Ducati Monster Forum

So as part of my through 15k checkup, I want to do a quick compression test to make sure that both cylinders are holding the same amount of pressure. That being said, I was planning on just using the starter to pump up the compression. I plan on disconnecting the sparkplug on the cylinder that I'm not testing but how do I disable the injectors? Can I just turn off the fuel pump or is there residual pressure in the lines. I'm assume there wouldnt be since the bike has a return loop on the fuel.

Any thoughts? I may want to run multiple tests but I dont want to flood the motor while doing them. Also for consistency I want to run the compression at the same engine temperature so I dont want to run the bike inbetween tests.

I would go for the fuse(s)...   [thumbsup]

You mean removing the plug; not just disconnecting the high tension wire, right?

+1 on removing the fuel pump fuse

For the cylinder i'm NOT testing, all I'm doing is disconnecting the cable on the plug. Is there a reason I should take the plug out? I want to obtain real life compression results as much as possible. Unloading the cylinder that I'm not testing makes the test less valid for a real life scenario. But if there is a reason I should I would be more receptive

I don't think you're looking at this correctly.  Ducatis are 90 degree L-twins.  The pistons aren't in compression at the same angle from each other.  One piston fires 450 degrees (I think) after the other, while that one fires 270 degrees from the first.  As a result, with the starter, one gets a bigger "run" at the compression phase than the other, if you leave the plug in.  Does it make a measurable difference?  I'm not sure, but I'm betting it does; especially with the smaller batteries.  If you pull both plugs, then you get just the affects of the one cylinder's compression stroke.

By leaving in the one plug that is not being tested, and no fuel is being released, then I would think that it would put excessive pressure on the starter to turn over correctly. By pulling both plugs the starter can turn over freely giving you independent readings on your compression gauge.

I don't know if leaving in other plug would hurt the results, I always pull all plugs, unplug spark and fuel

But these were in carb'd bikes not FI

Certainly no reason to leave a plug in, no advantage. 

Both plugs out, throttle open.

Crank the engine over with the pump fuse out first to relieve residual pressure.

i unhook the rotation sensor.  the ecu will do nothing as it doesn't know the engine is turning, and also that way it's not trying to spark unplugged leads.

a comp test is just a test.  it bears little relevance to what is actually happening when the engine is running, it just tells you if something is wrong at a speed slightly higher than normal cranking.

Doesn't matter if the fuel injection is on ( or carbs are full ), there's so little airflow during the test even with the throttle wide open, and a bit of fuel vapour won't hurt the instrument. Pull the fuse if you wish.

Put the plugs back in the HT lead caps and tie them to something metal, not a good idea to mess with the flow of the green smoke..........

A normal comp test is reassuring.

You guys are making sense.. I'll pull both plugs and pull the fuel pump. I'll probably just run the bike with the pump disabled and then measure when everything cooled down.

Thanks for the help