Knocking. Leaking exhaust?

[Tom and Maria]

Right. I am running a rebuilt engine which I did myself on a bit of a budget two years ago. Everything in the bottom half was replaced with a reground and rebore. I've covered about 8000 miles since then.

When I rebuilt the engine I reused the old valves, which weren't in tip top condition, I'd have liked to have replaced them but money didn't allow. I've been planning to rebuild the head since then but you know how it is...

Over the last 1000 miles it's become clear that oil consumption has gone up rather a lot and there has been a small but noticeable drop in power. I've put this down to leaking exhaust valves increasing pressure in the block and also dropping compression (confirmed with a comp test). Oil is leaking from the timing chain cover plus the dribble pin.

I'm also experiencing a knocking noise when accelerating.

Hoping that this was leaking exhaust noise I covered the pipe with my hand this afternoon to see what would happen. Once the pipe was block the revs dropped but didn't die. There was a loud hissing of escaping gas under the bonnet. I bunged up the pipe with a screwdriver wrapped in a cloth and moved the the front of the car. I was expecting the leak to be from the manifold or exhaust connection, but I was coming from completely the other side of the engine and sounded like it was escaping from the distributor!

Before I get on to my usual supplier for a new set of valves etc can I ask whether these symptoms sound like excessively worn valves/ seats? The knocking is loud, but doesn't have the hammer and anvil sound of big ends. Could this be exhaust noise?

Also, before someone asks, the oil filler and tappet breather are also chugging away under the bonnet with fumes.

I'm really hoping that the presence of the knocking indicates that this is valves not broken rings.

Thanks as always,

Tom

[bmcecosse]

I suppose it could be leaking valve stems .....but ..... that tends to be very minor. I would lift the head and check the valves and the guides. Possibly needs new guides - get the Mini ones with the small ridge to hold the better 'top hat' oil seals - only fit them on the inlet valves ! While the head is off you can have a look at the bores - and maybe pop the pistons to check the rings.......

[Tom and Maria]

Thanks Roy, that's exactly what I was planning on doing.

It's not burning any oil that I know of - no white smoke, so hopefully the new guides will do the trick.

I'll post back once this is done in the next week or so.

Tom

[bmcecosse]

And make sure there is plenty of suction on the crankcase breathers !

[Tom and Maria]

It just goes to the air filter at the moment, but I have an hs4 in the shed which might need to see action. Tried tapping the spacer block On the old, ailing engine which worked but gave a rough idle.

[kennatt]

Worn valve stems can cause oil consumption,but not very much crankcase compression,wouldn't think enough to cause fumes from filler.

[Tom and Maria]

Just back in from doing a compression test. With the engine hot and the throttle open, three pistons pushed 170 dry, the fourth only managed 115. I followed this up with a wet test on the bad pot, which raised the result to 140. Didn't bother doing wet tests on the others.

So this suggests I do have broken rings in one cylinder. Is it possible to buy small quantities of rings?

Another strange thing was that the two front spark plugs were very black, whereas the two rear ones were the nice coffee colour you'd expect. I check the plugs every few months, so this was a bit of a surprise. What would cause this? Blown head gasket?

Looks like it will be head off and piston number two out ASAP.

Tom

[kennatt]

A blow gasket would show up on the comp test IE Low comp but no increase on oiling,similar to burnt valves and usually water loss,and generally accepted that the offending plugs would be white or clear.Certainly not black.
I fear that you have a ring/bore problem,and I think individual rings are hard to come by from new but someone may have some on here,believe someone else managed to get some a short while ago .off with its Head and piston out I'm afraid.

[bmcecosse]

Yes - pop that piston to see what's going on.... Worry about the plugs later.

[Tom and Maria]

Removed the head today and pulled the piston and valves. Two of the exhaust valves were in quite a bad way and I'll be replacing the whole set. On pulling the offending piston I found that the top ring was broken in two. Luckily the bore was undamaged and still like new.

So, the question (which I'll also be posting on the wanted section) is - does anyone have a spare +20 top ring which they'd be willing to sell? I REALLY don't want to have to buy a whole set as I've just bought my first house and need every penny!

[Tom and Maria]

Spent the day today putting the engine back together after KAR in Barnstaple were able to supply me with the ring I needed. The compression is now equal across all cylinders and the car is much improved, although I can now hear and feel that the timing is slightly out - it's too far advanced and is making a roaring noise in mid revs. As there is no more room to adjust with the knurled adjuster I'll need to twist the distributor and reset the adjuster to remedy, but not a big job by any means.

[bmcecosse]

How do you know it's advanced? Is it pinking? Or kicking against the starter?

[Tom and Maria]

It's pinking slightly (jangling noise) and the roaring noise occurs when accelerating hard, so I figure it's when the vacuum advance is at maximum. I'll run a few test drives up a steep hill and find the best position from that.

[bmcecosse]

Accelerating hard is when the vac advance is minimal... Probably zero.

[Tom and Maria]

Ah, I'd always assumed it was the other way around, although what you say makes sense. I'll use the steep hill method to set it right, but will also use a test bulb to see what the end figure is, just for interest. From memory its about 7degrees advanced at the moment going by the notch on the crank pulley.