Got caught in a pretty heavy rain on Saturday night. The bike sat until this morning (Thursday). Started fine, got less than a mile from home and lug, lug, lug, the throttle stops responding and the bike won't rev and eventually stalls. This happened once before a couple years ago, also following a wet ride. Choke still works to pull the revs up. I nurse it at around idle speed to a gas station. Fill 'er up with just over 2 gallons. Give it a couple minutes of on and off choke to try to blow out the problem. I'm able to get it up to speed (40-50 mpg) on the road and it runs fine for a few miles, then won't rev again. Pull over, clear it up, and made it a couple miles to work.
You think it's water in the gas tank? Topping off the tank with new gas seemed to help. Where else would rain be able to infiltrate the system? Has this happened to anyone else? I kind of figure it must be a typical problem, maybe for carbies, and I'm comfortable dealing with it, but wanted to get it "on the record" in case anyone else experiences the same thing and is searching for a solution.
I had the same issue with my 97 M750. I never got any solid info on the problem, but I fixed it just like you did: new tank of gas, run the choke until the engine was warmed up, then just get it on a ride where I can keep it at a constant speed for about 10 minutes.
As for fixes...a new O ring between the tank and the cap. My best bet. I'm sure there are smarter people that know more about it.
I used to have some trouble with the front cylinder getting wet. The ignition system you know. After a while in the rain the twin sort of becomes a single..I quit riding in the rain on that bike. but I probably could have sealed some stuff up.
...do you have an open air box?
even if not, could be water got in your box via the "air" intake...water deposits in the lower part of the box then when it heats up, it evaporates and condenses back down in your engine...
just a thought
It maybe possible that the drains are clogged.
When the water collects under the cap it may not drain fast enough
and could possibly enter the tank.
I used to have weird running when the air filter got wet, so yes you could have water in your airbox.
Try draining your carbs by opening the drain plugs at the bottom of the bowls. Water got in on my buddy's' 01 m750 after a four hour trailer ride. Once we drained and cleaned the carbs the bike went from a dead horse to its normal peppy self.
I think you were somehow sucking water into the throttle body, or the filter may have gotten saturated so it couldn't get enough air... or both.
Water in the fuel is too easy of a solution. Given the nature of the Monster's styling, a great majority of your electrical system is exposed to the elements. Check your ignition coils, spark plug leads, and plug boots. A cracked coil will have a higher resistance generated, get hotter, and the crack will expand letting moisture in. Then bad things happen.
You can test coil resistance with a multi-meter. Most of the commercially available service manuals (Haynes, MBI, etc) will have the resistance values listed in them. It's not a difficult procedure to do. But as with anything, if you aren't sure how to do it correctly, take it to the dealer.
A cracked coil will do exactly what you describe. As will faulty/worn plug lines and boots.
Water and electricity no worky together.
One way to spot secondary ignition insulation problems is, with the bike running, spray the coils and wires and spark plugs with water, one at a time Use a spray bottle, not the hose [cheeky] When the symptom happens you found the problem.
In my 749 it was the vertical cylinder that ended up having water inside the spark plug hole and the coil cap was completely oxidised. I cleaned the spark plug and changed the coil in cap doodah and all went back to normal again.
Thanks for all the great suggestions.