DB18 compression test.

[davidian]

Hi The sooty plug problem has only arisen since I renewed the carb seals and needles, prior to that they had a pink deposit on the electrodes , If the engine was left ticking over and then revved there was some blue smoke, which soon cleared, I think the idea of doing a Dry & Wet compression test is to point to any problems with valves , head gasket , broken rings etc because the addition of oil into the bore would not improve the reading of a duff cylinder, Today I will go into the workshop and try to find the old carb needles and see if I can get leaner running with these .
Regards David.

[Brian-H]

I think the idea of doing a Dry & Wet compression test is to point to any problems with valves , head gasket , broken rings etc because the addition of oil into the bore would not improve the reading of a duff cylinder.

Yes it's standard practice to do a compression test "dry", and then squirt in oil and do it again. Those pressure readings seem about par, I wonder what they would be like with a fully reconditioned engine (new liners, new pistons/rings, valves and guides fully reconditioned) - would be a costly experiment just to see if those "wet" readings you got would be achieved on a "dry" fully reconditioned engine.

[davidian]

Hi This post seems to have solicited a lot of debate which is much appreciated. I know little of the history of my car prior to 1990 so have no idea whether any work has been done to the engine in the past,but can accept that at 70 years old its not in its prime,so oil will find its way down the valve guides and past the rings, but provided its not excessive I am quite happy . When I was a lad a trick that locals used to improve an engine was to squirt Redex into the bores, let it soak in overnight then give the engine a good revving , that certainly smoked out the neighborhood.
Regards David.

[grahamemmett]

Hi David
Quite by coincidence I did a compression test on my DB18 today.
All were at 130 in old money except number 2 which was at 118.
This cylinder is continually oiling up it’s plug resulting in the engine running only on 5 cylinders.
Looks like I’ve an issue with valve sealing or broken piston rings.
Luckily I have a spare head to try although the cost of the gasket set makes me hesitate.

Best regards

[davidian]

Sorry to hear of your problem, could you not just re use the old gaskets for testing purposes in order to pinpoint the cause then invest in new gaskets when you have established the fault be it valves or rings .
Regards David.

[Christopher Storey]

The omniscient Brian is not quite as all-knowing as he seems to think . In my quite long motoring career ( 1961 onwards and about 1.5 million miles ) I have seen a lot of blue smoke ! In the 1950s and 1960s it was common to see considerable amounts of blue smoke emitted , under load if it was worn rings, and on the overrun ( often in addition ) if it was worn valve guides. These worn engines diminished in number from about 1962 onwards when the 10 year test started to take its toll on very worn out cars, and this was accelerated when the tests for visible smoke etc were introduced later on

[qantasqf1]

Well now, isn’t this jolly fun? Just to add to the discussion/argument/squabble, Sir Harry Ricardo in his book The High Speed Internal Combustion Engine debunks the theory that worn valve guides oil up the engine on the overrun. He puts the issue down solely to worn piston rings. Just sayin’!
Steve

[Brian-H]

Yes it's been quite humorous in places

When I was a lad a trick that locals used to improve an engine was to squirt Redex into the bores, let it soak in overnight then give the engine a good revving , that certainly smoked out the neighborhood.

Yep Redex certainly did make a car look like a WW2 battleship trying to evade aircraft

FWIW my dad was a Chartered Mechanical Engineer, a lecturer in Mech Eng and Thermodynamics, I have his old recommended texts (all in imperial), and he did 10 million miles of motoring. I'll let you guess which one of those isn't quite on the mark

btw if he were still with us he'd confirm what I said - though he would, being my dad

[Christopher Storey]

Now let's see- assuming a 60 year motoring career - 10 million miles requires 160,000 miles a year, or 3,000 miles a week, throughout the entire period. Sure about those figures , Brian ?

[Brian-H]

Hi David
Quite by coincidence I did a compression test on my DB18 today.
All were at 130 in old money except number 2 which was at 118.
This cylinder is continually oiling up it’s plug resulting in the engine running only on 5 cylinders.
Looks like I’ve an issue with valve sealing or broken piston rings.
Luckily I have a spare head to try although the cost of the gasket set makes me hesitate.

Best regards

As well as the cost of the gasket set, it's the fact that removing a cylinder head these days seems daunting for many reasons, not least of which is the fear of scratching the body work, and the fear of alternatively removing the front wing assembly and putting that somewhere safe. Even when that's off, eyesight, short term memory, those hamper the progress (well they do in my case). Then there's the possibility of having to take off the sump to get the conrod undone to look at the piston rings. Having said all that, I'd never be certain that the job was done properly unless I did it myself, so if you've got a big enough garage with lots of spare room, go for it.