'93 M900 running poorly at steady speeds... [SOLVED!]

[NFG]

Problem Solved: replaced the emulsion tubes in the carbs.

Previous Title:   '93 M900 running poorly at low revs...

Updated to reflect changed analysis of the problem:  "It doesn't like to maintain any speed, it chokes and stumbles, the transition between on-throttle and off is very poor, keeping a steady pace is very rough no matter what the speed.  Hard throttle goes well, at least comparatively... "  Updated conversation starts at reply #10.  Any advice is welcomed.  =)

Original post follows:

Please forgive the newbishness of the question, I'm sure it's something simple but I'm kinda new to bikes and so on.

Anyway: I have a 1993 M900, ~35,000km on the odo, and it's been treated well and runs like a dream, starts immediately even on cold days, etc.

Recently I moved house and had to suffer a lot of construction delays on the new route home from work, and after a couple of days of very slow or stopped riding the bike started running very poorly.  Idles slower than it used to, doesn't pick up as quickly, sounds a bit rough.  It's actually very difficult to ride around the neighborhood as I can no longer tell when the power's going to come back on.  It's not a huge delay, but it's affecting my confidence significantly.  Everything sounds great once the revs pick up, and at decent speeds it runs as I expect.

I thought it was just in need of a good thrashing after all that sitting around in slow traffic, but a few days of cranking it up on the freeway hasn't made a change, so I'm inclined to think it was a coincidence...

I'm hoping for suggestions.  My father suspects the plugs, which would be nice if true and is high on my list of things to try, but I thought perhaps someone here might have more specific knowledge, and have some ideas for me?

I appreciate any ideas or clues.  =)

[ducpainter]

Listen to your Dad and put in a set of plugs...

or at least check them.

It sounds like you might be running on one cyl.

If you put your hand near the header for each cylinder you can determine if they're both firing.

[the_Journeyman]

Listen to dp.  I had an idle issue with just some poor running.  I was AMAZED a the difference $5 worth of NGK spark plugs made.

JM

[NFG]

Thank you for the advice.  As soon as I get back to town I'll pick up some plugs and give it a go.  I'll keep you posted.  =)

[NFG]

Well I'm back in the country, and the first thing I did was replace the plugs (well, after I slept half the day and ate a huge lunch).

Still the same problem.  Suspecting the single cylinder problem ducpainter mentioned, I pulled off the plug leads one at a time and presto, the top cylinder's not firing at all.

So tomorrow when the sun's shining I'll go digging for the cause (I have a connection problem SOMEWHERE, sometimes the starter won't turn until I shake the harness...) but I wonder:

Is there a particular point of failure to look for?  I'm good with electronics and wiring, but it's my first motorbike, so.  Any tips?  =)

[greenmonster]

Check wiring around steering stem.
Often wears through, causing f e sudden death.

Check wiring into ignition units.

Switch ignition units. Fault moves to lower, one unit is dead.

[Howie]

Also check the ignition pick up coils.  They should be 90-100 ohms.

[NFG]

Checked the pickups with the multimeter, 97.7 Ohms each, which seems to be appropriate.

Swapped the two ignition units, then unplugged the bottom cylinder, and it still ran (though not well).  Unplugged the top one, and it still ran...

So, both are firing now.  It seems to run smoothly when first fired up, but then the revs drop and it runs very poorly after 30s or so? 

It absolutely refuses to idle along, if it's not accelerating or decelerating hard it rumbles and skips and sounds dreadful.  I can't be sure both cylinders are firing while in motion, but I think I've ruled out the pickups and the ignition units as causes... 

[Howie]

You need to check the resistance quickly when the bike runs crappy.  They will test good when the bike runs well.  You probably have a pick up where the winding opens at a certain temperature.

[NFG]

When I ride to work, it runs poorly.  When I stop the bike and then think to check it 15-20 seconds later, I assume it's still hot, and I was getting both cylinders. 

If it's heat related I would expect it to still be rubbish...  Wouldn't that make sense?  I will check it anyway if you think it's still possible.

[NFG]

I'm starting to think I didn't really get the problem right to start.  After riding it more...

It doesn't like to maintain any speed, it chokes and stumbles, the transition between on-throttle and off is very poor, keeping a steady pace is very rough no matter what the speed.  Hard throttle goes well, at least comparatively... 

What does that suggest?

[Howie]

When I ride to work, it runs poorly.  When I stop the bike and then think to check it 15-20 seconds later, I assume it's still hot, and I was getting both cylinders. 

If it's heat related I would expect it to still be rubbish...  Wouldn't that make sense?  I will check it anyway if you think it's still possible.

If you restart the bike in the same 15-20 second time frame and the bike runs like poo you can probably rule out pick ups. 

I'm starting to think I didn't really get the problem right to start.  After riding it more...

It doesn't like to maintain any speed, it chokes and stumbles, the transition between on-throttle and off is very poor, keeping a steady pace is very rough no matter what the speed.  Hard throttle goes well, at least comparatively... 

What does that suggest?

This description leads me to think fuel related.  Your Ducati can run amazingly smooth on one cylinder, just extremely slow.   

[NFG]

As you can see I'm sorta having trouble describing the problem, which makes it exceedingly tricky to search for clues or solutions. 

Assuming it's fuel related, what am I looking for?  Carb adjustment? 

[koko64]

If the bike has the original needle jets then they may well be causing the rough running. Ensure the choke cable returns fully. The bike is old enough to warrant a carb service, with a clean, new needle jets and if you like a jet kit.
Components like coils and ignition leads can degrade over time too.
You can give the old girl a freshen up. She is the original 1993.

[Dellikose]

For cheap insurance, change out your fuel filter. It could be clogged, giving you fuel delivery problems.

You can use the Napa Gold filters (Part #  FIL-3032) as a good replacement.