Poor Compression

[Village Idiot]

My son and I have just completed compression testing all four cylinders. Results in PSI were 30,15,25,27. With oil injected into each cylinder these rose to 75,45,67,60. I then checked the pressure gauge against a high pressure bicycle pump and up to about 85 psi, there was agreement. The engine actually seems to run rather well, with less power than a modern car, but perhaps not as much as it should have. (Top speed is 70 mph on the level, but by that point perhaps the speedo was interacting with the suspension to fool me!)

Clearly there is some leakage past the piston ring seals, but it looks like there is also some from the valves. Any thoughts?

[bmcecosse]

These readings are DESPERATELY low - I find it hard to believe the engine actually runs....... Were the tests done on a hot engine/all plugs out and throttle propped open ?? But if it can do 70 mph - and you don't get pulled for causing a smoke screen - you may as well continue to use it! Meantime - start looking for another engine.....

[Village Idiot]

I thought the readings were desparately low too, which is why I did a check on the gauge. There is no smoke from the exhaust and the plugs look quite good - a sandy colour, not black and oily. You are probably right about another engine, but I'd quite like to have a look at this one. I assume that both the valves and the piston rings have gone, but I'd have thought there would be oil everywhere and a black cloud from the exhaust. I suppose it's possible that the gauge is reading right but that there is a leak in the hose that feeds it.

[bmcecosse]

Certainly the one that was reading '15' (really - are you sure??) has a particularly serious problem. That could well be a burned valve - so it may be worth lifting the head and grinding in the valves.....

[Village Idiot]

I tested by taking all plugs out, air filter off and with my right hand lifting the carb piston, pressed the red starter button. I gave each one a good ten or fifteen seconds cranking until the gauge pressure stopped increasing. Thanks for your advice so far. I think I'll take a look at the valves over the next few weeks. (I try to keep my son involved, so have to synchronise with his school and social life!)

[bmcecosse]

Lifting the piston does nothing - the throttle needs to be open - a brick on the pedal works well but remember to remove it after the test. However - I doubt it will make a huge difference - so, off with it's head!

[Village Idiot]

Many thanks. I did it with a warm engine (sorry for not answering your previous question fully) and assumed that lifting the carb piston would ensure a good airflow into the engine. Like you, I don't think there will be a great improvement, so yes.... "Off with its head".

[mike.perry]

Try cranking the engine over on the handle with the plugs in. If those are the true readings you will soon tell.
I am surprised that it even fires.
What size engine?

[Village Idiot]

It's a 1098 cc engine. I'm not sure what cranking by hand will do that cranking on the motor will not. The good news is that the engine still works! If I hadn't done the test, I'd not be too concerned. I'll check tomorrow with a brick on the throttle, but I'm not too hopeful. Perhaps it wouldn't start so easily if the compression was right!

[mike.perry]

If you crank it over by hand you can feel the compressions. With those readings there should be very little resistance

[Village Idiot]

The compression when hand cranked is reasonable and is pretty much the same for each cylinder (perhaps "equally poor for each" might be a better description.) I've repeated yesterday's tests this morning with the same results, so something is amiss. But on looking carefully at my compression testing kit, I think there may be some problems. (Bad workman blaming his tools?) There is a "dummy sparkplug" that connects to a hose via a "click-on" fitting. The dummy sparkplug seals not on a washer, but on an O ring. I took a washer from a spare spark plug and threaded it on, but I'm not sure if it's sealing properly. Also, the fitting into which the dummy spark plug fits may be leaking. The good news is that it all still runs. Thanks for all your help, but at some time, I'll remove the head to have a look.

[mike.perry]

My compression tester consists of a dial, a rubber tube and a brass screw thread in to the plug hole, sealed with a rubber O ring. I don't know what a dummy spark plug is for.

[bmcecosse]

If #2 always reads considerably lower than the others - no matter what the actual readings are - there is something amiss in there. So - head off time.

[Village Idiot]

Absolutely right! I'll get on with it and look forward to buying a bag of spare parts. Many thanks for your advice.

[bmcecosse]

All you need is a head gasket ~ £5/6 at your local autoparts place - ask for one for a 998 Mini.......

[daveyl]

You could try the gauge on another car. As said, those readings seem too low for it to run.

[MarkyB]

You can use your thumb over the plug hole, it doesn't give a reading but if you can keep it there while turning the engine over on the starter then the compression is too low.
Just keep an eye on where the HT leads are when you are doing it!

[kennatt]

try another tester if you can get one,with readings as low as that I can't see how there isnt clouds of blue smoke and excessive oil comsumpsion,it would also be blowing fumes out of the oil filler(is it) and if true ,from cold I don't think it would even start,let alone do 70.

[Village Idiot]

I've just tried again with a different gauge. Results for 1to 4 are 178, 148,162,172 psi. With oil added to cylinders, these inceased to 218,178,195,210. I think this gauge must be reading high ( I had assumed that 150-170 psi would be great for a petrol engine.) This confirms I have a problem with #2, but my real concern is the increase in every case when there was a drop of oil sealing the pistons. Have I a more serious problem with the piston rings?

[MarkyB]

Do you get blue smoke from the exhaust?
The head will have to come off anyway to sort the valves out, then you can examine the sate of the bores.