Cylinder pressures

[Johnny]

My 1108 cc engine develops a mid range misfire. It seems to idle smoothly enough.

When I checked the plugs and measured the cylinder pressures I got the following results (with negligible leakdown in any cylinder):

Cylinder 1: a tan coloured plug and 190 psi
Cylinder 2: a tan coloured plug and 190 psi
Cylinder 3: a much darker plug, wet too and 235 psi
Cylinder 4: a tan plug and 240 psi.

Does anyone have any suggestions as to what may be going on please?

Johnny

 

[AdamWilkes]

The pressure numbers depend on engine compression ratio - my 1108cc F6 van is low compression (8.3:1), so my pressures would be lower than with a saloon engine (9:1 or higher).
I've read that 20 times the compression ratio in psi would be roughly new cylinder pressures.

Higher than expected pressures could indicate carbon build-up on a piston or in the combustion chamber (carbon build-up reduces volume, hence increases pressure when the piston rises in the cylinder). To me, the cylinders 3 & 4 numbers seem too high. Possibly the engine needs a de-coke (cylinder head off, clean everything inside).

Possibly there is oil leakage into cylinder 3; given the high pressure I wouldn't expect broken piston rings or head gasket leaks in there so would look to valve-shaft seal leakage. If you are going to do a de-coke, replace the valve-shaft oil seals as well & job done!

Otherwise, try 'Oil Stop Leak' or similar poured in the engine oil - it worked for me once with a 26-year-old SAAB probably on original (age-hardened) valve-shaft seals. This stuff swells & softens old seals where they may have hardened and then worn a gap.

Misfire from burning oil ? Or maybe pre-ignition due to carbon build-up in cylinders 3 and/or 4 - compression ratio now too high, or carbon staying red hot ?

 

[orsome]

The pressure numbers depend on engine compression ratio - my 1108cc F6 van is low compression (8.3:1), so my pressures would be lower than with a saloon engine (9:1 or higher).
I've read that 20 times the compression ratio in psi would be roughly new cylinder pressures.

Higher than expected pressures could indicate carbon build-up on a piston or in the combustion chamber (carbon build-up reduces volume, hence increases pressure when the piston rises in the cylinder). To me, the cylinders 3 & 4 numbers seem too high. Possibly the engine needs a de-coke (cylinder head off, clean everything inside).

Possibly there is oil leakage into cylinder 3; given the high pressure I wouldn't expect broken piston rings or head gasket leaks in there so would look to valve-shaft seal leakage. If you are going to do a de-coke, replace the valve-shaft oil seals as well & job done!

Otherwise, try 'Oil Stop Leak' or similar poured in the engine oil - it worked for me once with a 26-year-old SAAB probably on original (age-hardened) valve-shaft seals. This stuff swells & softens old seals where they may have hardened and then worn a gap.

Misfire from burning oil ? Or maybe pre-ignition due to carbon build-up in cylinders 3 and/or 4 - compression ratio now too high, or carbon staying red hot ?

Hi, i did some pressure test on my restored 845cc engine,all readings were exactly 140 psi, where i can find original values what those should be?
Another thing, sparkplugs look like this and to me at least cylinder 1(which is towards driwer) burns best? Any assumptions what could be wrong on 1-2-3 , inlet valve clearances too tight, distributor /distr cap most close to cyl 1, diff spark plugs quality, ignition wires(of course lengths are diff. I am questionning this due petrol owerconsumption? difficult to start and jerky drive.It takes revs fast and runs finally good and has electric ignition....and also tremendous Zenith 28 IF carb.
Some advices are highly appreciated.
Yesterday i did check and clean sparkplugs and check clearances. All Ok.Then i did check the distributor cover(it's not the original one) and looks like there is some kind of wearing(or electric cavitation on each of the nips inside the cover.Nips are here of aluminium and the original was of copper.What do you think- does the rotor arm and distr cover work together fine and/or the distributor sfat has worn out that it is woffling around? Motor started fine w-out choke after cleaning of the sparkplugs but it was not very smooth more like it was running with three cylinders at forst and could not resist more gas, like it was suffocating but when use choke very carefully started to run better(all the tiome w-out air filter and teh housing.