Morris Minor Owners Club

Right, I've checked the firing, and she's not firing on no.3 cylinder. Leads are OK. Going to have a look at the dizzy cap and the no.3 plug. Last time (when I lost a chunk of a valve on no.3... ) I was able to jolly her along for a bit, but the plug was coming out very wet. Didn't notice it last night, but that may be the case now, as I've not long got home...

Okay, plug no.3 is soaking. Dizzy cap is fine (swapped with an old one). Tried a clean plug in no.3 and she still wouldn't fire on that cylinder. Am I right in assuming that this would indicate a problem with the inlet valve, i.e. that with a spark present (which there is) and petrol present (enter first witness, a damp plug ) that there must be an absence of air?

Thanks for all the help so far, chaps!

You CAN do a compression test yourself - just turn it over on the starting handle (ignition off) and feel if there are 3 good ones and one soft one ! And I agree with the various posts above - it's almost certainly an exhaust valve. Dead easy and cheap to fix

Turn it on the starting handle...

Stupid boy, Pike.

Mind you - with an exhaust valve problem the plug will still fire - it just won't be doing much good. It could be rings/piston problem - but usually that gives cloud of blue smoke. If it is a head gasket 'slightly blown - you can be sure it will not heal up ! It's going to get worse - and may damage the head/block too if you keep running it.

Gareth, is it oil or fuel that's wetting the plug?

...or water?

Odds on its a burnt valve, off with the head and find out! ;)

I got a compression tester from Lloyds motor spares for £8 earlier in the year. Its another tool that everybody should have

The plug's wet with petrol. I've been given a tip about a quick compression test - use a cork (not the traditional cork, but the reconstituted plastic ones as they don't flake off, possibly causing damage). If it pops out when turning the engine, there's compression. Trust me - there IS compression. Took me ages to find the cork again after it bounced off the walls a couple of times...

Wet with petrol might be something simple like electrics / bad plug (hopefully) but can also be fouled up due to oil.
When the plug is fouled it then gets wet with ptrol as the petrol doesn't get burned.

I was going to suggest using the starting handle and your finger over the plug hole, but the cork test sounds much more amusing!

Right - an update.

We have:

• spark (changed leads and plugs, used colourtune on her),
• combustion (as shown by colourtune - bright blue flame, going to yellow when revved),
• compression (cork-test),
• slightly smoky exhaust (wasn't running engine for long - most smoke generated with choke out).

Running with the colourtune in, I took the no.3 lead off, and it made no difference. Still ran like a bag of nails... It's almost as though there's compression on the upstroke, but the downstroke is creating insufficient power.

It's almost as though there's compression on the upstroke, but the downstroke is creating insufficient power.

something fairly serious then (Valve, gasket, rings, piston...)

It really is time for a compression test with a guage/gauge!

It really is time for a compression test with a guage/gauge!

Whats the difference ;)

Just the spelling. Amazing the number of 'guages' on the Mini boards !!

Well Minors don't come with guages do they. Maybe Minis do

Took Phyllis up to the Moggy Factory today, and they did a compression test: 110, 100, 20, 100. There was sufficient on no.3 to blow the cork out (though I could have pushed it in a bit tighter, I suppose) but as the readings show, it was well down.

Took the head off, and sure enough the exhaust valve for No.3 cylinder was chipped. They swapped the valve for another one, and she's purring happily again. Hoorah!

Excellent! Compression test reveals all!

You must have been able to feel this on the starting handle ! But the readings at 100/110 are miserable too - either all the valves are leaking - or the rings/bores are badly worn. You should get 160 psi compression on a healthy engine. Surely when the head was off anyway - they ground in ALL the valves ? Did they do another compression test at the end of the work ?