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

• Introduction
• Testing the Compression
  • Procedure
  • Interpreting the Results
  • Zero Compression
• Reasons for bad compression (not F650)

Also check these FAQs:

• New Engine FAQ
• Classic Hard Starting / Poor Running FAQ
• GS Hard Starting / Poor Running FAQ
• Rotax 655 Troubleshooting FAQ

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.

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.

• If the two numbers are close your rings are FINE.
• If you get similar (poor) compression results both before and after the squirt of oil, you probably have a valve sealing problem, either worn, misadjusted or burnt.
• A new engine should have something between 10-12 bars. After a while this reduces to 8-10 bars. Service is needed when it drops below 7, but you'll recognize that because the engine won't run correctly (no power).

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.

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

• What I would check first is the tightness of the sparkplug. Second look at the plug to see if it has any melted metal attached to the electrodes (indicating a melted piston crown). Third check for proper operation of the starting de-compressor. Anything else is likey to be a major problem. Richard #230
• Do you know the guy? Is he reliable? Call me suspicious, but when you hand keys to a total stranger and he says ZERO compression, bells would start ringing in my head. I'd want to see proof of ZERO compression.

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

• Incorrect valve adjustment/leaking valves. Have the clearance checked. echo

• Check the compression. Pour about a teaspoon of oil down through the spark plug hole and turn the motor over about three times. Then check the compression again. If the second measure is significantly higher than the first, the rings/bore are the problem. If not... might be the head gasket, might be valves.

  Better yet, perform a leakdown test. Set the motor on TDC. Put the TDC bolt in. Attach an airhose to a fitting that screws into a spark plug hole. There should be a pressure gauge on there. Set the regulator for 100PSI. Read the gauge. Subtract the reading from 100 for % leakdown. If it is more than about 5%, take a stethascope and place it at various points on the motor to determine where the air is leaking. Flash 412 (CO)

• What the others said and, I will add, if the compression loss is from the ring-bore relationship you should ask about the air filter and check the condition of it. Machines ridden in dusty conditions are subject to inhaling mucho abrasives which wear down the parts mentioned. Real enduro riders put a lot of emphasis on having additional clean air filters. Additionally, I have seen motors with poor compression because the spark plug was either cross threaded or loose. Valves/valve seats or rings/bore or spark plug hole other that a cracked cylinder or a tiny hole in the piston= all the options, IMO. Ike647