How to do a compression test

[cleverusername]

I am getting suspicious of my Sprint engine, still won't fire, so I tried a compression test.

Got a reading of 90psi from cylinder 2, but not sure how accurate that was. The cheap Chinese compression tester self destructed before the end of the test. Ironically the compression fitting couldn't take the pressure.

So what is the correct procedure? Oh and how do you prevent the thread adapter for the spark plug hole getting stuck in? I had to resort to thread lock to remove it, that was after the connection on the gauge went bang.

[James467]

On a Sprint engine get yourself a Gunson Hi-Gauge compression tester. easier to screw in down the plug tube compared to my sealey sectional kit.

http://www.gunson.co.uk/product/G4101

I'd say you need a minimum of about 10 bar cold, about 140 psi. But I have had as low as 50 and the engine still started, that was on DTR.

Remember to put your foot on the accelerator fully and crank at least 10 times to get a good reading.

[tony g]

Also, when screwing gauge in, nip it up using the gauge itself as a tool and when the hose starts to twist thats tight enough. Its more about comparison between cylinder at this stage so dont be disheartened if you get 4 100's. It may be different when hot. Good luck

Tony

[cleverusername]

On a Sprint engine get yourself a Gunson Hi-Gauge compression tester. easier to screw in down the plug tube compared to my sealey sectional kit.

http://www.gunson.co.uk/product/G4101

I'd say you need a minimum of about 10 bar cold, about 140 psi. But I have had as low as 50 and the engine still started, that was on DTR.

Remember to put your foot on the accelerator fully and crank at least 10 times to get a good reading.

I did it with the throttle closed.

[tony g]

Not a major problem at this stage. Get 4 decent readings and assess from there

Tony

[James467]

Common theory is that the throttle must be fully open to ensure that the cylinder gets fully charged with air.

[tony g]

Yes I agree James, what Im getting at is it's rarely low compression that stops an engine from running. If theres a hole in a piston then that wont help of course. If this engine has been idle for a very long time compression will be lower due to no oil on the bores and rings hence me saying just getting a comparison for now is a good check.
If there is spark, fuel and some compression chances are it will bump start then go from there.

Tony

[James467]

Sorry Tony, I was replying to the previous comment.

You make a good point though!

[tony g]

Tony

[tinweevil]

Also test with all spark plugs removed. Testing with the others fitted will exaggerate the differences between cylinders.

[Karlos]

I always test with all plugs removed too. If you get a low reading add 2 tea spoons of oil into the offending cylinder through spark plug hole. Test that cylinder again, if the reading is the same suspect leaking valves or holed piston, if you get an increase in the reading suspect leaking piston rings as the oil has created a temporary seal and so produced a higher reading.

[cleverusername]

Yes I agree James, what Im getting at is it's rarely low compression that stops an engine from running. If theres a hole in a piston then that wont help of course. If this engine has been idle for a very long time compression will be lower due to no oil on the bores and rings hence me saying just getting a comparison for now is a good check.
If there is spark, fuel and some compression chances are it will bump start then go from there.

Tony

I have a new 015 battery to replace the knackered 038, so it is now cranking fast enough for a timing gun. I thought it was dead on, but in reality my guess was about 40 degrees out. Now at TDC with dizzy at max adjustment, so need to move that back a tooth.

Not convinced the fuel mix is right, so that is next. Then spark strength, I have switched to a 12v non ballast system, might have been a mistake.

Could have bad compression, but the pistons are sound with new rings, so I don't think it would be enough to stop it firing. I have to admit I much prefer fuel injection and ECUs, vastly superior technology to old SU's.

[tony g]

Is this a new rebuild? Or a long lay up?. Either way with a good battery and somewhere near tdc it should run. Fuel wise, on my sprint, if left for a couple of weeks it cranks quite a few times before firing. I pull choke and open and close throttle while cranking (to alter the venturi effect) and it always goes.
A 12v coil will now be running at less voltage when cranking so the ballasted coil would be my preference. Charged battery is a must of course.

Tony

[lazeruspete]

In my line of work we do compression and leakage tests, the main thing id say is its relatively meaningless without the engine being warm. but if you cant start it then having a set of cold readings is a start.

I've never heard of holding the throttle open whilst you do it though.

[James467]

Na, ive canged my opinon, just take the plugs out and stick your finger in one of the plug holes.

If you loose a finger it's got compression!