Ducati Monster Forum

Hi, I've got a carbed 96 monster that's having a small problem. Stock mikuni carbs. It starts and runs great, isles great, makes good power. Runs fine until you hit the gas hard and drain the carb bowls at which point it falls on it's face.

Ease up in the throttle and when the bowls fill back up it's has tons of power again for a few seconds of hard acceleration, then sputters again. He jetting is good, the mixture is good, the bike ran fine with the existing setup but sat for a month or so and this problem appeared.

The actual carb jets themselves seem fins as I have access the the full spectrum of throttle usage, but it seems like the carb bowls just aren't filling fast enough to keep up with demand.

I have tried changing the fuel pump out for a good known fuel pump, fuel filter does not appear to be clogged, there are no kinks in the fuel lines, the vacuum petcock has been replaced with a motion pro fuel valve.

I've pulled both of the carburetor bowl's, and the floats are not stuck at all, nor is there any varnished gasoline residue on the floats or in the bowls at all.

The only thing out of the ordinary that I noticed, was that when I drained the bowls prior to removing them, the right-hand side bowl let out less than half the amount of fuel that the left hand side bowl drained out which leads me to believe that the right-hand side bowl is not filling correctly while the left-hand side one is.

Really hoping to avoid having to pull the carbs out as it is a huge pain in the ass, but I'm not sure what to do at this point. My knowledge of carburetors is very limited. Any help would be greatly appreciated.

There is a filter in the inlet where the fuel line attaches to the carbs. Have you checked that?

I have not. Is it just in the fuel line itself?

It is in the part that swivels, where the fuel line attaches. Once you're there, you'll know it.

Awesome. Thanks. I'll check it out when I get back home.

I always remove them when I find them and trust my in line filter. Some stale fuels act like a resin over the mesh when they go off, or its just crap build up over time.

It means any in line filters with the porous bronze elements  require more regular replacement or they will be glued up by modern fuel additives. I soaked one in carb cleaner overnight and it looked pristine, but wouldn't flow gas.
That T piece connector filter mesh is very fine.

Awesome. Thanks for the help guys. Will check and report back!

So, there is no filter in line on the carb inlet... any other ideas?(https://uploads.tapatalk-cdn.com/20170529/2a431930f943f05ced3872ebba2aced3.jpg)

Are you running the vacuum petcock, or a manual unit?

Motion pro manual unit. Same unit I have on my other m900 with fcrs. No issues with it on the other bike. I've check the lines, there's no kinks in them anywhere.
(https://uploads.tapatalk-cdn.com/20170529/ffa2bd2a22ccbefa142934227870bf00.jpg)

Sorry...missed that in your OP. [bang]

Are you getting strong vacuum at the pump? Vacuum line in good shape?

What are the float levels set at?

eta...Is the vent hose free of constrictions/kinks?

Is the hose from the fuel pump to the carb original?  If so, it might be collapsing when warm in the area around the V.  Another area to check is the float bowl venting.

No worries :) as for the floats, I don't have any idea on the float levels. I don't know my way around carbs very well at all.

I know everything is exactly as it was and the bike had no issues at all prior to the bike sitting for a month or so.

With the fuel line disconnected from the carbs, crank the starter to verify that fuel spurts from the hose. The pump has way more capacity than a Monster requires.

I can't explain why this would be an all of a sudden thing, but a month shouldn't be a long enough time for problems internal to the carb/float bowl...but who knows? I'd check delivery. If you have fuel, I'd look into setting float levels. Your description of one bowl having more than the other is an indication one is wrong. I'd also verify the vent hose is free of kinks.

Ok. I'll check those things now

If it's only one bowl giving trouble then I would check that bowl's float valve and needle for an obstruction.
The T piece filter can be hard to spot.
Hopefully your pump  and vacuum lines and hoses are ok There is a list of things to check.
If you cant progress it soon and just want it done asap give it to a carb guy. Make sure the shop knows carbs because in this day of electronic fuel injection many dont get carbs or know how to deal with them.. If you dont mind taking the long slow way in order to learn about the fuel system then the community here can talk you through it. [thumbsup]

Koko, there's definitely nothing in the fuel inlet of the carb.

So the fuel pump sends out plenty of fuel. As for the floats, I have no idea what I'm looking yet. I suspect if I go tinkering in there I'm going to break something. How would I go about checking the float valve for an obstruction? Can this be done with the carburetors on the bike?

Which vent hoses are you referring to? The gas tank vent? The gas tank vent hose just go straight down out of the bottom of the tank.

What are you referring to the carb vent hoses? I'm not sure which ones those would be what are you referring to the carb vent hoses? I'm not sure which ones those would be. Are those the ones with the metal coils around them?

They are supposed to go into those little gray airbox things. At the moment the hoses are tucked up above the carburetors in front of the battery.  They were connected to the gray boxes mounted to the frame, I forgot all about those.

Come to think of it, those are only thing that has changed. I had removed them and followed some write ups about routing the hoses up underneath the tank area as others had.

Could this be a cause of this problem?
I tried to find the proper routing for them between the left and right carb and the left and right boxes, but was on able to find any truly specific information on how to route the hoses and where each one goes in relation to the left and right side gray box.

I had reinstalled one box with both hoses going into it, and it didn't seem to make any difference at all

Short answer...yes. Those hoses are the diaphragm vents. If they are kinked, or in the free airstream, it can prevent the slide from lifting. If the slide doesn't lift the bike won't pull hard at full throttle. It's like the choke is on because the motor can't get enough air. The plugs should show the bike running rich if that's occurring.

Just make sure they have no constriction, and the routing doesn't allow air to blow directly in to the hoses.

I edited my post.
Checking the float needle requires some dismantling. If its the slide vacuum hoses then that would be a good outcome.

It does pull hard a full throttle, than falls on its face after 2-3 seconds of hard acceleration.

If I hit the gas hard from a part throttle  cruising speed or idle the bike takes off like a rocket and has no fuel issues initially. Seems perfectly normal. Then it seems to me that it sputters like its starving of fuel. Ease off the throttle for 10 seconds or so and hit the gas hard and it'll go like hell again for a few seconds then falls on its face again.  Easily repeatable scenario.

It's sounding more and more like the inlet to the bowl is being obstructed somewhat :/

While you're checking, take a close look at the vacuum line that goes from the pump to the vertical cylinder intake manifold.

Those eventually perish, compromising the strength of the vacuum signal.

Ok I'll look at that as well. How involved is checking the needle valve seat and associated fuel paths for obstructions?

That's a task best done with the carburetors off.

Don't be afraid to do this task. I just removed and soaked mine for a few days (gunk that gelled in float bowl), cleaned with carb cleaner and then used compressed air. Good now, just need to tune it better. This is new for me.

As has been suggested, if you have original fuel and vacuum lines, while the carb is off, replace them. I did and don't have to worry about cracked lines.

I don't suppose there's any chance of running something like sea foam through the system making any difference in this area, right?

You could try it and see.

Well, I poured some in, and went for a ride. Gonna let it sit overnight and take it out tomorrow to see if it made any difference. If not, I guess I'll be pulling the carbs...

There are filters in those hoses that can clog.  Very rare.  There is also a smaller vent hose that went to the...I think left pod. 

Does anyone have a diagram of how the carb vent hoses are supposed to be routed with regards to the grey frame mounted pods? The carb vent hoses seem to each have a one way valve inline on them. Are these necessary?

They are filters.  Necessary?  I would want them in there.  Will the bike run without them?  Yes. 

They were routed as you have them on my bike, no issues.  They just need to be out of turbulent air

This sounds like a carb vent issue. Try putting those vents & pods back to stock, incl. the little vent things on the vent lines before the pods.
I had similar problems w/ my 98 M900 after the epa stuff & carb vent pods were removed, leaving carb vent hoses hanging straight down. Stalling out can be common at higher speeds on these carbs if the epa vents & pods are removed. They have to be terninated in TOTALLY still air or the fuel flow will be blocked as described above. This happened to mine only above 70 mph and with a headwind. It would go away and run fine again if I slowed to 60 mph for a few mins. Would restart upon higher resuming speeds or a big headwind. Suprisingly, those little white vents on the carb diaphragm vent hoses don't work at all. Any air up these tubes caused me problems, even w/ the vent thingys on.   I got sick of that & went to FCR carbs.

Ok. I agree, my other m900 has fcrs and they're WONDERFUL :) im really trying to resolve this so I can sell it

I'd love to put them back in and plumb everything correctly. Do you have any idea what the correct plumbing for the carb vent lines looks like? Do both carbs  vent hoses go to the same pod? Or one to each?

I do not, the epa pods were gone when I bought my bike. Check the factory manual if possible. (search here for a link)
Prev. owner left the vent hoses hanging. Started causing me issues 2 yrs later for some reason. I never got rid of the issue until I got fcrs.

I'm no mechanic but I'd try long hoses run all the way to the back of the bike w/o using the pods, see if that gives still enough air. If so, remove the ugly pods.   

Long hoses aren't necessary. I ran my 900 like that for years, I had the hoses tucked up by the battery.

As most of us did.  Like I said, out of turbulent air.   Anyway, the two large hoses with the spring and filter go to the right pod, the small hose to the left.
                                                                                                                   

I've got mine tucked inside of the battery straps behind the battery.

Well seafoam definitely made a difference in the ease of starting the bike. It caught almost immediately, when i tried to start it with only a single backfire before it was running.

Previously, it would be cranking for quite a bit before multiple backfires and then actually catching. 

It also felt like it ran for longer at full throttle before sputtering and losing power...

Maybe I just need to run more seafoamed fuel through the system.

So, a quick update, after taking it for a couple more quick rides, it's starting immediately now with no fuss and no backfires.  While riding, light throttle application seems perfectly normal, part throttle use is still bogging and sputtering. Seafoam seems to have made quite a difference...

The difference I'm noticing now, however is when I push through the bogging and whack the throttle wide open, The blogging and sputtering continues for a second or so, then stops and the bike takes off takes off like it should.

It seems that I definitely have some clogging somewhere in the carbs, in whatever Circuit controls part throttle application. Where would I want to start looking for this? I'd really rather not take the whole carburetor set up out of the bike if it can be avoided. Is it possible that I can drop the bowls and remove the Jets that are affected to alleviate the issue?

Or am I back to having to pull the carbs apart and thoroughly clean them?

I would pull the carbs.  While there replace the needle jets and needles if they show any signs of wear.  Better idea, tell us what sizes all the jets are  buying anything.

There's no getting away from it really as Howie said. Carbs need periodic maintenance. You could keep running Seafoam or some other brand carb cleaner additive for a few tanks and hope for gradual improvement, but there is every chance you will be pulling the carbs anyway. Find a trustworthy mechanic old enough to know carburettors ;)