Hi all. Calling out to carburettor experts! I'm rebuilding my BDSTs and ordered some new needles from Allens. According to Haynes I needed 5c19s. However they are quite different to what are currently in the bike. They have much less taper. They do fit snugly in the emulsion tubes whereas the current needles are a rattling fit. Does this mean current needles are worn or incorrect? The reason why I stripped the carbs is that the bike would not pull at high revs. Felt strangled at the top end. Many thanks for any help.
The 5C19 are the stock Mikuni needles. If there is no size marked on the thinner needle it is a Dynojet kit needle. It will help if you post a photo of the spring to confirm it is a Dynojet kit. Dynojet will replace their needles FOC if they are worn and if you send them to Dynojet but you will need to know which kit is fitted.....the main jet size will tell you that....or I will. NB. You cannot mix and match needles etc from a Dynojet kit with the stock Mikuni items. They are calibrated using different methods.
Thanks Arquebus. Most helpful. Attached a picture of spring, main jet and emulsion tube. Looking at dynojet kits online they dont seem to include an emulsion tube so I presume the standard one is used? The needle profile looks the same as dynojet so I'm sure you're correct. The bike had got a k and n filter and a cut off airbox top. Is the staining on the needle an issue?
The emulsion tubes used with Dynojet kits are normally the stock Mikuni ones....if the hole (not the one in the side) is round they are probably OK, if it is oval, near the top in particular, change them. However, the big jet you show in your photo isn't the main jet....that is the starter jet which is a Mikuni one (note the square logo). The main jet is the one held under the forks of that small plate. It would be helpful if you could lay the spring down by the needles so I can get a proper look at the coils because the angle you have it in your photo certainly makes it look like a stock Mikuni one. Here is a comparison photo which you can use to judge for yourself.... The fact that your airbox lid is cut about or missing suggests you may have an Stage 2 Dynojet kit fitted.....depends if your exhaust cans are straight through. But I am beginning to think you have a part Dynojet kit fitted, which is why the main jet will be useful to see.....no logo on it, only a size.
No logo, so given you have Dynojet needles it is most likely a Dynojet one...which at that size it would be from a Stage 2 kit. You have four options.... 1) Try to live with what you have got (if it runs OK) 2) Get stock Mikuni jets (size 140) and fit with the stock needles 3) Tell Dynojet what you have got and perhaps ask for some springs to suit and fit them instead of stock. (One thing I don't know about Dynojet Stage 1 & 2 - or 3 which isn't common - is are the springs and needles the same for each kit....I think the St 1 needle is slightly different to the St 2. 4) See if anyone has got used spares for a Dynojet kit....make sure you know which bits you are getting just in case they vary as per 3) above. I fitted a Stage 2 kit from a 900ss in my 750ss carbs using the Stage 2 750ss jets; and it ran fine.
Following this thread with interest. I have had a sh1t load of grief trying to sort my carbs on my monster 900
Cheers Arquebus. I've got no interest in more performance, I just want the thing to run nicely. The problem was that the bike started fine and ticked over OK once warm. Ran well in the lower rev range but felt very flat from mid range up...kind of strangled. It would seem I have a bit of a mongrel set up. But I don't want to go chasing my tail adding non standard parts. It seems I have standard springs, standard emulsion tubes and main jets only just a bit bigger than standard. So I propose to fit the standard needles and go from there. Maybe the slight upjetting will balance the missing airbox top? We can but hope! I have standard silencers. I have stripped down and thoroughly cleaned the carbs and blown through all the jets and passages with an airline. The only blockage I found was a bunged up pilot jet.
For the few that fit Dynojet and have a peachy time, there are thousands that have had a life of grief. As far as success with Dynojet kits goes, much depends on patience, a will to fiddle and a certain amount of understanding carburettors/knowing what you're doing. Over the last nearly 30 years, whenever I've heard the word Dynojet with reference to the 900ss, it has almost always been in relation to someone putting a bike they've bought back to standard.. People that know most about the kits will invariably have been successful in fitting and running a Dynojet setup. They are few and far between IMHO
Ziff....don't make the mistake thinking that the Dynojet 144 is only a bit bigger than the Mikuni stock 140. Dynojets are measured/calibrated in a completely different way to Mikuni. Apparently one is measured by the hole diameter; the other by flow rate; however it seems that the chamfer around the hole also affects things. In fact, the 144 might be smaller than the 140 or a lot bigger. The consensus of opinion normally seems to be that Dynojet are larger than Mikuni......but in my simple mind I would have thought that the narow taper of Dynojet needles and a larger hole could well lead to flooding problems, so maybe the spring is much lighter allowing the slide to drop quicker thus shutting off the excess flow. I would give a Dynojet Technical guy a call and ask him. I think you will find the stock needles will not work that well. IMO the best thing you can do is find some Dynojet springs because the stock springs are causing the lack of power inasmuch they are too strong and don't allow the slide to lift quickly enough. Alternatively get some stock 140 jets and fit the stock needles, plus fit an airbox top. Standard exhaust won't help with the current set up either. I don't know all the answers to Dynojet set ups, but it's a bit like the current vaccine programme queries.........I just do what the manufacturer's recommendations say.
A quick update....having gone through all my SS files (of which there are loads)......Dynojet say that you can use the stock springs, but you will have a slower throttle response. But I still think you might have problems with the stock needles and the Dynojet jets together....
Thanks arquebus. Your insights are so valuable. I think I will put everything back to stock. I went to some expense to find and fit original silencers. It had monster ones when I got it. Does a missing airbox top make much difference? The top has been cut off so I would need to source a complete top.
I think you will find it was for drilling out plugged access to the pilot screws, but I will check tomorrow.....when I go through all my files.....again.
Zuff....I ran my 750 with straight through Laser cans, Stage 2 Dynojet kit, god knows what air filter, and airbox lid with no trumpets......
