My bagger has 57,000 miles on it and I want to check the compression. I found the thread on the DIY forum, but it says to ground the wires from the coil. How do I do this? Leave plugs in and then attach to engine? or just remove plug wires at coil?

 

To ground the plug wires is simple, i would imagine just fix it so that when they fire, they fire over to a ground someplace on the engine, pull the plugs out of the head, put the plug wires back on them, and lay the plugs somplace where they are grounded..the spark will jump normally between the electrodes then..i dont know why this is important really?

dont know about bikes yet, but a car engine i pulled all plugs, screw the tester into the plug hole i wanted to test and watch the needle as i crank, needle jumps 3-4 times maybe 5, i cant remember its been so long, you should have your reading..put a little oil in the cylinder and the pressure will come up higher..better wait for someone here who knows, to tell exactly how its done.

 

I haven't done it on a bike yet but on my trucks it's just a matter of pulling a spark plug out and attaching a compression tester. Then pull the wires from the ignition coil (low voltage side) and hit the starter to turn the engine over a few times and read the tester.

 

Be sure to have the throttle wide open when checking compression.

 

Check first without adding oil. If you find pressure is on the low side you can then add a little oil into the cilender.
If pressure goes up after adding oil the rings are leaking some, a little or a lot.
there is also a leak down test where you see how long it takes for the pressure to leak down.

 

Not sure what year your bagger is, instead of grounding plugs, unplug ignition module, open throttle wide open, screw in compression tester, turn motor over 2-4 times. wha-la your done. hope this helped

 

Take a wire and ground the plug wire to a ground. THAT is what is meant by grounding a plug wire. Taking a spark plug and grounding THAT will cause th ECM to see the engine as in a cannnot fire condition.

The ECM SEES how a plug fires anymore. This ain't your fathers Oldsmobile. The ION sensing circuit can tell if the plug is trying to fire under cylinder pressure or not. No such thing as an 'ignition' on a bike with an ECM. If it has EFI and its newer type bike with Delphi ECM, ground the plug. THIS will in turn tell the ECM that you are working on the bike and will NOT even try to fire the injectors, plugs, etc. Before the engine even finds its compression stroke, it will 'see' the ground. A newer bike will not KNOW which stroke is combustion and which is exhaust. Everytime one hits the starter, the bike has to find that out using the crank sensor and software. Grounding turns THAT system 'off'.

A wire with two alligator clips is good. Or simply strip two ends of a hunk of wire, ball one end up and stuff into plug wire, take other end and place under a grounded bolt and tighten.

If it is an 08-10 touring bike, those have drive by wire throttles. Twisting the grip will NOT open the throttle plate. One needs to take off A/C filter and place something inside to HOLD the plate open. Plastic screwdriver handle comes to mind. IF... the ECM sees that plug grounded, it WILL open the plate like 20%, but I, me, do not think that is enough for a good comp reading. It does not hurt the plate motor to gently open the plate manually. If you use a finger, like me, be prepared for that plate motor to try to snap shut when you hit the starter. NOT enough power to hurt your finger, though, and CAN hold it open with NO problem.

Remove BOTH plugs for this test, even though only checking one cylinder at a time. I usually crank like 6-8 times, but I very well be wrong in that. Also... after you do both cylinders dry, if you squirt a few squirts of oil in the chambers and the reading changes a lot... one should do a 'leak-down' test next. A leak-down test is a compression test in reverse. Air compressor air is shot into the cylinder. There will be two gauges. One will tell how much air is being injected... like 100psi. The other gauge will say how much air is being CONSUMED. The consumed air is the 'leak'. Going past the rings or going past the valves, or going out that torn headgasket!!! Most likely can borrow a leak-down tester form Autozone, etc.

When one sees guys talking about CCP, that is nothing more than Cold Cranking Pressure.... the compression test! Hot Rods are 200 psi and above. Best all around "cam and pistons" bikes are 185ish. Under like 125? (not sure.... look in manual), may be a problem. A good 180-190 CCP bike will have plenty of power. NOT be gasoline or ping sensitive.

AND... it DOES matter what year bike. Do NOT ground a bike with an ignition. If it has a carb... it has an ignition. And... I have NO idea of how one does the early MM injection found on early injected bikes.

Im just a dumbass playing it forward, you know?

 

Thanks, guys. Lots of good information. It's a 2001 fuel injected.

 

mmmmm

That may be a MM fuel injected bike. Simply remove the plugs, let the plug wire dangle, run your tests, leave the plugs out afterwards like an hour to dry out the cylinders from the injector pulses.

You would be good.

Is you tests for normal maintenance to 'check' the condition of the bike? If so... NOTHING beats a comp check as part of that check. You ARE on the right track.

If there may be a problem with your bike? Share what you feel it may be, and maybe one of us can help.

I have found, on a sum total, on the harley boards, there is ALWAYS someone else who has either broken their bike in the exact same manner, or screwed one up, just the same too.

Fire away my biker friend!!!

 

What is wrong with the bike that you are concerned about the compression. Usually there will be some indication that you have lost compression when you ride the bike.