Identifying some mystery parts/fittings on my Mikuni BDST 38 carbs
In the process of restoring an old Ducati, I cleaned and re-jetted the carbs. I know my way around carburetors in general but there are a few parts on these that I don't understand. The carbs in question are a side-by-side pair of Mikuni BDST flat sliders.
https://imgur.com/a/UCFco in this pre-restauration image, at the bottom there are two brass pipes with hoses attached to them (one has an anti-kinking spring on it). These hoses had a little plastic air filter fitting attached to them and ended in a small plastic drain container. They were all cracked and stuff so I assume they aren't important for function, however I can't find out for sure what they are. Possibly plumbing for a carb heating system? Or some sort of vent to do with the emissions control systems? My bike has neither, so what should I do with these pipes? Block them off or leave them open?
The other one I am wondering about is the little fitting next to the float chamber drains. This picture is taken top down, bowls are on the right, throttle cables and fuel hose in the middle. The thin plastic hoses are the drains for the float chambers. The thicker rubber hose with the circlip on it, at the top left in the photo, is the one I can't identify.
The picture is also pre disassembly, and as you can see the hose is only attached to one of the carbs. It ended in the same empty plastic container as the hoses I was talking about above.
Posted Dec 21, 2017, 10:46 PM UTC
Good to see some familiar parts on here! You're correct that the top set of hoses are Vacuum pipes, necessary as these are CV carbs not flatsliders. These should be routed to the breather box as you have them.
The smaller fittings below are LH: Carb Overflow and RH: Carb Drain. The setup on mine is with the hoses on the left fitting for normal use and I'll switch them over as and when the carb needs draining. The overflow will have fuel coming out of it so should be routed to the ground.
When you get to starting the bike again post restoration the function will likely become clear, I had teething problems with fuel leaking from the overflows and it's a common experience post rebuild.
Excellent that solves the mystery of the small hoses! I like your idea to have only one pair connected for normal operation. I will find some rubber line and route these below the engine somewhere.
So if these are the bowl overflow ports, what is the function of the second plastic T connection behind the main fuel supply? Mine currently (pre and post rebuild) has nothing connected there
I believe that setup was how they came from the factory but mine had 6 previous owners pre rebuild so who knows!
The second T piece is a vent hose for expansion gases and to prevent a vacuum in the float bowl under heavy use. I did wonder about the lack of it in your photos, put it down to age difference but we both have '94s on reflection.
Can't comment on whether it's essential, wouldn't imagine it being a problem apart from the potential of debris ingress.
ProTip I can give you is to ensure you've got a functional fuel tap, hopefully not necessary but in my case fuel drained into the cylinders when they overflowed even after the ignition was cut.
Thanks again. Mine also has an unknown history before me, and at least one previous owner was definitely a hack. I spent a lot of time fixing his little sins during this rebuild.
I'll try to get a short piece of hose on the second tee to alleviate the debris issue. Tank is going back together today so I'm planning to check float levels and such, hopefully the float valves don't leak. I did already fix the fuel tap just in case, going by what I found in there it wasn't meant for frequent operation though.
Always best to replace if in doubt, I got new needles (float valves) for about £10. As they are finicky I'd recommend it! This is a good guide for the float height.
I went to an additional inline tap for piece of mind but definitely replace the fuel filter while you're in the tank. I'm not an expert but been through many issues so feel free to message me in the future.