HELP 748 spark plug issue!!! - Ducati.ms

ceye04 · Jan 6th, 2012, 2:08 pm

So I recently had a problem where the 748 would not start. I did the usual diagnosing and decided to pull the spark plugs since I know I haven't looked at them since I purchased her in May 2011.

When I pulled the plugs, the rear plug was fouled. The ground electrode had snapped off the plug, causing her not to start.

Well I purchased two new NGK DR9EIX Iridium plugs, since I did some research and found many members here using the same thing.

After putting the NGK's in, she fired and ran beautifully. For about two weeks.

Just the other day, I went to take her to work and she would not start once again. So when I got home that evening, I decided to pull the tank and change the fuel filter and clean out the inside. After doing so, she still would not start. So I decided, last thing I changed was the plugs. So last night I pulled the plugs again.

Sure enough, brand new plugs and the plug on the rear cyclinder has the same issue. The ground electrode is broken off again!!!

What could be causing this?? Any help would be much appreciated!! I have added a picture of the brand new spark plug with broken ground electrode.

Shazaam · Jan 6th, 2012, 2:58 pm

Perhaps a fastener got dropped into the throttle body intake and embedded itself in the top of the piston - with just enough reduction in clearance in the combustion chamber to strike the plug's low point, the ground electrode.

ZetaTre · Jan 6th, 2012, 3:34 pm

You can't tell whether the ground broke off or melted. In the first case it could be some interference, in the second would be a lean condition...

Can you tell if it broke or melted?

ttpete · Jan 6th, 2012, 4:18 pm

If you bring the piston to TDC, you should be able to look down the plug hole and see the piston top.

pjonte · Jan 6th, 2012, 5:14 pm

Quote:

Originally Posted by Shazaam

Perhaps a fastener got dropped into the throttle body intake and embedded itself in the top of the piston - with just enough reduction in clearance in the combustion chamber to strike the plug's low point, the ground electrode.

Yea, but wouldn't it smash into the center electrode? It looks pretty much pristine in the photo - weird? Hopefully the engine excreted the broken bits.

ttpete · Jan 6th, 2012, 7:34 pm

One thing you might do is put a lump of clay on the end of one of those busted plugs and put it back in, then turn the engine over and see how close the piston really comes.

ProphetPVD · Jan 6th, 2012, 8:04 pm

Using the probe on a set of Vernier or Dial Calipers is probably your best bet. Please do NOT use a "lump of clay"... the last thing you want to do is actually drop something in there, especially if there is nothing physically wrong with the combustion chamber components to begin with.

The bike needs to be on a stand... remove both plugs, put the bike in gear, extend the probe on the calipers and insert into the threaded plug hole. Rotate the rear wheel with the bike in gear and at some point the piston will hit TDC. The recorded distance on the caliper is the minimum distance between the piston crown and the top face of the head where the plug threads.

If there is a locking screw on the calipers, lock them with this distance and grab an undamaged plug. Using the outside jaws (the big ones) put the inside face of one of the jaws at the point on the plug which is flush with the top face of the head. Depending on the age of the plug the area above this will most likely be discolored/weathered... this should help you find that point. If the inside of the other jaw is close to or flush with the plug electrode, then you have a problem. If not, then you have a problem other than a foreign object embedded in the crown of your piston.

Personally, I think this entire scenario is totally unlikely, as I dropped a 2-56 nut into a cylinder once and the noise it made as it shot peened the inside of my cylinder was not something you would ever miss.

In any case, best of luck and let me know if you have any questions re. this approach.

P.S. ttpete... I mean no disrespect. Just trying to help him prevent a bad situation from potentially becoming worse. And your first suggestion is actually what I would do before anything else. Not sure what the clearance is between the piston crown and electrode on a 748, but I am guessing that it is not insignificant.

ttpete · Jan 6th, 2012, 8:26 pm

If done properly, the clay is not going to fall off. Have done it many times.

davy.j · Jan 6th, 2012, 9:54 pm

Go to your local airport & borrow a boroscope from the mechanics there, stick it down the plug hole & then you'll know for sure what's going on.

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asimetrixr02 · Jan 6th, 2012, 10:02 pm

I imagine these plugs come pre gapped? I ask becuase you said you are using something besides OEM. If the piston contacts the electrode fast enough it could it could brake it clean off. Just trying to suggest idea's and not step on any mechanics toes

Also wonder how tight you are putting the plugs in.