Hi all, I've just become the owner of a 1996 750ss. The previous owner had fitted two ramair filters and bos exhaust cans to the bike, but not bothered to dial in the carbs to compensate for the new parts so it was running very lean. The fuel air screws were 6 turns out. Having found it difficult to locate a stock airbox, I have jetted up from a 140 main to 150 and from a 37.5 pilot to 45, unfortunately this hasn't really improved matters and I'm still getting a lot of surging and rough running (tried the screws everywhere from 2 turns to 4 turns out). My question is, have a gone too big on the jets? I just wanted to know what other people's experiences are of jetting the Mikuni BDST 38's, and whether anyone has found a good balance. Any advice would be apprecaited. Loz
I would be fitting a stage 2 Dynojet kit, getting recommendations from people on here as to their settings and using that as my starting point, then get it set up on a dyno.
From my own experience the 140 main is fine, they tend to be too rich as standard. The pilots about 3 to 4 turns out. You would do better with a Factory Pro or Dynajet kit for the needles and springs though. However, if the needle jets are worn, and they usually are by 20,000 miles, you will not get a smooth transition from a closed to open throttle. This can be compensated to some extent by closing the pilot screw a little but it fucks up the idle. So new needle jets (also called emulsion tubes) might be worthwhile.
Taking on board what Gilps and Derek have already said above......... Firstly, I would establish what you have got in your carbs......you say you had 140 main jets fitted originally - this would appear to be a stock/OEM Mikuni jet. You can tell if it is by the little square logo stamped on it.....the same applies to the main jet needle which should be 5C-19 with a Y6 emulsion tube (I think I got those the right way round). The idle jet should be a 37.5 and the starter jet should be 70 (also with the square logo). Dynojet jets and needles don't have any markings at all; plus the size measuring is different to the OEM Mikuni. This means a 140 Mikuni main jet is a different size to a 140 Dynojet main jet. However, if someone has been sodding about in the carbs previously, the main spring may have been changed and the only way for me to tell what you have is for you to look at the images below. Once you know what you have got; you can either set them up as the stock/OEM carbs or fit a Stage 2 Dynojet kit for the best performance. Stock idle/pilot screw adjustment should be about 2 to 2.5 turns out; Dynojet at least 3 turns out. Bear in mind the float / fuel height setting is fairly crucial......this is because the fuel height is part of the idle circuit and the carbs don't really lift the slide until the revs have reached about 3000 rpm. You will find reference that the float measurement is supposed to be 14mm measured above the gasket face.....IMO 14mm is wrong and it makes the idle circuit run weak..........I suggest it should be nearer to 12mm. My 750 runs a Stage 2 DJ kit with straight through exhaust and the trumpets removed from the airbox.....Floats are at 12.5mm (but I should have set them at 12mm) so that means the pilot screws are approx 3.5 - 3.75 turns out. Don't expect both the pilot screws to have the same settings, though.......they can easily vary by 0.5 to 0.75 of a turn.
This is solid gold guys, thank you. I'll get a look at the needles this evening and also set the float height to 12mm. I know that all the jets I saw in there had the square logo on them, I might try putting the 140's back in there and seeing how it runs with just the larger pilot jets, I should of followed the golden rule and changed them one at a time to begin with. I guess checking the sync would help too, time to borrow some gauges...
I doubt it will run that well with stock jets etc, if the airbox top has been cut about and your exhaust is straight through.........but make sure the holes in the emulsion tubes at the top (where the thicker part of the needle sits when the slides are down) aren't oval. And as an aside......make sure the little O rings under the black diaphragm caps exist; plus the tiny O rings and washers on the pilot screws are still in there and in good condition.
Looks like dynajet needles! They were set to the third rung from the top, and with the new 150 main jet and 45 idle jet it's running way rich. Any recommendations? The reason I switched from the 140 main and 37.5 was that I had to run the bike with the fuel/air screws out to about 5 or 6 turns and the low end was rough.
Looks like needle and spring are both Dynojet....compare the spring with my images and see what you think...... If you agree with me, you want Dynojet main jets at least 148 or even 150.....third notch from top is correct for DJ needles....make sure you have them assembled correctly.....3 turns out for the pilot screw starting point. If you don't have any DJ jets, talk nicely to DJ UK Technical .....you might find them useful.....they also do a free exchange for worn main needles........but don't be surprised if they turn out to be 900SS ones........or not even suitable for a Ducati...........you never know what people have been doing with the guts of carbs. Change the idle jet back to 37.5. That's all assuming the main diaphragms and air shut off diaphragms; plus float needles and all O rings are OK....
I recommend morgantune gauges for balancing. Remember that the stock set up is more about emissions regs than smooth running or performance.
Thanks for the advice, I've checked floats and set to 12mm (they were at about 10mm) also done a quick bench sync with a guitar string and re installed the 37.5 idle jet. Diaphragms all look fine and the main jets have new o-rings. Going to throw everything back together and annoy the neighbours
When you checked the floats, did you reverse the float bowls so that they are fixed with two screws to make sure the float assemblies stay in place while you check? 10mm would make it run too rich IMO although one of mine was found at 12mm and one at 9.5mm....it would always start without choke.
Right, so I got my hands on a digital vacuum gauge and got the carbs synced at 3 turns out each with the needles on the third clip. Bottom end of power is very weak and choppy. Lurches a lot at thirty in second gear and bogs at 30 in third. I'm going to double check my float settings again because the front cylinder is running way rich now wheras it was the back cylinder before. I'll get back on it tomorrow night and let you know my findings! It idles really good but running is a bit rough still. Quick blasts of the throttle are producing a puff of black smoke too (probably that front cylinder). Thanks for your continuing advice!
"Lurches a lot at thirty in second gear and bogs at 30 in third"............... Yes, Sir...........they all do that........... You will find you are constantly changing up and down between 2nd and 3rd at 30mph. I don't know if you have managed to get hold of some Dynojet main jets ( rememember they are different to Mikuni) or if you have changed the pilot jets back to stock.......and of course we don't actually know if your main needlea are Dynojet 750SS ones or 900ss ones. Also, my advice is find a non-cut about airbox lid and fit it without the trumpets before you keep fiddling with the carbs. But it sounds as if you are getting nearer to solving things.
I'd like to thank you all again for your input on this. Floats are both bang-on 12mm, I've balanced the carbs and gone with the 150 main and 37.5 idle jet with the dynajet needles set to 3rd clip from the top. With both of the mix screws out at 6 turns it's idling at it's strongest so I've left them there. The low end is a little rough and it pops through the air filters and cans a little sometimes, but it pulls smoothly on acceleration and runs smoothly when cruising around 40mph+. I'm going to try and track down an airbox to replace the previous owner's foam ram-air pod-filters and keep the un-baffled bos cans. In the meantime, do you think there is anything to be gained by going back up to bigger idle jets and getting the mix screws back to about 4 turns? Again, thanks in advance!
A coil kit can help with smooth running and idling. I've even seen it suggested that a starting circuit cable kit can help as well, it will certainly help with starting anyway... Dropping the gearing is a big help as well and a 14 tooth front sprocket is a quick, cheap way to achieve it. I never did get my SS to idle and pick up perfectly but I did get it a great deal better than the standard set-up. Hope you do better than I did.
My 750 idles at 1000 rpm when hot although in really warm weather it will rise to 1100 rpm. It picks up instantly when the throttle is opened but has a slight 'rough spot' around 1800 rpm....once it hits 2000 rpm it will rev very cleanly. Until it gets hot, it will spit back in the airbox very occasionally on a small opening of throttle. I would leave yours as it is until you get the airbox cover fitted......that will have the effect of richening the mixture, therefore the idle screws could probably go back to 4 turns out.
IMO, when there are carb / fuelling problems it is always better to get back to a known combination............for example an OEM set-up using stock jetting etc; or making sure any Dynojet kit that is fitted has the correct components....... .......at the moment, someone else has been buggering about in the carbs prior to the OP and it seems to me that the OP is trying to get the DJ kit to work, using Mikuni main jets at least 10 sizes greater than the stock size. With pod filters, no airbox, a mix of DJ and OEM carb components, plus a non-stock exhaust; I feel the OP is trying to cure / solve the issues all at the same time, whereas it is much better to change one thing at a time and alter the jetting / settings at each stage.
Haha That's a bit like replying to someone asking for directions by saying that you wouldn't start from here. I know what you mean though and in this instance, I can't help wondering if the solution might be to take the bike to someone with a rolling road set up in order to get it properly sorted?
