Help a newbie...mechanical issue on recently acquired '99 M750

[Saxofoto]

Hi everyone,

I recently bought a used 99 Monster 750. I planned on riding it a bit more before I started asking questions here but I ran into a mechanical issue this evening that surprised and worried me. I rode the bike Saturday without any troubles, but when I took it out tonight I noticed that the engine seemed rather weak. I wasn't getting nearly the pull when starting and while it's never very smooth in first gear, the choppiness seemed stronger than usual. It really struggled to go fast at all, topping out under 30mph (though I wasn't trying to push it too hard). It was idling like it wanted to stall, and sure enough, it stalled several times as I rode just within a couple blocks from my house. Had a really hard time restarting it, and when I did I let it sit a bit longer, revved the engine some and noticed the strong scent of gasoline (I always smell a little though). I'm not sure if I didn't let it warm up enough or if there's something more serious going on. I'm new to riding and know even less about the mechanics of motorcycles (something I need to learn so I'm open to suggestions for resources). I'll probably need to get it looked at by the Ducati dealership nearby, but I thought I'd see if there might be some ideas you all had that I might investigate further before taking her in. When I bought the bike I had the previous owner meet me at the dealership where a mechanic took a very quick and superficial glance at the bike and asked many questions about the maintenance history (which was consistent with the paperwork) and said it looked like a great bike. Not sure what my first step should be. Hopefully someone here can talk me through a couple ways to troubleshoot and maybe we can narrow it down. Your collective expertise will be greatly appreciated. Thanks in advance!

[koko64]

Any gas overflow to the ground from the carb hoses?
Does the bike blow black smoke when it misses?

[BastrdHK]

Pickup coils/sensor.  Sounds like the bike is dropping a cylinder.  Quick troubleshoot, when the bike starts doing this again, pull off one of the spark plug wires, if the bike dies, reconnect, restart.  Try the opposite wire, if the bike continues to run with no apparent change in roughness at idle/throttle.  You aren't getting spark to that cylinder.  The pickup sensor is a known failure on this model (I have replace two on carbed M750s).  Check and report back.

[Slide Panda]

Follow that ^^ first.

On an older bike it's also quite possible your carbs need some TLC. Parts like the needles in the carbs do wear over time and can use poor or no running. Floats can get stuck or restricted etc etc. I'd first track a known trouble maker like HK noted, then start to ensure everything was is proper order after. Spark plugs replaced and gapped, check the air filter and intake. Replace filter if needed etc etc.

[motoxmann]

x2
if it's been a while, it'd probably be a good idea to clean and/or rebuild the carbs either way. it couldnt hurt, could only make things better.

for the pickup coils, you can unplug them from the ignition units (the two-wire plugs) and check the resistance between the two wires. it should be 95-105 ohms.
the actual ignition coils could also be an issue if they are original, might want to look into replacing those with aftermarket ones if all else fails.

whats funny is I have a '00 M750 with the original pickup coils, and one of them is actually cracked, but still is within spec and has never missed a beat :p

[Saxofoto]

Thank you all for the suggestions. I'm gonna try to troubleshoot the pickup sensor per your advise, HK...that is, if I can figure out how from the manual I downloaded. I don't suppose there's a book on Monster Maintenance/Repairs specifically, is there?

I'll have to inquire with the previous owner about the condition of the carbs. I'd have thought the Ducati mechanic would have pointed that out or taken a closer look just to see if something needed attention. I know he had the 6k tuning as soon as he acquired the bike last year, and it was a little late at 7500 miles (he put about 4000 miles on it since then). I didn't notice any gas dripping or black smoke. I'll take a look in the morning and report back if I find anything; my boss has tools he said I could borrow if I need them but won't have access til later in the day. Thanks again everyone! The info here will prove invaluable in no time.

[motoxmann]

I don't suppose there's a book on Monster Maintenance/Repairs specifically, is there?

yes, there is, I have it sitting right in front of me right now. that's where I got the specs from and the testing procedure that I posted above.

It's the Haynes Manual, number 3290. titled "Ducati 600, 620, 750 & 900 2-valve v-twins '91 to '05". it covers all '91-'02 M750's, I bought it on ebay for around $30, I recently used it to do a complete overhaul on my '00 M750, including a complete engine overhaul/rebuild, and to diagnose a few minor issues it had previously as well. I literally rebuilt my entire bike from the ground up. It's extremely easy to understand and provides PLENTY of pictures, and even has full color wiring diagrams

there is also a spec for setting the ignition timing and the air gap between the pickup coils and the flywheel, but that should not be an issue because that doesnt change unless you actually physically move the parts yourself, so it wouldnt be something that would just start to happen on its own.

[Saxofoto]

Ok, so finally today I had a chance to troubleshoot the problem as recommended by HK. And thanks also to motoxmann for the tip on the Haynes Manual. I ordered one from Amazon for $35, had a gift cert, and signed up for prime to get it asap (arrived yesterday). As predicted, the engine died pulling one spark plug wire (horizontal), but not the other (vertical). When pulling the vertical plug, I noticed that there seemed to be angles where the power was delivered properly and the engine revved higher as a result. That happened by accident; as I pulled and the wire finally gave, it came out of my hand a recoiled back toward the spark plug from whence it came. It landed in a way that seemed to rev the engine higher. I killed the engine at that point not knowing what that implied. I decided to retest the process though to be sure. Given that info, what is the likely culprit thus far? Is there a temporar or alternate solution? I called the local dealer who told me pickup coil replacement would cost around $350 with parts and labor, and they can't even run a diagnostic for two weeks (I'm gonna have them inspect the bike then anyway) plus another two weeks to get parts for it. I'd like to see what else I can manage on my own; see if I can get it ridable, otherwise I have nothing to ride for a month. What do you guys think?

[Ddan]

I'd probably put in a new set of plugs first, but it sounds like you have a bad spark plug wire, or perhaps the connection with that wire and the coil.

[brad black]

the spark plug caps screw onto the plug wires.  if they rotate easily on the wires, pull them off, cut the end off the wire and screw them on again.  use a lubricant like silicon spray to make it easier.  this is something i often do at service.  replace the plugs too.

always start with the bits you can see.  they tend to be cheaper and it's much less effort.

[dlearl476]

With a bike that old it could be anything, so start with the cheap stuff. 1st, go to an electronics store and buy a can of contact cleaner. I like CAIG ProGold, because it coats as week as cleans, but there ate a lot of brands.
Replace spark plugs (after spraying contacts with contact cleaner.) see if your problem goes away. If not replace spark plug wires with some good ones. Problem remains, then think about replacing expensive electronic parts. $350 for OEM COILS? BS. Buy a pair of Dynacoils and install them yourself. (After checking them as above, of course)

Good luck. Electrical problems can be tough.
FWIW, I have a can of CAIG and a tube of dielectric grease and I clean/coat every electrical connection within reach every time I dig into the bike. It's amazing the problems you can avoid.

[Howie]

And new spark plugs.  At low engine speeds increased gap means higher voltage, which helps a fouled plug fire.

[Saxofoto]

Thanks for the ideas. It definitely makes sense to fix the cheaper things first and try to clean the contact points. I was just messing around with it with a friend who rides and is only slightly more mechanically inclined. It seems more and more clear that the problem is at the interaction between the wire and the spark plug. If I keep the plug just barely on where there's no metal connection and the rubber just starts to cover the insulator, the cylinder will fire and I can hear the spark plug clicking. It seems crazy to me but as soon as I push it on more the connection is somehow short and the cylinder drops. Could it just be the plug end of the wire? Where would I get a replacement part to test this? I might replace the spark plugs too but they actually look fairly new, probably less than a year old. I feel better that this probably isn't going to be a major repair.

[bigiain]

I don't suppose the plug cap is damp or dirty? From your description, the problem could easily be moisture or dirt on the plug cap conducting to ground when the rubber+dirt/moisture connects to the head. Try thoroughly cleaning and drying the cap - WD40 and elbow grease to clean it, maybe followed with denatured alcohol to dry it.

As Brad mentioned, the plug caps screw onto the wires, so one diagnostic option would be to swap the horizontal and vertical cylinders plug caps, and see if the problem changes cylinder or not.

big

[brad black]

i wouldn't increase plug gap on a carby bike.  anything over 0.7mm with std coils impacts cold and low speed running.  even with dyna coils my 600 liked 0.6mm.

the ca cycleworks coils are well worth the money should you suspect the coils.  they're worth it anyway. plus you get new leads and caps with them.