1999 M900 Carb and breather issues

[motorcycles]

Hi all--

I'm sorting out a 1999 M900 I picked up on craigslist a few weeks ago. Tuning the carbs has me banging my head against the wall.  A bit about the bike:

Previous owner claims to have picked it up off a friend who had used it as a track bike; he had been in the process of converting it back to a street legal machine.  According to him the bike would need a carb rebuild, fresh timing belts, and an oil change.

Salvaged title; the bike had been down before.  Came with the old tank and a spare set of carbs (I presume the seller had given up on them and thought he could get away with a swap).

The bike appeared to be in good shape and far enough along that I could probably get it back on the road for under $2000 total cost of the bike and the parts I'd need.

I emptied the carbs and tank, put some fresh gas in the tank and it choked to life for a minute.   RPMs at idle were all over the place,  it would rev up real high without touching the throttle or choke and then drop RPM and stall out. 

Installed fresh timing belts, tensioned them using the allen key method.  Checked the valve lash, I'm no expert, but all four appeared to be in spec.

Removed the carbs (the original mikuni's), cleaned, separated and rebuilt (stock jets were already installed and remain installed). Rebuilt the external fuel pump.  Changed the fuel filter.  Reverted from some carbon fiber obnoxious loud pipes back to stock. Changed the rotting air filter to a K&N---annnd here we are today.  If you read all that your all up to speed.

Now, when I fire the bike up it does the same thing, BUT it will idle for 10 or 20 seconds until it backfires out the carbs and a woft of smoke rises out of the airbox. I spoke with a wrencher at my local ducati dealership and he suggested the carbs are out of synch, the diaphragms carbs suck and I should get myself some keihin flat slides (I'm waiting on my winning lottery ticket).

Unless I'm missing something obvious here, my next move is to bench sync the carbs with a guitar's E string and see if I can track down a set of fresh diaphragms to fit to the top of the carbs.  From there maybe I can get a strong enough idle to synch them with some gauges. 

Now the other issue is the breather:  It seems the previous owner has taken off those little plastic pyrimid breather triangles on the sides of the frame.  The breather hoses now just hang off of the carb...  I see that each of these breather components is different and I cannot for the life of me find a diagram of the proper routing of these hoses!  Does anyone have any photos of the stock routing of the breather hoses to each of the breather components?  Are these necessary to have hooked up or was this guy onto something when he removed them?

Any and all input is appreciated, thanks!

[Speeddog]

How many miles on the bike?

The Mikunis are fine carbs when in good condition.
Like all carburetors, in bad condition they work poorly, if at all.

Are the diaphragms ripped, cracked, or?
If they have been left uninstalled for more than a day, they can shrink and are very uncooperative at assembly.
IME they hold up quite well in general.

Yes, bench sync them.
But that will only get you within shouting distance.

Diaphragm vents go to the right side, bowl vent to the left side.

Likely the nice formed diaphragm vent hoses perished and they've thrown away the little filters as well.

[motorcycles]

Thanks speeddog,

I can't be certain of the bike's actual mileage.  It has a new instrument cluster which reads "0000...", but at the time it was salvaged it had 3200 miles on it.  I believe it has been on a racetrack since then, though I'm told only for a few days here and there. 

The diaphragms are not visibly cracked, though I suspect they may have shrunk a bit. Fitting them into the groove was a sensitive process.

I have the little triangular filters, though as far as I can tell they're just plastic housings (should there be foam in there or something?).   

I don't seem to have the original tubing though.  The bowl vent seems easy enough, and the hose seems to fit alright on the filter.  Does the filter then connect back to the main breather?

As for the right side vents-- there's a bit of a variety for ports on the filter.  The hoses coming off the carb have some little barrel shaped filter with a crude screen about the same mesh size as a window screen zip-tied to the end of them.  I honestly cannot tell if these were added to the hoses by the last guy or if they're meant to be there.  Are the hoses meant to be held in place in the triangular filter by some kind of grommet? or rather just slid in a ways so that the excess hangs loose inside?  Also does this triangular breather then route back to the main airbox?

Thanks!

[ducpainter]

The triangular housings were just a method to provide still air for the diaphragm vents. At speed, air rushing past, or into the vent hoses can affect the way the slides lift.

The screen is an add on, and doesn't serve any purpose. You can replace the hose from the filter to allow the vents to reach the housing. IIRC they just slide in.

The housing on the left side was just for symmetry. The bowl vent doesn't require still air.

[greencow]

Coughing from the carb indicates a lean condition, even if for a moment.  A cold engine combined with the higher revs can do that.  I wonder if you have a air leak somewhere. 

Have you looked at the choke as well?  Plungers move freely and are all the way in with the choke disengaged?

[motorcycles]

Great, thanks ducpainter. I'll just rig up some fresh tubing and try to set it as far stock as I can. 

Greencow, I'm not sure about the choke.  It's at least operational in that the cable works to engage/disengage, but I'll take a closer look for a leak when I bench sync. 

Though the airbox does not have the rubber boots over the top of it like I've seen in some pictures of other monsters.  I'm not sure how much of an increase in air flow this would cause, or if they're found on the '99 to begin with. 

[greencow]

Those upper boots wouldn't cause a high idle like that.  I know it doesn't want to stay running for long, but you could try and diagnose it, in the 10-20 seconds you have before it stalls.

Take some WD-40, or something similar, and spray it around the carbs.  Especially around the rubber boots that connect the airbox to the carbs, and the carbs to the intake runners.  If the idle changes at all when you're hitting it with the spray, indicates an air leak.

Chokes on these bikes enriches the mixture, bike will idle up, but it shouldn't cough/pop from the carbs while doing this.  If you don't engage the choke, and give it throttle, or there's an air leak somewhere, the idle will rise, chop up, maybe cough from the carbs and then die back down.  Cold engine with to much incoming air and the RPMs will "porpoise" like that, rise/chop/fall and repeat.  At least until the engine comes up to temp.

If the carbs are still on the bench, set the idle screw so the throttle plate is halfway across that first little hole in the carb.  Adjust the sync screw so the other plate is similar.  That should get you in the ballpark at least.

[ducpainter]

All monsters had the snorkels. Them missing will lean the mixture very slightly. The K&N shouldn't have any effect on mixture. The factory filter flows very well.

[koko64]

Good suggestions and info from these guys. This is a great community.

Use the choke to get it going then turn it off and manually keep a fast idle at warm up. The choke needs to be turned down progressively or off depending on the weather where you are. The revs will rise then start to falter when the choke needs to be phased out before it fouls the plugs as the motor warms. Make sure the choke cable is correctly adjusted to operate.

What's the baseline jetting and float level? The spraying of intake joints is a great test for sure.

Yes spitting is usually lean from too much air or not enough fuel.

What do the plug colours say?

Check it running with the filter out to see if the carb slides rise together and correctly as revs rise. The shrunken diaphragms can be pesky as stated.

There's every chance the carbs are so out of synch that someone cranked the idles speed right up so they dance around out of time. Watch out for sticking and kinked throttle cables not returning.

Truth is Keihin FCR carbs cost as much as the bike nowdays.

[motorcycles]

Great suggestions, thanks all--

I dug into them a bit more last night, found that one of my diaphragms did indeed have a small tear I had missed on the first go round.  The other had become loaded with some black dirt-crud (perhaps disintegrating rubber?).  Oh boy.  Looks like I'm going in for a second cleaning. 

Checked out the diaphragms on my spare set, but they've shrunk a bit too much for comfort, I've read alot about some cheap methods for reviving these, but I decided to bite the bullet and order new ones.  Partzilla was the cheapest I could find ~$26 each. Part number 13507-07D01 if anyone else is looking. Purchased for a suzuki but I hear they're the same carbs (Mikuni BDST 38), sooo fingers crossed.

I don't recall the actual jet #'s, but the jets installed were the same in my spare set and the the carbs installed.  Also the same as the sizes included in the rebuild kit I purchased.  I presume these to be stock-- but I suppose its possible they're not.  Jet needle hangs 3rd position from the top. 

Plugs are brand new, I'll see if I can run it for a bit and check for coloring with the new diaphragms.

Boots on the carbs look to be in good shape, but once I get it buttoned back up I'll try a spray test.

Hoping the diaphragm fitment goes well and resolves much of the issue.  While I wait for them to arrive I'll do what I can to route breather tubing to a proper setup.

[koko64]

 

That's a good price for them too

[greencow]

Make sure those breather filters are clean, too.  One of mine disintegrated, wound up throwing a whole bunch crap into one of my carbs as well.  If those filters are original, you can always replace them with a small inline fuel filter.

[koko64]

I don't think they filter so much as they're more of a baffle. The perforated internal mesh looks like it evens out air pulses. If they are filters then they wouldn't stop dust or grit just larger stuff. Have you tried fuel filters and how did they go? Interested to see how they worked because they would actually keep the carb clean.

[Howie]

I don't think they filter so much as they're more of a baffle. The perforated internal mesh looks like it evens out air pulses. If they are filters then they wouldn't stop dust or grit just larger stuff. Have you tried fuel filters and how did they go? Interested to see how they worked because they would actually keep the carb clean.

They do sometimes clog though.  When they do it is a big problem.  Ducati does call them filters, though I agree they won't filter much. 

[greencow]

Yah, not so much a filter, like you said Koko, when I said make sure they're clean, I meant more a long the lines of not falling apart due to age.  I took one off, tapped it on the bench, and all the foam material that was sandwiched between the mesh screens just came falling out in pieces. 

I haven't personally used fuel filters, but I've seen it done.  I just took my old ones, ripped the junk outta 'em, cut a piece of generic sponge to fit, and crammed it in there.  Filters better, and still allows the slides to open.