Ducati Monster Forum

Here's hoping the collective wisdom of the board can solve my problem..

I have a 2000 Monster 750 just under 20k miles. Carb was rebuilt/rejetted 5k miles ago. New belts, spark plugs, fuel filter, 50 miles ago. Last week it started acting like it was running out of gas when I was at about 1/4 tank. I thought maybe I had some old gunk on the bottom of the tank, and refilled it with fresh 89 octane. It ran slightly better after that, but still acted like it wasn't getting enough gas, and would frequently cough and misfire when opening the throttle. I even went so far as keeping the choke on slightly to make sure it didn't die when I came to a stop at a light.

That Saturday, I checked all of the vacuum lines and fuel hoses, and found a spot where the main fuel line from the tank was making a very tight corner into the fuel pump. I alleviated the situation, and the bike went back to riding normally. No problems starting, running, accelerating, etc. Power felt smooth through all regimes of the ride and I shut it off confident I could go back to commuting on it the next Monday.
This morning (the next Monday) it would start halfway for a couple seconds then shut off. Nothing has changed since Saturday when it started up and rode perfectly. The kicker is I am on a business trip and living in an apartment without a garage or many tools to work with for the next month.
Attached is a 30 second video of how it acts when I try to start it; first two attempts with choke, then without. I also apparently need to work on my videography skills.

https://youtu.be/k30XjcRf86Y

Are you running the OEM vacuum petcock?

Sure am

Get rid of it and install a manual petcock.

Also make sure the vent line isn't kinked.

Sounds like a logical step/eventually necessary upgrade... Any recommendations for where to get one and which one to buy?

Motion Pro makes one. Parts Unlimited also sells one.

You need a 5/16" barb fitting.

You might check with Jeff at Monsterparts.

Replaced the vacuum petcock with a manual one from parts unlimited. Result: The bike would start and run long enough to determine that the vertical cylinder wasn't firing. After much fiddling with wires and multimeters, it appears there may be a minor fault with the grounding wire off the vertical cylinder's ignition coil. I've tried removing it and retightening it. There also doesn't appear to be any severe kinks or frayed wires anywhere, however it still occasionally drops to one cylinder while riding - sometimes idling at a red light, sometime while accelerating on the highway >4500rpm. Maybe vibration is causing the faulty wire to short enough to drop voltage to the spark plug?

One other thing I found was a small chip off the top of the same ignition coil. I've heard that small physical damages can cause  those things to not work properly due to internal trauma. I am leaning towards replacing the coils, but if the grounding wire attached to the main wiring harness is bad, would that help in the long run? How do I determine which is the cause, and which is coincidence?

Additionally, my analogue tach on my DP gauge cluster has been working on and off erratically for a few years. On another thread, a rider was having that issue along with some rideability and performance, and he seemed to think it was the ignition module upstream of the coils.

Did you try a new plug?

Fresh plugs are a tune up secret!  ;D Ive had guys tell me Im a magician asking what I did to make the bike run so well. Er, um, change two five dollar plugs [laugh].
The damaged ignitor box is sus. The tach runs off one of the box wires, so follow the tach wires to that spot and check it out. Consider swapping boxes to see if the problem migrates, worth a try. Also some ignition components fail when they warm up and this can often be a coil issue as a cracked winding opens.

There would be numerous threads on this stuff via the search function.
Probably for the cost of an oem coil I would buy Exactfit coils from a thread sponsor. The carbed coils are getting long in the tooth and all things being equal the bike will run better. We forget that these bikes are getting older and some parts were built down to a price and not up to a standard. A ten to twenty year old bike deserves a freshen up of old parts.

Just to add, if you are sure one plug doesnt fire, the worst scenario is an ignition pick up fail. Not too technically difficult just a pita.

Continue to work in order from easy/simple to difficult/complex fixes with your diagnostics.

Spark plugs were both replaced about 50 miles ago. Could be worth a shot anyways... Might have gotten a bad or damaged one.

Do fix the ground.  A bad ground could kill an ignition module.  Easiest shade tree way to diagnose a module or coil is switch the suspect component to the other cylinder.  Misfire moves to the other cylinder you found the culprit.  Pick ups can be checked with an ohmmeter.  95 + 5 ohms.  Check quickly after misfire occurs or they may test good.

 [thumbsup]

Koko, thanks for the pointers. I didn't see your post before I mentioned the age of the spark plugs.

It was actually the search function that led me to identify the possibly faulty ground via this thread http://www.ducatimonsterforum.org/index.php?topic=58702.0

My plan going forward is to check the modules and pickups with a multimeter, and by swapping cylinders to check performance. I'll check resistance on the ground wire to see if it needs to be replaced somehow. I will be heavily dependent on the info on this board, since I am without my normal tools and manuals. Thank God for knowledgeable and helpful people like you guys!  [beer]

I thank God for this place too. So many knowledgeable and helpful folk, many of whom have a truckload of years loving, riding, modifying and maintaining the Monster.

Update...

As I removed the multimeter from my travel tool bag, the wire ripped out of one of the leads and broke. Since I already have a higher quality one at home, I can't convince myself that I absolutely NEED to go buy another backup multimeter...

Through empirical evidence, however, I think I have confirmed the grounding wire is the main problem causing the vertical cylinder to not fire ~50% of the time. It's also binary... It either works perfectly, or not at all, which to me sounds like an electrical problem. I've found that by keeping the ground wire oriented up out of the coil mount, it works almost 100% of the time, but if I mount it like the other cylinder's wire, it works 50% of the time. The picture below shows the misbehaving wire circled and mounted upright, and an arrow pointing to the dent on the top of the same coil.

My biggest problem is how to fix the wire that seems to be shorted somewhere inside. It disappears into a bundle of other wires, and I am NOT an electrician by any stretch of the imagination. Of course, I still am without a garage or normal tools until the end of the month, as I'm still traveling for work.

(http://i1268.photobucket.com/albums/jj579/downwthequickness/orca-image-1210228897.jpeg_1533692058288_zpsk1wzmaxr.jpeg) (http://s1268.photobucket.com/user/downwthequickness/media/orca-image-1210228897.jpeg_1533692058288_zpsk1wzmaxr.jpeg.html)

You are dealing with an open, not a short.  If wiggling the wire at the coil causes the misfire that is probably where the open is.  Cut back from the connector a bit, add wire if needed and a new connector should do it.