F650 Compression Testing FAQ

compiled & edited by Kristian #562
Dan #943, Hombre sin Nombre, Flash #412, 28/11/01
Please read the Disclaimer before attempting any work in this FAQ.
Last Updated: 15 April 2006, Winter #1935

Also check these FAQs:


If you have oil pouring out somewhere and believe the crank-case is getting pressurized, your first thought may be "Ah, it is blow-by past the rings". However before you check your compression, check the following FAQ. Counter Balance Seal Failure FAQ. You might also like to check the Oil Pressure Sensor FAQ, it is not related to pressurized casings, but is related to unknown oil leaks.

Testing the Compression

Unlike your car engine you can't just put a compression tester in one of the Spark Plug Holes, turn over the Engine and get a compression reading. The reason is because the F650 has a Compression Release Mechanism which essentially holds open one of the Exhaust Valves and activates at starter motor speed, so the engine will turn over for easy starting. If you try and take a reading you will not get an accurate one.


If your engine is starting and running reasonably well:

  1. Warm your engine to normal operating temperature.
  2. Remove the Tank and Remove the Valve Cover. You need to test your compression while you have the valve cover off. You can "restrain" the Compression Release Mechanism with a piece of electrical tape in order to do the test.
  3. Remove one Spark Plug.
  4. Insert the Compression tester in the hole.
  5. Unplug the other Spark Plug lead. REMEMBER that this is a dual plugged motor so unplug the second plug lead! (This is very exciting if you forget and bad for your compression gauge. Essentially the engine fires and will blow all hell out of your gauge and possibly your eyes, so be careful!)
  6. Twist the throttle WFO while compression testing. This simulates (worst-case) driving conditions and ensures no vacuum back through the carbs.
  7. Write down the number on the compression gauge.
  8. Pour about a teaspoon of oil down the plug hole and turn the motor over about a dozen times and then check compression again. When you are squirting in oil for the test, use engine oil or similar, NOT WD-40 or penetrating oil.

Interpreting the Results

Note that the above type of compression check (with warm engine) is fine for an engine that is starting and running reasonably well, and is a valid way to test compression. While the engine should be warm/hot to check true compression, in an old and marginal engine, the biggest problem may be getting it to start. Hot compression in an engine may be irrelevant if it will not start! One of the first noticeable symptoms of poor compression is hard starting. Starting problems in an old engine may be due to poor compression, and measuring the compression on the hot engine may not be a true indicator of engine compression while starting cold. Many engines with poor compression due to worn rings will start and restart when warm, but are hard to start while cold. So on a hard starting engine do a cold compression check first to see what you get.

Zero Compression

I am writing because my bike literally died yesterday on I 91 near North Hampton, MA. I was just cruising when it started to lose power. Got it to a mini mart and left the keys for a mechanic. He called me this AM. Says, ZERO compression. The bike HAS been sitting for 9 months. I was over in Asia for a while, but I took all the proper precautions prior to leaving, so I didn't expect any problems. Stabil in the tank. Changed the oil and plugs before the sit etc..

Anyone had something like this happen? I will take a closer look at the FAQ's and pray there is a simple answer. The guy at the bike shop is pulling the head now looking for a stuck valve or burnt piston. Could this have something to do with the cylinder compression valve? Jeremy

Reasons for bad compression (not F650)

I am going to buy another bike. An enduro as acomplement to my F650. I just saw an ad about an Husaberg FE400 1992 (4 stroke, single cylinder), which could be an option. The seller (I haven't talked to him yet) stated that the compression was bad, but the bike could be ridden and apart from that in good condition. He also stated that it was probably due to a bad cylinder head gasket. I am thinking that another reason could also be from bad rings/bore. Other reasons? spakur #1117