Ducati.ms - The Ultimate Ducati Forum banner

1 - 20 of 25 Posts

·
Registered
Joined
·
198 Posts
Discussion Starter #1
I'm planning to rebuild my Mikunis including replacing all the O-rings & Gaskets as well as the Emulsion tubes / needles & Jets..
Anyone who's done this have any tips or tricks or "No No's" ?
I have a Haynes manual plus from what i see it all looks like it should be a fairly easy task...Also Re: jetting what's been people's experience with larger jet sizes for running an open airbox..?
 

·
Registered
Joined
·
7,104 Posts
Just open air box alone with no exhaust change might not require any change. If you're buying an jet kit, this is not a drop in affair, anyway. While you're installing the new gaskets and kit parts, check your manifolds for flatness at the head flange. Mine were not flat, a potential air leak. Also , the air tubes to the vacuum can collapse and should be replaced with something stouter. If these tubes are not connected to a clean air source, they will suck dirt into the diaphragm . I used cleanable fuel filters and reinforced fuel line, which I routed to draw air from inside the tail. I think there are some videos on u tube showing the float adjustment, which is tricky. I found these carbs to be difficult to get right, but with a little patience it can be done. If you were thinking about an exhaust change , now would be the time. That will definitely require a retune, no sense doing it twice.
 

·
Registered
Joined
·
198 Posts
Discussion Starter #3
Thanks duc96cr ...great insights & advice ..just what i needed..:D
The bike came fitted with Staintune mufflers with restrictors (not fitted..)
I'd hazard a guess that the P.O never changed any jetting & the bikes always seemed a little "gutless" but runs pretty well generally & so i figured i'd see what jets are in there now..I have a selection of Main jets ranging from 137 >142.5 & a 42.5 Pilot jet ...so was thinking of trying a 142.5/42.5 combo...does that sound reasonable?
The 900SP came stock in Australia with 140/40 i think so i reckon that's whats inside now..
 

·
Registered
Joined
·
99 Posts
The Haynes manual will tell you the floats need to be set at the dealer as it requires some special equipment. 14mm seems to be the general consensus ( I went 13mm). I would take a measurement before you remove the floats and write it down just in case. I'd like to know what those numbers are actually, I've heard of mismatched float levels from 9mm to 15mm...
I've never tried the rebuild kits but I have heard there are one or two o rings not included with the kit. Possibly under the slide body and there's also an o ring under this metal plug that sits under the idle transfer ports. I'm guessing most people dont take those off.
Also, I don't know if you have rebuilt carbs before so forgive me, but its very important to use the correct size screwdriver. I'm talking the thickness at the end so it fits real snug in things like pilot jets.

I have a dynojet stage 1 with Micron cans and KN airfilter, running 124 mains and 37.5 pilots
 

·
Registered
Joined
·
7,104 Posts
I set mine at 14 but one leaked when hot, so I lowered it a smidge. I'd replace all o rings, even if you had to buy seperately. The float carrier seals were leaking on mine. All of them looked dried out. "if you don't have time to do it right, where will you find time to do it over ?"
 

·
Registered
Joined
·
3,977 Posts
i think the float level thing is very overrated myself. you can check them with the carbs assembled and fitted by putting a clear plastic tube on the carb float bowl drain and opening it. with the tube looped up to the side of the carb the fuel level should sit at the middle of the cast lump, either the little lump or little recess. make sure all the air is out of the tube. you usually need to remove the tube and let some fuel run.

i've run a few 900 without the lid on the std 140 mains (mikuni, not dynojet), but you need the kit needles or they fall over badly in the midrange. i have run them with 35 pilots to clean up the low speed as they can get very rich there with the factory pro needles are dynojet stage 2 needles (same root diameter). dynojet stage 1 needles are just a larger root diameter. but i'm told that can hurt higher rpm coming back onto the throttle. not experienced it myself.
 

·
Banned
Joined
·
378 Posts
Carbs

About half of all things are told to you are not that important. ex breather tubes. i just cut them as short as possible and stuff or wrap a bit of foam so it keeps the debree out.
all and all its a easy job you will have no trouble, (that is if you can buld a lego set) lol
 

·
Registered
Joined
·
198 Posts
Discussion Starter #8
The rebuild Kits i have are Mikuni Oem for a Yamaha super tenere ytz750 (made in Japan)
I got these for a song & also because they have all the O-rings , gaskets & springs & even a replacement air bleed plus an array of jets & the emulsion tubes & needles.
I'm now thinking if the existing jets are the standard 140/42.5 i may be wise to clean it all up & just re-use them. I'll replace the emulsion tubes of course & gaskets /Orings etc..
I have rebuilt & fiddled with old Dellorto carbs but the Mikunis are somewhat of a mystery still...but am learning & reading helps shed some light also..Next step is to just do it...
 

·
Registered
Joined
·
198 Posts
Discussion Starter #9
Am about to re-assemble everything & just wondering what setting the clip goes on the new throat/slide needle? there's 5 or 6 notches & on the old one it's only one plus a plastic washer/shim...Also the new needle is a few mm longer than the old one..
 

·
Registered
Joined
·
7,104 Posts
Use all the shims, etc. as stock. Put the clip in the middle groove, or measure the needles, new and old, and put the clip in a groove that gives you the same depth into the emulsion tube. You could also measure needle diameter and try to match it at that point. It will be a miracle if you don't have to make adjustments, though. Part of the process.
 

·
Registered
Joined
·
198 Posts
Discussion Starter #11
:p
It will be a miracle if you don't have to make adjustments, though. Part of the process.
LOL...Thats what i was afraid of...:p
 

·
Registered
Joined
·
198 Posts
Discussion Starter #12
Ok so i've installed the jets & the bike starts & idles fine. a little rich & high..but when blipping the throttle it gags & there's a slight delay in response.

Settings are as follows & i'm thinking i need to raise the clip on the needle one maybe 2 notches as the first step? Lower the needle into the emulsion tube...
(needle has the Oem plastic shim & washer fitted only)

Main jet: 140
Starter jet:70
Pilot jet: 42.5
Air screw: 2 1/4 turns out
Jet needle is on the 2nd notch UP^ from the bottom of the needle (the thinner end)

So my limited knowledge tells me it's a needle height issue i should be addressing first?
 

·
Registered
Joined
·
198 Posts
Discussion Starter #14
Yeah i've already fiddled around & tweaked the idle screw for quite some time starting from fully wound in & various small increments outwards... & 2 1/4 turns out "feels" about right . The engine is nice & warm too btw while i am doing this.
Irresepective of any idle tweaks the slight delay in response is always there..
 

·
Registered
Joined
·
7,104 Posts
Check plugs but yea, raise needles a notch sounds right. I mean put clips in a lower notch.
 

·
Registered
Joined
·
7,104 Posts
Even after synching at idle, you should watch the vacuum slides as you slowly open the throttle to make sure they are opening at the same rate. My vacuum caps were a hair warped. Remember these aren't connected to the throttle or each other. In general, if it doesn't take gas well or the idle hangs, open the screws. Mine are 4 turns out. Adjust these out 1/2 turn at a time, blipping the throttle, til the hang goes away.
 

·
Registered
Joined
·
636 Posts
A hanging idle can also be caused by a vacuum leak. Make sure everything is reconnected properly and tight and then do the WD-40 test. While idling, spray some around the manifold to head connections (one at a time) and listen for a slight increase in idle speed. Also check the carb to manifold joint. A change in idle speed tells you you've sucked WD-40 into the engine via a leak.
 

·
Registered
Joined
·
198 Posts
Discussion Starter #19
I lowered the needles into the jets & the whole thing seems to be coming together now more or less how it should..
The mixture screw is now out to 3 turns & there's little or no delay on throttle blip ..
Didn't have time for any finer tweaks or a ride but will today..I will try those Tips too thanks guys..but wouldn't it be SO much easier without that blasted Airbox /battery lump in the way?..It's such a pita to access anything!!!:mad:
Seriously thinking about a lithium battery & relocation options / pods after this little exercise...although i know once it's sorted i'll forget the pain..

Merry Xmas to all as well btw...we're almost a day ahead so it's now 25th..:D
Safe & Happy Holidays for all..
 

·
Registered
Joined
·
7,104 Posts
Yea, I know. I don't bolt all that stuff down til I get it right. Every time I play with the carbs, I think about pod filters, but the loss of filter cfm is a concern. You've got me thinking , though. Maybe I will seperate the air box from the battery ,coil unit so I don't have to unhook all that crap every time I want to tweak. I have noticed after you take it apart a few hundred times, you get a lot faster at it.
 
1 - 20 of 25 Posts
Top