A motorcycle needs to be able to compress the air/fuel mixture that enters the piston cylinder. Each cylinder on the motorcycle must do this to a certain pressure range (which varies engine to engine) in order to be able to produce a decent amount of power to turn the crankshaft when the spark plug fires. A number of symptoms, from running rough to losing power could indicate a compression problem. It is really easy to test the compression on the motorcycle yourself, but you will need a compression testing kit to do this. I got mine for $26 posted on ebay: First you need to prepare your work area. Make sure you have good access to the top of your engine. I had to remove the fuel tank, air box, thermostat and thermostat hoses to get to the top of mine. This will vary engine to engine. (Note: I have also removed the valve cover on this bike. You do not need to do this for a compression test because oil will still be pumped up to the cams and potentially drip down the side of the engine making a mess. I had my cover off already so I put measures in place to catch the oil that dripped out.) On an engine that does run, it is best to do this test warm, so warm up your engine for a few minutes until you can take the choke off (if applicable) and the bike is idle. On an engine that doesn't run or hasn't run in a while, you may not be able to do this but the test is the same. Once you have done this, unplug your spark plug leads and remove your spark plugs completely from the bike. Be careful not to drop them as they are fragile. Next you need to assemble your compression tester. Some of these are "universal" and therefore come with multiple adapters. Check one of your spark plug threads against an adapter to ensure you have the right size. You do not want to go forcing the wrong thread in your spark plug hole, this is very expensive to fix. Next assemble your compression tester by putting the adapter onto the hose and tightening it until the o-ring seats properly. Ensure this is relatively firm (finger tight is enough) so you do not get any air leaks, and also so when you come to unscrew it from the engine it doesn't come detached leaving your adapter in the top of the engine (otherwise it will be an absolute pain to get that out!) Then insert the other end of the hose into your pressure gauge (not the end you just attached the adapter to). Notice this is a collared connection which will require you to pull the collar back before firmly inserting the end of the hose. You should hear a "click" and the collar will be firmly over the join and it should not be loose. Once you have done this, ensure you have no residual pressure in the gauge by pressing the pressure release button on the side. The gauge should read zero when you start. Next, prepare a record. Label your cylinders so you know which is which. Usually, if you are sitting on the bike, #1 cylinder is on the left, with #4 on the right although some manufacturers reference their cylinders differently. This is not that important provided you know which reading came from which cylinder. Remember to write down the pressure unit of reference you are using so you don't get confused. On this gauge there is PSI and kg/cm^2. It doesn't matter which you use as a reference because they are interchangeable and you can convert either reading to the other unit (google an online converter). Check your manual to see what your motorcycle uses, because you are checking to see each of your cylinders falls within the manufacturer's range. You should also plan to do the compression test twice on each cylinder, one "dry" and the other "wet". I will explain why below. Screw the adapter end of the pressure guage into the spark plug hole of cylinder #1. Make sure it is quite firm so twist it around until you feel it seat and the o-ring engage. This will ensure you do not have a leak and give you an accurate reading. Turn the key in your bike so that the ignition is on, and place the gauge somewhere where it can sit and not get damaged (you can hold it if you want to). Hold the throttle fully open and press the starter button for around 10 seconds. If your bike has a kick start only it is a bit trickier, but essentially you need to kick it over a few times (I would aim for 5 or 6). As you are cranking you will see the needle on the valve jump up in pressure, and it should settle and max out at a certain point. That is your reading for this cylinder. Record it. Repeat this process for each cylinder, remembering to press the pressure release button on the gauge before installing it on the next cylinder. Eventually you will have readings for all cylinders recorded. Now you need to do the test again wet. For this you will need some motorcycle engine oil (whatever your bike uses). I used some of the old oil from the last oil change so it's nasty and black. Not ideal, but it will do the job. I put about a teaspoon of oil in each cylinder through the plug hole before doing the exact same pressure test again. Record all your readings and note any differences and your motorcycle service manual to see what readings you should be getting. Usually the manual will give you a max-min in kg/cm^2 or PSI, a value to show an acceptable variation between each cylinder and a "use limit". The "use limit" is the minimum pressure required in the cylinder for the motorcycle to be operable. Now it's about knowing what the readings mean and if you are troubleshooting where to look next. Note: "significant difference" in my mind is >5% variation. Scenario A: If each cylinder on the dry test is within the manufacturer's range AND doing the wet test did not make a significant difference to the readings you obtained from each cylinder, your compression is probably OK. Scenario B: If each cylinder on the dry test is outside of the manufacturer's range AND doing the wet test did not make a significant difference to the readings your obtained from each cylinder, it is likely you have valve seating problems. Next step is to check your valve clearances and adjust where required. Scenario C: If each cylinder on the dry test is outside of the manufacturer's range AND doing the wet test DID make a significant difference to the readings you obtained from each cylinder (like mine did above) you have a larger problem. It is likely that I will need to look closer at the engine now to troubleshoot a problem with the pistons and rings. It may mean I need to partially rebuild it, but I'm up for the challenge. The reason the "wet" test is an important part of this process is because the oil seeps to the edges around the top of the piston and fills in any voids/pressure leak points that wouldn't have been there during the dry test. It allows the cylinder to build much more pressure during the short space of time you are cranking. If there were no voids or leak points, there is nothing for the oil to fill and therefore no significant difference in the readings. Happy troubleshooting (I hope you don't end up with the same news as I just did!)
yep, exactly what i said but in more words.whether hot or cold the gauge gives a reading. i use auto tranny oil but engine oil is the same..i operate on a 10% variation but all cylinders should be less than 5psi.to set up new rings the ends need to be inline with the piston pin.oil ring first with end gap opposite each other, second compression ring opposite to top oil ring, 1st or top ring end gap opposite to 2nd rings. now they might need rotating depending on spark plug hole as it must not have the end gap under the flame travel.just rotate each ring till the first is on opposite pin rod line to the spark point. then rotate 2nd and oil rings to the opposite end gaps on ring above. oil rings and fit piston into bore. remember rings need some end gap and are not meant to be pulled and pushed. the compression stroke of the motor pushes the rings against the bore.hope this helps.
I'll fiddle some more over easter, see where we get to. It's not worth a lot to me so I could still wreck it and break even I reckon.
Compression tests are usually taken with the throttle wide open. A slightly more difficult task with CV carbs as the slides only lift with sufficient vacuum. You will have to remove the carburettors or hold the slides fully open manually.
