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  #16  
Old 5 Feb 2008
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Performing a Compression test

Run the engine until it reaches normal operating temperature. Tests done on a cold engine usually show lower readings. Remove the spark plug leads and take out the spark plugs. The ignition system MUST then be disabled. If this isn't done it will continue to generate high tension voltages into the HT leads which will have nowhere to go with the plugs out. These high voltages will find another route to earth and can damage the ignition system or even the bikes ECU. Unplug the low tension ( 12v )connections to the coil . If you aren't sure how to disable the ignition system on your own Bike then check with a dealer before you start. It is also good practise to unplug the fuel injectors or disable the fuel pump,( just pull the fuse to the fuel pump) especially on Bikes fitted with a catalytic convertor. This prevents unburned fuel getting into the exhaust system during the test.

Screw the gauge into cylinder 1 and rest it somewhere you can see the dial while you crank the engine. Open the throttle fully either by opening the throttle wide open. If the throttle isn't open then air can't get into the cylinder and the readings will be far too low.
Crank the engine until the gauge stops rising and count the revolutions while you do so. It should normally take no more than 10 engine revolutions (5 compression cycles) to get a full reading. You can count the cycles by watching the gauge too - each jump of the needle is one compression stroke.
Write down the final reading and also make a mental note of how quickly the gauge rose on the first few cycles. Then just repeat for the other cylinder. Make sure that each cylinder reaches its highest reading after the same number of engine revolutions. If all readings are good then the test can end there.
If any cylinders are low then a "wet" test can be done. This involves squirting a few ccs of oil into the cylinder and repeating the test. The oil will help seal bad rings and increase the reading but won't affect it if the problem lies in the valves or head gasket.
By doing the above test you will know if the problem lies with the piston rings or worn bores

An engine in good condition should have readings within the specified range (preferably at the upper end of it) and with both cylinders within 10 % of each other. A perfect engine might have almost identical readings on both cylinders - it is certainly possible to achieve this on a really well blueprinted competition engine. A good cylinder will reach about 2/3 of its final reading on the first compression cycle and reach the full reading after only 2 or 3 cycles. If the rings are worn you often see a gauge rising in smaller jumps of 20 to 30 psi per cycle rather than one big initial jump and also taking more revolutions to reach a peak reading.
If both cylinders show similar psi but are below the minimum figure then this usually indicates excessive ring and bore wear due to high mileage.

One cylinder low means a bit more detective work. If the wet test improved things back to a normal reading then the problem lies in the rings or bores. If not then its usually either valves or gasket not sealing properly. To an extent I fail to see the point in worrying overmuch about exactly where the problem lies. Most of the time the cylinder head is going to have to come off so you might as well do that first and see what shows up.

Hope this helps ya!

Martyn
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  #17  
Old 5 Feb 2008
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yes, margus gave me some good pointers, I already speak with my mechanic and in the next 2 weeks will check all, I will write the result.
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  #18  
Old 19 Feb 2008
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Today I made the compresion test and was 11,1 (left one) and 11 (right one), so there are no problems, the mechanic told me that sould be because these last 3.000km I was riding almost all inside the town.
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