QuestionHi I have a 1971 Mk IV spit that I intend to restore, I was wanting to run a compression test, but I'm afriad I'm a complete novice in this respect - I've seen that I need the throttle fully open - is this a case of just pushing the pedal down? and do I need to disable the distributor and fuel lines to prevent flooding and spark? I could really do with an idiots guide to this process - I have been told that I should expect 120ish psi on all cylinders. Sorry for my ignorance and thanks for your help in advance!
AnswerHi Karl,
First pull all of the plugs and remove the small wires on the coil. (only one side is necessary but remove the wires that are colored white with a black strip on them) This disables the ignition system and so there are no loose hot wires and no sparks anywhere. You don't normally need to disable anything in the fuel system.
Have a friend open the throttle and be sure they spin the engine over at least 5 or 6 revolutions with the compression gauge in. The reason the throttle is held open is that a closed throttle lets very little air in and you can't compress air that is not in there thus you get a false reading.
This compression test is called a "Dry" test. When you finished and write down the readings. If you get one that is low take a second test of that cylinder.
You should now do a "Wet" compression test. This is done the same way except this time just before you put the gauge in, put about a teaspoon of engine oil in the plug hole and run the compression test again on each cylinder and write down the readings. If you have a oil squirt gun about 4 or 5 good squirts is about the right amount. There will be a difference between the wet and the dry test and that difference should not be more than 10% to 15%.
Generally the readings should be 125 to 180 PSI on all cylinders on the dry test. When a cylinder gets down around 100 PSI it just about will not fire. If you have one very low cylinder and the wet test didn't bring it up then it is usually a valve problem. If you have a low cylinder and the wet test brings the reading way up, it is most likely a ring problem. The wet test will only bring up a bad rings reading it can't bring up much on a valve problem.
A second and very important test that you can see after doing these tests is just spin the engine (right after doing the compression tests) and watch the oil pressure gauge. You want to see about 35 PSI on the gauge with the starter motor only spinning the engine. This is a rough test of the bearings and oil pump system.
All of these tests are 95+ % problem indicators. There is a 100% sure test but you need compressed air to run that test.
I hope this helps, let me know the results,
Howard
