Problems with '94 900SS Carb Diaphragm Operation - Ducati.ms - The Ultimate Ducati Forum
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post #1 of 12 (permalink) Old Jul 6th, 2019, 8:22 pm Thread Starter
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Problems with '94 900SS Carb Diaphragm Operation

Hope I have this thread in the right place. Haven't been around for a while, but bringing a 1994 Bimota DB2 out of hibernation. The engine is basically a '94 Ducati 900SS engine complete with Mikuni 38mm CV carbs. I've been through the carbs with new gaskets, O-rings, and diaphragms. If I blow through the diaphragm vent hoses, both slide/needle assemblies lift as they should so I'm reasonably certain the diaphragms are seated correctly. No leftover parts, no leaks, no drips, etc.

Engine starts, idles and revs on the stand, but one slide in one carb doesn't lift as it should when you rev the engine. Also, when the engine is running, the side with the non-operational slide has a lot of "pulsing" in the diaphragm vent hose while there is what feels like no airflow or pulsing through the other vent (the side that is working). What am I missing here? Any help here appreciated.

Thanks.
Chuck S.

Last edited by wallaman; Jul 6th, 2019 at 8:28 pm.
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post #2 of 12 (permalink) Old Jul 6th, 2019, 8:35 pm
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Quote:
Originally Posted by wallaman View Post
Hope I have this thread in the right place. Haven't been around for a while, but bringing a 1994 Bimota DB2 out of hibernation. The engine is basically a '94 Ducati 900SS engine complete with Mikuni 38mm CV carbs. I've been through the carbs with new gaskets, O-rings, and diaphragms. If I blow through the diaphragm vent hoses, both slide/needle assemblies lift as they should so I'm reasonably certain the diaphragms are seated correctly. No leftover parts, no leaks, no drips, etc.

Engine starts, idles and revs on the stand, but one slide in one carb doesn't lift as it should when you rev the engine. Also, when the engine is running, the side with the non-operational slide has a lot of "pulsing" in the diaphragm vent hose while there is what feels like no airflow or pulsing through the other vent (the side that is working). What am I missing here? Any help here appreciated.

Thanks.
Chuck S.


Check the vent hoses for kinks or blockage. You could disconnect the slide vent hose and see if the suspect slide lifts when applying throttle. In addition, make sure the small O ring on the diaphragm cap didn’t fall off during assembly.


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post #3 of 12 (permalink) Old Jul 6th, 2019, 10:34 pm
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Member *Wallaman I see you live in Phoenix. I live in Yuma, and have lived in Tucson (over 38+ years all tolled). There is a small bee that lives in these parts of the world, I don't know it's actual name but folks around these parts call them "leaf cutters". They are small bees, about the size of a medium house fly. They LOVE LOVE LOVE small, round holes, preferably about 1/8" (0.125"). They lay their eggs inside of small round holes like that. Then they take small bits of flower petals and sortof roll them up like tiny little "doobies" and pack them in the same hole that they placed their larvae inside of to protect their unborn young. Those rolled up flower petals and their larvae will plug a breather or vent hose solid.

It's actually a very common problem with propane powered RV generators and propane BBQ grills. Those little critters will plug the vent hole of a propane regulator with their little "package" and prevent the regulator from working. That vent hole in a propane regulator essentially operates like the vent line in your CV carbs. If plugged the diaphragm cannot move freely.

So, as Member *walduc suggested blow out the vent tubes. Make certain to remove the tubes so when you blow them out whatever may be in the tube won't get packed into the carb.

Those little leaf cutters get into everything out here. Any type of round hole that roughly 1/8" in diameter is precisely what they search for. I've found their little packages inside of hoses, vent holes, oxy-acetylene twin-hoses, et al. Especially in areas where there are blooming flowers since flower petals seem to be what the little guys prefer to use.

It's just a thought.


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post #4 of 12 (permalink) Old Jul 6th, 2019, 10:47 pm
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the chokes also pull air through those hoses under the diaphragm, so maybe it's a choke airflow interaction?

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post #5 of 12 (permalink) Old Jul 8th, 2019, 10:09 am
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the chokes also pull air through those hoses under the diaphragm, so maybe it's a choke airflow interaction?
I didn't kown that and it made me think.

Last winter I serviced the carbs on my bike successfully. The bike runs better, starts better,...
There's just one thing that's changed since I mounted the carbs again and that's the choke.

At first: I pulled the choke all the way when starting, the bike started and revved up to +/- 3000 rpm, then I released the choke till the 'click' halfway and the revs dropped to +/- 2000 rpm. At that time I started the ride and after a few hunderd meters I turned the choke off.

Now: when i pulled the choke completely the bike still revs to +/- 3000 rpm but stays there even after I pull the choke back to the halfway click. From there on the revs drop to 2000 rpm when I pull the choke halfway towards the off-position.

So since I serviced the carbs there is no change anymore between full choke and halfway choke.

before: choke 1/1:3000rpm, choke 1/2: 2000rpm, choke 0/1: 1200rpm
after: choke 1/1:3000rpm, choke 1/2: 3000rpm, choke 1/4: 2000rpm, choke 0/1: 1200rpm

The only thing I changed is that I removed the air-filters in the diaphragm lines. I should put some new ones in but I haven't done that yet. ( I could not find the OEM ones and I'm not sure I can replace them with simple inline fuel filters)
Could the absence of these air filters cause the change with the choke?

(there's no problem with the choke cable or the choke on the carbs, all works flawlessly)

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post #6 of 12 (permalink) Old Jul 8th, 2019, 10:48 am
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The stock hoses are thin and prone to kinking and collapsing. They may look ok but when they get hot and soft it can happen. I replaced my diaphragm hoses with reinforced fuel line and the type of fuel filters that you can unscrew and clean. The hose ends should be tucked somewhere out of the air flow and if they’re hidden the critters won’t lay eggs in them. Always replace the o rings under the caps. Check the slides for free movement. My plastic caps weren’t flat. I flattened them with some 600 grit paper on a pane of glass.
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post #7 of 12 (permalink) Old Jul 8th, 2019, 5:45 pm
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if you removed the carbs i expect you removed the choke cable, and that when you refitted it it was in a slightly different position?

also, the choke idle rpm is far more influenced by ignition advance with the single stage step than the actual choke opening. it tends to be a lot more linear with an ignitech.
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post #8 of 12 (permalink) Old Jul 9th, 2019, 7:39 am Thread Starter
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Quick update. Pulled everything apart again and no obvious issues I could see. I blew out/cleaned all the small passages again (they all seemed to pass air before and after). No signs of "insect residue". Checked the diaphragms to be sure I didn't tear one and the tops to be sure they were flat. Checked against the exploded diagram I have to make sure all the parts were there.... they were. After putting it back together, same result. I did remove the small filters from the diaphragm vent hoses, no change. You open the throttle and one carb slide goes up, and the other doesn't. Interesting tid bit... If I block the diaphragm vent on the carb that doesn't work properly, after revving the engine and closing the throttle, the slide lifts as the engine slows down (lot's of manifold vacuum in that situation)! I know that is telling me something, but not sure what....

Appreciate all the suggestions, but I'm still stumped. Anyone know of a write-up that explains what each passage does on these carbs? I've worked on a variety of CV carbs over the years, so I understand the principles of how the diaphragm slide works, just not sure where all the little passages go in the carb body itself.

Thanks (again).
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post #9 of 12 (permalink) Old Jul 9th, 2019, 8:00 am
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is it really out of balance?

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post #10 of 12 (permalink) Old Jul 9th, 2019, 12:46 pm
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@wallaman do both throttle plates visibly open at equal rate when the throttle is actuated?

If you split the set at any time...you assured the small linkage spring between throttle synch levers is present and assembled?

(note in splitting, subject spring becomes loose and sometimes will "go missing", its critical to throttle plate synchronization, its presence needs to be confirmed)
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