Monster carb question

[Furberger]

I have a Ducati Monster, its only a 400 and it was built for the japanese market to comply with some licensing nonsense or other.
The bike seems to have no bottom end - at all. Nothing happens till you hit 6k rpm. I've ridden a few 4oo singles and they all seem to go better.
Today a took the airbox apart to see if there was some restrictions in there to comply with the licensing nonsense. Looking into the carbs I could see what I thought was the butterfly, however, when I blipped the throttle it slid, not rotated, and didn't respond exactly to the throttle opening. When I killed the bike and opened the throttle it didn't open at all, so it must be opened by the incoming air, some kind of damper perhaps.
Anyway, i've not seem this before, so apologies if I'm being dumb, but is this affecting the low end grunt and throttle response? When you give it a handful with no load its very slow to pickup.
Is there any way of getting rid of it and what will happen?
Thanks!

[Pumaracing]

It's clearly a vacuum operated slide throttle like an SU carb and those are in fact very good for low rpm torque because they only open far enough to satisfy the engine's air demand. Your problem, if there is one, lies elsewhere.

[Furberger]

I was hoping for a less sensible answer like rip it out and with the right resistor off ebay it'll be good for another 20bhp, but what you've written makes sense.... cheers!

[GreenV8S]

I am visualising that as working on the same principle as an SU carb to maintain constant depression and high turbulence across some sort of jet.

Transient enrichment on an SU is achieved by a damper which prevents the slide from opening quickly and increases the depression as it opens, so more fuel is sucked in. If this wasn't working (for example lack of oil in the damper) it could produce a lean condition when you open the throttle. If you find there is a flat spot after opening the throttle, it would be worth investigating to see if yours is similar.

[Pigeon]

They are Mikuni BDST 38s which are indeed constant-depression carbs.

AFAIK they use an accelerator pump for transient enrichment rather than delaying the piston movement with a damper.

[Simon Says]

Furberger said:

I have a Ducati Monster, its only a 400 and it was built for the japanese market to comply with some licensing nonsense or other.
The bike seems to have no bottom end - at all. Nothing happens till you hit 6k rpm. I've ridden a few 4oo singles and they all seem to go better.
Today a took the airbox apart to see if there was some restrictions in there to comply with the licensing nonsense. Looking into the carbs I could see what I thought was the butterfly, however, when I blipped the throttle it slid, not rotated, and didn't respond exactly to the throttle opening. When I killed the bike and opened the throttle it didn't open at all, so it must be opened by the incoming air, some kind of damper perhaps.
Anyway, I've not seem this before, so apologies if I'm being dumb, but is this affecting the low end grunt and throttle response? When you give it a handful with no load its very slow to pickup.
Is there any way of getting rid of it and what will happen?
Thanks!

Chances are if its a low mileage import is that the carbs have gunked up through long storage times anything up to or over 12 months,the fuel jets in that size of carb are very small and usually its the pilot jets(idle)that suffer 1st,the fact the float bowls hold so little fuel this causes the fuel to go stall that bit quicker,this is a very common problem with bikes that are left idle for long periods,me personally i would have the carbs out and drop the float bowls if you find them full of sediment or brown gunk then chances are you have found your problem,just because it idles and runs is not proof you don't have carb issues,some import specialist have ultrasonic cleaning facility's for this very problem its that common,also if its a jap import it may be worth checking jetsizes with manufacturers specs the Japanese are well known for unskilled meddling on there motorcycles,i have seen some pretty grim modifications on grey imports especially the big four(Honda,Suzuki etc)that generally compromise reliability or power outputs

Edited by Simon Says on Tuesday 21st July 09:44