On BDST carbs fitted to an M900 which side of the float should be measured to , I’m pretty sure I know but don’t want to bias any answers and these photographs contradict each other
In case anyone finds this lonely little thread in the future it’s this side of the float that you should measure to, if anyone thinks I’m wrong please shout up!.
there's much conflict over this Mark, elsewhere and on here. It's best to do this before stripping but, do your adjustment, if you feel certain that it needs it and then confirm with the tube method :- http://bradthebikeboy.blogspot.com/2016/03/setting-float-level-on-mikuni-bdst38.html I can certainly relate to, and agree with :- "I don't think I've ever checked one that has been wrong. Certainly not one that hasn't been played with anyway. If someone has been in there trying to make it right it's usually not."
Hi Chris, it’s definitely been messed with, not least by me!, when I first opened them one was at 11mm and one at 12mm, it was running rich so I set them to 14mm, I replaced the emulsion tubes etc etc at the same time, the O rings for the float needle carrier that came with the rebuild kit from NRP were clearly not the correct ones so I reused the ones that were already fitted, the horizontal cylinder is fine but the vertical is very rich, my best guess at present is that the reused O ring on that carrier is leaking and allowing petrol into the bowl even when the needle valve is closed, I called Allen’s performance today to get new O rings and he said “oh the ones nobody can get”, that explains why NRP sent me the wrong ones I guess, so I bought a new carrier assembly which comes with the O ring already fitted, £26, so hopefully my guess is correct, it’s so rich at present that when I pull the plug it’s black and sooty and wet with fuel!. PS I will check the level with a clear tube before I take them off this time.
Hi Mark Re: the leaking 'O' ring, just checking - is it definitely flooding that bowl or are you going by fuel weeping out of the overflow nozzle/pipe on that bowl? If it's the later type of bowl with overflow and drain galleries amalgamated into one outlet nozzle, then it's possible that the drain screw housing is cracked around the taper-seat (quite common/easy to do) and that's why it's weeping.
No fuel overflowing Chris, I’m at my wits end with this and can’t think of any other cause now, I’ve checked and rechecked the float heights and they are crack on 14mm, replace all of the brass bits on the carbs and checked everything is clear, I’m sure I read on another forum a while back that someone had a similar problem so I’m hoping it’s that!
Sorry, I only half-read your relevant post ^, I doubt seal leaking is the cause, it looked intact when you fitted it didn't it? I re-use them every single time if they look ok and not aged, they are over-kill for the job they have to do. It could be so many things causing it to run rich, even from new you won't get a clean appearance in the exhaust. People always "home-in" on the float levels thinking it must be to blame for running problems but it rarely is.
Yes, I’m far from convinced it’ll fix it Chris, run out of things to try though, the other cylinder with its carb identically set up is fine and is giving a good plug colour, this one is really getting black sooted and wet with fuel which is something I’ve never seen before, so it’s slinging fuel out for some reason , it ticks over fine and rides fine, I’m tempted to take the carbs off my 900ss and fit those to prove that it’s the carb but that’s a lot of messing around, I’ll get there in the end but it’s just hit that frustrating point at the moment
after hearing that, were you happy with the idle mixture screw? e.g. was the 'O' ring ok and did the tapered face look "true" and sealing evenly? A bit extreme but you could try swapping idle mixture screws or even floats and needle valves if you are convinced it's carb related just to see if it changes from one cylinder to the other. Are you happy with ignition pick-ups and their timing. Have you done compression test on each cylinder?
Mixture screws and O rings are new, timing and compression I haven’t checked , not saying I shouldn’t check those next but I’m thinking they are less likely than a carb fault at present, if it rains at the weekend (and I’m guessing it will) I’ll have the carbs off again and fit the new float assembly, plus re-check everything else on the carb, if it still runs rich I’ll move onto the other stuff, mpg is also way down I forgot to mention that, again it’s not conclusive evidence it’s the carb but put together with strong smell of fuel in exhaust gas, black sooty plug, plug wet with fuel, the other plug being a good colour, the bike starting straight away on the button and not missing a beat when running I’m still favouring a carb issue, at least it give me something to do on a rainy day I suppose!
your rich symptoms can be down to a pick-up coil partially breaking down just as much as a carburrettor at fault. Swapping both carbs might be the quickest way to rule this out. How's your vacuum pump hose?
Vacuum hose is good, refitted it recently to rule it out, given the bike runs well throughout the rev range with no backfires or hesitation do you think it could still be an electrical issue, I’m not saying it couldn’t be electrical but I’ve put investigating that side of it to last because it runs so well.
From what you've said it sounds as though there's nothing left to chance on the carbs, most jets, internals checked and/or replaced on both carbs? Diaphragms and small 'O' ring checked? After you've done external float level test on "bad" cylinder, then it won't be a waste of time to try compression test. Both coils/leads/exciters/plugs ok? You can do a resistance test on pick-ups, my memory is failing but documented elsewhere on here and easily Googled.
Cheers Chris, I might see if I can build up the enthusiasm to do the clear tube check one evening this week, cold in that garage though!.
I’m thinking of buying one of these Chris, I’m pretty sure that if I wear it whilst working on the bike I’ll get straight to the root cause.
You were right about the fuel level not being the issue Chris, I did the clear pipe check and as you can see in the photo it’s correct, I decided to swap the K&N needles that came with the bike for the standard needles that came with the rebuild kit, didn’t really think that would fix it as the horizontal cylinder was already giving a good plug colour, when I took the carb tops off I found the diaphragm for the vertical (problem) cylinder covered in fuel, also fuel in the recess & channel that the diaphragm sits in (ref photos) so that’s got to be a big clue, trouble is I don’t know what the clue is pointing at!, anyway I didn’t strip the carbs any further deciding to use this attempt just to rule out the needles (and more honestly just wanting to get it back together and go for a ride ), result is predictably the same, horizontal plug nice tan electrode, vertical plug black as soot and wet, the choke mechanism all seems to be working properly but how is that fuel getting up there?, I’m thinking could the primary jet have unscrewed itself and dropped out, total guess and I’ll obviously be taking the carbs off again and investigating but any suggestions are very welcome, cheers Mark.
Fed up with messing around, just bought another set of carbs, seller is a Ducati owner and vouches for them being in very good working order, so here’s hoping the carbs are the cause and not a symptom!.
