Misfire Poser - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 22 (permalink) Old Jan 10th, 2017, 7:00 am Thread Starter
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Misfire Poser

Here's a New Year teaser for everyone. My 900 bevel has had a problem for the last six months; it goes like this:

The bike starts instantly on the button, I warm it up for five minutes and ride away, engine running perfectly, then about half a mile down the road one cylinder misfires and cuts out. I carry on and the cylinder chimes in and out until by another half mile it has cleared, to give no more problems. Until today. This time it did not chime in again and I was forced to complete a circuit on one cylinder until I could get it back home. Bummer.

I've not bothered to try to find the problem before now, because once it starts running ok again I forget all about the misfire. When I got home today I found that it was the rear cylinder that was not running, so I took out the plug, which was soaking wet, and checked for a spark, and boy was there a spark! So no problem there. After fitting a new plug to the offending cylinder, she fired right up, but I didn't feel like taking her out for a test run.

I use iridium plugs and I'm wondering if they are more susceptible to fouling at low temperatures? I'm also wondering if the engine is oil fouling the plugs on start up, although if I check the plugs after a run, they are a lovely clean, dry tan colour, as are the piston crowns, and the engine does not use any oil at all. But, as the engine is 33 years old and has never been touched, the valve guide seals may be a little brittle and hard by now! Maybe it's time for a grade hotter (running) plug.
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post #2 of 22 (permalink) Old Jan 10th, 2017, 7:32 am
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Is your oil level where it should be (not overfilled)?

Tom
Sophia, 2001 ST4 (Ohlins, Race Tech, Cycle Cat, Russell, etc).
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post #3 of 22 (permalink) Old Jan 10th, 2017, 7:38 am
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Clogged fuel filter or pinched line to the back carb?

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post #4 of 22 (permalink) Old Jan 10th, 2017, 8:28 am
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Fire tier plugs

I like your valve seal theory assuming it isn't overfilled (T.M. Roe post) Is it is smoking when you fire it or after adding throttle after a downhill coast?

Your proof that it fires immediately after failure when you change plugs rules out a lot of things but too bad you didn't ride it to make sure it didn't foul again. If it had, a compression check would be in order.

33 years without engine work. You must take good care of your bike and a bevel certainly deserves it.

As far as the iridium plug being more foul sensitive; haven't heard that

As far as looking at the exhaust, oil burns white, fuel burns black.
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post #5 of 22 (permalink) Old Jan 10th, 2017, 9:17 am Thread Starter
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Right, let's see... The sump's not overfilled and if it was it would tend to affect the front cylinder more than the rear. There is no smoke on start up, which would appear to rule out my theory of oil getting past the guide seals.

If it is fuel starvation, how does it normally manage to clear itself?

A good description of what happens is that it feels exactly like when a bevel runs out of fuel - the cylinder stops firing, you turn on the reserve tap and then after a couple of attempts, the cylinder chimes back in. Maybe it is fuel starvation but how? And why was the plug soaking wet today? Grrrrrr.

BTW, I've been running the iridiums for about five years, so if that is the trouble, why has it not manifested itself until now?
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post #6 of 22 (permalink) Old Jan 10th, 2017, 10:11 am
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Bad coil overheating, then cools down and works again ? Or a piece of mouse poop in the carb that floats around and wedges in a jet, then falls out when there is no flow ( really ). Swap coils on the cylinders and see if the problem goes with them.

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post #7 of 22 (permalink) Old Jan 10th, 2017, 10:20 am
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Intermittent short? Just got my 750 GT running after sitting for 12 years. Real PIA tracking down electrical gremlins.

What oil are you using? I have always used Castrol GTX straight 50w. No longer avail or hard to find so use Valvoline Racing 50w. If I use 20/50 it can intermittently foul.

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post #8 of 22 (permalink) Old Jan 10th, 2017, 11:16 am Thread Starter
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Intermittent faults certainly are a PITA. I always used to run on SAE50, but for the last five years have been using synthetic 20/50. The problem only started six months ago, so I don't think that it is oil connected.

I'm leaning towards spark plug fouling after starting, but why now?
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post #9 of 22 (permalink) Old Jan 10th, 2017, 11:49 am
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Originally Posted by Duccout View Post
Intermittent faults certainly are a PITA. I always used to run on SAE50, but for the last five years have been using synthetic 20/50. The problem only started six months ago, so I don't think that it is oil connected.

I'm leaning towards spark plug fouling after starting, but why now?
You can be pushing oil past rings w/o obvious smoking. Time for that compression check?

BTW, I also pulled both my coils (saved for that collector one day) and put in new ones. Why dick around?

"If Evolution is Outlawed, only Outlaws will Evolve."
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post #10 of 22 (permalink) Old Jan 10th, 2017, 2:58 pm Thread Starter
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I would have thought that oil fouling would be more likely with a hot engine and the oil thinner.

I'm wondering if it may be due to flooding, although there is no sign of eight stroking. I'm thinking possibly accelerator pump connected - too much fuel being pumped in and fouling the plug. Unfortunately it is the dead of winter here, so there is no point in to solve it at the moment. All I can do for now is have a check round for anything obvious and fit a new pair of plugs.
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