Ducati Monster Forum

Hey guys, been a lurker for awhile now, but I'm kind of at a loss on this, so I thought I'd ask for your advice.  Bike is running just a tad rich on the horizontal cylinder, but super rich on the vertical.  Started a few months ago, and just got worse.  To the point that I'm fouling out that plug every week or two. 

Pertinent specs;

Jetting

Main - 140
Pilot - 40
Needle - Position 4 (second from the bottom)

Fuel enrichment screws - 3.5 turns out.

Drilled airbox

Arrow cans

NGK DCPR8E plugs

8,500 miles - all city driving.


Bike was starting to puke fuel out of the carb overflow, so I took them apart 2 months back.  Cleaned everything, replaced all the gaskets and orings, synced carbs with a DIY manometer.  Fitted a manual petcock while I was at it.  Now I don't know if it's just coincidence or not, but this problem slowly started after this.  I didn't change any jetting, the needle jets/emulsion tubes looked OK, but after 8,500 miles, all city, they could be shot and I just didn't realize it. 

Vertical plug sooty black, sometimes slightly wet with fuel.  I've been checking it after my commute home.  Bike fires up right away, hot or cold, needs minimal choke to start, but it doesn't want to stay running for long, without a bit of throttle.  Very rich exhaust smell, slightly smoky at start.  After it warms up, it'll idle and run fine, still smells rich, no smoke though.  Until the plug gets so fouled that it's popping and backfiring. 

At first I thought weak spark, so I replaced the plugs/wires, problem went away for a week or two, then came back.  Swapped the coils and ignition control units around, problem didn't move with that, so now I'm figuring its carb related, and not electrical.

Weird thing is, like I said, the bike ran decent-ish when I first got it.  Not perfect, but OK.  After I went through the carbs, and tuned everything, it was running like a champ.  A month or two goes by, and this starts to happen.  As of now, all I can figure is the needle jets are ovaling out, but I don't know for sure.

Did you replace the orings on the float valves?

I wonder about the float height on that cylinders carb.

Hey Speeddog, yah, orings and gaskets for everything. 

Koko, float was at 9 and 11 when I opened the carbs up, reset both to 14.

Did you clamp the float valve down when you set them?

Yah, used the float bowl "trick".  Unless I overlooked something, and the levels are set way outta whack.  I'm not getting any fuel coming out of the overflow, if that would indicate anything.

I'll use some clear tubing to check the levels, just to verify they aren't set wrong.

Assuming the floats are set correctly and one needle jet is not worn more than the other it could be your enrichment plunger is leaking.

Is there any way to test for that?  Short of just tearing it apart and replacing the seals on the plungers?

The only way I can think of is physical inspection.  I don't remember if you can get to the plunger with the carbs still on the bike, but you probably want to go through the carbs again anyway.

You'll need to get free access to the carbs to pull the vert cyl enrichener.

Not much more needed to have the carbs off completely....

Your bike have a vacuum or manual fuel tap? V cylinder manifold take the vacuum taps vacuum hose?

Fuel tap?  Are you talking about the fuel petcock? 

I replaced the vacuum petcock with a manual one, then capped off the vacuum line to the horizontal cylinder with a rubber nipple.  The only other vacuum line running from the vert cylinder is going to the fuel pump. 

Maybe pull the vacuum line to the pump first to see if there's any fuel in it? Perforated diaphragms aren't common, but it's possible.

What he said^.

Checked the quick/easy stuff first, gonna have to wait till the kiddos are sleeping before I can dig into the carbs.

Vacuum line to the fuel pump is bone dry, verified the floats are set correctly.

Gonna pull the carbs off tonight, if I can.  Ordered a set of FP needle jets as well, figured at 8500 miles, might be worth it anyways. 

Got the carbs on the bench, upon closer inspection, looks like the PO fitted a Dynojet needle and spring.  4th groove down from the top.  Definitely not the stock Mikuni needle, this one is tapered, and has more grooves than stock.

Other than that, the needle jet looks pretty good still, maybe slightly ovaled.... Very hard to tell, if it is, it's minuscule.

The choke/enrichment plunger looks in good shape.


Side note, the slide housing o-ring fits a little sloppy.  I used a KL supply kit to refurb these carbs, if that o-ring is ill-fitting, could that cause excess fuel to bypass the needle jet/slide housing?

Perhaps, but I've not seen that happen.

Check the rubber tip on the float valve needle.
I had one with a small blister on it, very troublesome.

 [thumbsup]

So, while I had the carbs on the bench, gave 'em another thorough cleaning.  While I was taking them apart, noticed a LOT of gas on surfaces I wasn't expecting to see it.  Mainly the slide housing, in and around it.  I had drained the carb bowls before taking them off, so that stumped me a bit. 

The only thing that was causing some concern, to me, was the new slide housing o-rings.  Fitment was super sloppy, and the o-ring itself was to large, in diameter.  Threw the old o-rings back on there, just for giggles.

Put about 60 miles on the bike so far, and it's definitely running less rich.  Vertical plug is still a tad to rich, for my tastes, before however, it would foul out with soot after a 15 minute ride.  To the point that I could scrape off a layer of it.

So, dunno for sure if that was problem, as of right now, seems to be.  I changed nothing else since having the carbs off the bike.  Still gonna throw some new needle jets on there, once I get them.  I'll keep you gents updated, thanks for all the input guys!  [thumbsup]

 [thumbsup]
Those o-rings should be a tight fit so it's good you put the old ones back in. They take a set over time and not all rebuild kits are equal. I leave them be if they look ok, but change most of the others if required. Needle jets should be replaced with Factory Pro needle jets as oem can wear quickly. Look up the Dynojet site for needle position recommendations for your modifications.

So, just to keep this thread updated in case anybody else comes across issues like this. 

Been chasing my tail for over a year with this problem.  After the carb rebuild kit, new needle jets (emulsion tubes), brand new ignition coils from CA cycleworks, etc....  I was still having rich issues, to the point of changing plugs every few hundred miles, or the bike would run like garbage.  Point of note, didn't really have this problem until I did the carb rebuild kit originally. 

Bought the full kit from Litetek, just had the bike out for a spirited ride, and boy does it ever run great.  Time will tell if that was the problem, but seeing as this only started to crop up after using crappy, ill-fitting seals from the first rebuild kit, I'm leaning towards yes.  Pulled hard, felt great all through the powerband, and no more rich puffs of smoke from the exhaust.  Props to Litetek for a solid kit  [thumbsup]

Well that seems to back up what I'm seeing. Customers using cheap knock off kits are getting the level of performance they paid for. There are plenty of rebuild kits on ebay of questionable quality at too good to be true prices. Avoid "unbranded" crap.
If oem gaskets and o-rings are in good shape I leave them be. Quality kits from reputable mfrs are pricey, but the only ones worth using. The longevity of oem components is quite remarkable and the needle jet seems to be the main wear item in BDST carbs.
I serviced a dirt bike FCR recently and it was so badly clagged from sitting in old fuel that every jet was blocked and the float bowl was full of blue gel. Disassembly and thorough cleaning both manually and with a sonic bath restored it. Every rubber part was good and stood "proud" of the gasket surface. Incredible quality. The only part replaced was the float needle as I couldnt envisage how to clean it in a timely way without dissolving the rubber tip. I noted how expensive the genuine kit parts were.

There is also the phantom O-ring which can interfere with the carburetion.

It is an O-ring internal diameter 5.5mm and toroid of 1.8mm.

(https://i.postimg.cc/jSCzQ68R/20210317-165413.jpg)

Yah, that's the pilot merge cavity o-ring I believe.  Pretty sure that wasn't even included in the first "kit" I put in the carbs.  The Litetek kit comes with a pair.