Trouble tuning Mikunis

[Foxtee]

I´ve fitted some mikunis that I got off a Paso being wrecked in the US (M750 originally), but I cann´t get the buggers to run right.
The bike shop said that was to lean when I took it in for a WOF , so I stripped them down and check trough all the post in this forum. From what I could tell they had the original jetting etc , so got some 140 mikuni mains and reset the clip back to the 3rd groove on the needle - still ran rough maybe to rich this time as the plugs were black & wet.
Got a Dynojet stage 1 kit for M750 and have fiited the new needles adjusted the mixture screw to 3.5 turns as well tried both sets of jets.
The bike starts ok, but idles rough until warm , coughs back though the carbs & back fires (with flame) out the mufflers, apart from scaring some children I haven´t had much joy with the tunning.... any help would appreciated.

ps 12v mod to the coils done & check coils are off a carb´ed M750 , new belts fitted

[paso750]

Before messing with the jetting did you check if the carbs are ok ? There should be no air leaks, you should make sure the choke is completely closed when the lever is turn to its off position, too.
Did you keep`the fuel pump and fuel return line to the tank ? Did you synchronize the carbs properly ? (!) What`s your airfilter setup ?
I assume the bike ran ok with the old carbs, hence the valve clearances are ok?!
Looking at your avatar your exhaust is not original, this and the airfilter will be one of the reasons why a standard jetting may not work properly.
As it`s backfiring I`d check the points mentioned above first, first of all synchronization.
If you then individually want to determine your jetting check the SUDCO manual.

SUDCO Mikuni tuning manual
http://www.box.net/public/njc0psmd7x

G.

[Foxtee]

Hi G
The carbs looked almost new when I got them, came with everything except the throttle .
Balance checked with a good set of gauges , 60mm pod filters,choke action good , the mufflers I was told are of a 900SS (I went for the Dynojet kit as should work with non-standard exhaust) .
I did have probs with the webber as well, also rust in the tank now fixed with a POR15 seal kit which did a great job on removing it.
I´ll double check for air leaks , fuel pump and return line back into tank are still in use


I´ve read the tuning manual, but the valve clearances I hadn´t thought of I´ll go there next

[paso750]

check also if the fuel inlet and float chamber overflow line were not swapped by accident. Furthermore you`re missing the tubes with the one way valve that are connected to those bent breather tubes on the right side of the carbs.

[Tamburinifan]

I would simply try w removing those air filters and see what happends.
They look too small and could be too restrictive.

[micklm]

Hi,

A couple of thoughts, first of all to agree with previous posts about syncronisation, jetting for an aftermarket exhaust/air filter and connection of the breather hoses (to the frame mounted black boxes on the SS and monster models). See below:


Also, have you connected the vacuum sensor line from the digiplex to the inlet manifolds on the Minkunis? See above photo and http://forums.ducatipaso.org/viewtopic.php?f=2&t=3387

Another thought - I had issues with Mikunis on my 600SS that were at least partially attributed by the Mech that solved it to worn emulsion tubes (not commonly replaced in most jet kits). See http://www.factorypro.com, search for CRZ-EMU-100120K or under \Ducati\M750\

Sometimes ditching the airbox can make sync issues/poor idle worse and can leave a big hole in the midrange - do you know anyone good with fibreglass? Maybe changing the bottom of the airbocx to match the Mikunis might help - there are some photos of this modification in the photo gallery on this site
http://gallery.ducatipaso.org/index.php ... ZiYzYxN2Mw

Final thought (I promise). I used to live in NZ (New Plymouth) and I had severe carb icing issues with Mikuni carbs on my 600SS due to the long inlet manifolds. I ended up insulating the inlet manifolds and adding a Ducati carb de-icing kit that ran oil from the oil cooler outlet/inlet plugs on the lower RHS side of the engine to special carb bowls with internal heating passages. This helped ease (but not eliminate rough idle/engine stumble/plug fouling issues. There is also an electric bowl heating kit that came out later.

Good luck - let us know when (and how) you exorcise the gremlins!

Cheers,

Mick

[paso750]

Also, have you connected the vacuum sensor line from the digiplex to the inlet manifolds on the Minkunis?

Very few P750 had a Digiplex ignition. If you have a Kokusan (2 BB1105 ignition modules) there`s no connection to the carb.

[Foxtee]

Yup I have the BB1105 ignition modules so no connection to the carbs,
the air box had been replaced with an ill fitting K&N filter to webber carb (1 of the reasons for the change) before I brought the bike.
I´ve ordered the one way valves for the breather tubes 3-4 weeks from Italy $15NZ each (not to bad ) and I´ll try running with some larger foam filters + check the valve clearances on the weekend.

[jcslocum]

Do you have the skills & tools to check cam timing? There is a thread in the 906 forum about a always having a rough idle until the cams were degreed in.

[micklm]

Oops - no Digiplex eh. Maybe I should stick to the 906 forum Sorry for the red herring.

[Mustang505]

Hi Fox
have just fitted a set of Mikunis to my sport and started it for the first time last week. Your symtoms sound almost exactly like what i had. Same as you once, warm it almost is ok and all the coughing and backfiring disappear and it will then idle. Needs a lot of running on the choke to get started and run. To me these systems are too lean but I could be wrong. my carbs were fairly used, so i replaced the emulsion tubes, fitted 140 main (175s fitted came of a 900 monster). I also have 60mm foam pods fitted. the rest of my jets are - pilot jet 40 know standard for 750 is 37.5, main air 50, and pilot air has 1.4 on it, and needle is set at 3.
I have removed the fuel pump and it runs off gravity feed. how important are those breather pipes and valves you have ordered? mine does not have either.
let me know how you get along - i'm going to continue fidling this weekend.

PS there is guy here in CHC who runs mikunis on his paso and it runs sweet - will try to find him again and see what he is running.

[88WhitePaso750]

Do you have the manufacturer / part number for the air filters you used. I have been looking for some short one's like those with no luck?


Thanks,
Chris

[Foxtee]

Going to try the foam unifilers I got with carbs and see if they breath easier , the pod filters I got off ebay, just did a search for 60mm pod filters paid $50NZ including shipping for the pair.
I´ll check valve clearances- timing as well , I´ve got a pair of 140 mains plus the original 132 and the Dynojet kit jets , I think the other Paso in CHCH has posted his setup on this forum, which I think was the Dynojet stage 1 kit with different air jets.
This cann´t be any harder than a Xerox iGen4 to sort .
Thanks for the input so far I´ll update with results

[Foxtee]

Fitted old foam uni-filters, cleaned then best as I could as they soaked in a sticky oil (they could just about stick to the wall) .This has made a big difference , starts with a little chock and once warm idles perfectly (1100rpm approx) no back firing or flame when rev´d up.
I haven´t had a chance to ride it yet , work & weather holding me up, but have order some new filters

[Mc tool]

Those missing hoses ( twice mentioned now ), I believe are the breather tubes for the float bowls, and previous experiance ( I nearly took to it with a shotgun ) tells me that any change in pressure seen at these vents will drastically effect mixture. I removed all the anti polloution gear on my VFR700, connected all the carb breathers together and poked the hose down the back of the motor with all the others. It ran like a swiss clock ( I'm sure finnish clocks are very good too ) till I got moving. I eventually reduced myself to riding up and down the road with this hose stuffed into my helmit, sucking or blowing into the hose , even as gently as I possibly could drastically effected the motor .
I would suggest fitting the hoses , valves and still air boxes off a 900ss , I am sure this will help , sorry Foxtee I didnt realise the significance of the comments ( by others ) re the missing hoses till I was re reading the post just now,