Compression Test An engine compression test will tell you if your cylinders have good compression. An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily. Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled. How to check compression with a gauge To check compression manually with a gauge, all the spark plugs must be removed. The throttle must also be held open. The engine is then cranked for a few seconds using the starter switch while a compression gauge is held in a spark plug hole. The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders. The individual cylinder readings are then compared to see if the results are within specifications Rings or Valves ? If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a teaspoon of engine oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings. If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve. Leakdown Test A leakdown or cylinder leakage test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss. A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order. A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder. An engine is great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble. The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the crankcase breather vent would tell you the rings and/or cylinders are worn. A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems. A cylinder that has poor compression but minimal leakage usually has a valve problem such as a worn cam lobe or broken valve spring. If all the cylinders have low compression but show minimal leakage, the most likely cause is incorrect valve timing. The timing chain may be off a notch or two. If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel or ignition issues. I'd like to say I wrote this, but I must credit http://www.aa1car.com
Good post. First thing when buying a bike...do a compression test. My 2 cents... Recently did a compression test on a V-twin 250cc engine that had been laying around for some time (2+ years) and I had never heard it run. Compression was ZERO for front and back. Did the oil trick and front came up to 30psi; back still zero. After looking for large holes in the block and not finding any I rang my local Motorcycle head guy and asked the question. Short story...he said spray a heap of Inox down each cylinder and into the carb inlets and then crank the crap out of the engine (1 minute minimum) with the plugs out. Blow out whatever's left in the cylinders and redo test...180psi back and 150psi front. Thank you God. I'm not sure of the science behind this, but am grateful not to have to buy another engine. By the way, you can do a compression test on any engine even if it's out of the frame and doesn't have carbs hooked to it. All you need is a working starter motor and a battery pack (and a compression tester).
Great story The rings obviously became loose with the Inox in the bore, as they loosen up expansion takes place and "PRESTO" you have compression back. Sometimes life is just that easy, happy days!! Good for you mate, nice to have a win. Next time I have a pot with low or no compression I certainly am going to try this out. Great tip..................Thanks
Thanks Phil. I thought that spraying it into the carb inlets probably gives the inlet valves a clean and the up stroke probably pushes some inox through the exhaust valves too. Either way, you're right...it is nice to have a win! Cheers
Good article. The only additional point to note is that ideally, a compression test should be done when the engine is warm.
