply because even cold with no throttle they're being tested under the same conditions.

Still, if everything's good on a hot test, I wouldn't worry

[MoggyTech]

Another possability is the test on #4 was done with the compression tester not being scewed into the plug hole completely, or if push on by hand type, at a slight angle.