A compression test is a good starting point before doing any engine service work. There are lots of videos and information available on this subject on the internet. Notes: You need to do the test at full throttle. With CV carbs the slides will not lift unless the engine is running, so best to lift them manually or disconnect the carbs. Don't use a $10 eBay compression gauge and expect accuracy. Your el cheapo gauge will give a comparison between cylinders but the numbers might not be accurate enough to compare to your service manual. If you are going to buy instruments or gauges then look for good quality items. If the cylinder has low compression you can add a squirt of oil and retest. If the compression improves then you have worn rings. Some ignition systems do not like being run without the spark plugs being grounded, so put your plugs in the caps and let them spark as you crank. Make sure that you move them away from the spark plug holes or they could ignite the fuel air mixture that is being pushed out. If you can crank with the kill switch on then you don't need to worry about this. Ideally with a warm engine. The leak-down test was invented by Smokey Yunick. If you ever want to read some good books about engines then read his. Yes, his books are about car engines, but you can still learn a lot. Here is a good video on using a leak-down tester, this one is a Snap-On brand. Notes: The narrator says TDC (Top Dead Centre) but he means TDC on compression not exhaust. As you rotate the engine to bring it to TDC on that cylinder, you can place your finger over the hose attachment to feel the pressure on the compression stroke. You will feel nothing on the exhaust stroke. You will need an adaptor to suit the size of your spark plug thread. Bent valves are very rare on OHC engines (except when owners bend them while doing maintenance). On a motorcycle it is often a riding valve (no clearance between valve and camshaft) or a burnt exhaust valve. All comments and suggestions to improve this thread are welcome. cheers Blair
Compression tests should ideally be done when the engine is warm as well - that's pretty important. What is the reason why you need some sort of "certified" pressure gauge? I'm not sure I agree with that statement... I might see how my $25 chinese set compares to a certified gauge at work to attempt to reassure my skepticism that the variance wouldn't be significant. Digital gauges on the other hand, I agree that caution is well placed.
Thank you @joker I will edit accordingly. With regard to the quality of the gauge, when I post some advice I err on the side of caution. An inexperienced owner might buy the cheapest piece of junk gauge they can and after using it decide they need to pull there engine into a million pieces But you are right I am going a bit overboard so I edited that as well. cheers Blair
lol well let me at least add in my reasoning! With a compression test I'd say you're testing between something like 120 and 180 PSI with a service limit maybe of 115 (I'm just pulling figures off the top of my head as an example). The reason why I say the variance shouldn't matter much is because as long as you're between 120 and 180 - you're still within spec. A motor will still work at the lower end of that spectrum, but it just won't work as well (especially on startup). If you get a low compression motor spinning fast enough, it will probably still run but you'd have to keep the RPM up for it not to cut out. It's only when you're on the bottom of that range where the accuracy starts to come into play, but if you're that close I think you've got bigger problems than whether or not you're 10-15% off on your pressure reading
