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 Post subject: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sat Dec 21, 2013 9:30 am 
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Joined: Wed Mar 25, 2009 12:06 pm
Posts: 135
Location: Brisbane, Australia
model: 906 Paso
year: 1990
Here is what I learned when tuning my Mikuni BDST38 CV. Because I am using a Wideband sensor and controller to get AFR readings as I tune, this may be of interest to some.

I bought a second hand Mikuni BDST38 CV and added a Factory Pro Mikuni jet kit. I also swapped to a Mikuni vacuum pump (thanks to Gerhard). I have had the Webers tuned nicely (thanks to Higgy) making some modifications to the emulsion tube (ET) etc, so I am not thinking the Mikuni is better, but I hope I will get something from the Mikuni for giving up the accelerator fuel pump that the Weber has. I am thinking the Mikuni will need less tweaking and maybe there is an advantage in the manifolds (Webers have a 90 degree turn on one manifold) to get better fuel economy. Time will tell, maybe in time I will get to afford and want to try the FCRs. In the seat I think I might have lost a bit of power compared with the Weber when it's tuned in the sweet spot.

Some background on light acceleration/part throttle for the Mikuni: I found there is a natural rich load at the point of light cruise acceleration. I expect this rich load is designed in by way of the emulsion tube (ET) to allow for the option of quick acceleration. In a way it is like a default accelerator pump jet i.e., if you were accelerating hard from a stop the rich load at that time would be needed. Only, this rich load on light throttle is a compromise because it is also there on light acceleration such as when cruising and you just add a tiny bit of acceleration/part throttle - that is not what you want. Since there is a lot of driving on the idle circuit and a lot of light throttle going on for the normal rider, I would want to limit the rich load for example to not drop into 10 AFR (Air Fuel ratio). That is the only point where I cannot reach my goal to get an AFR between early 12s to late 13s (12.8 seems the optimal point to aim for).

Some background on the main jet's influence on the idle and off idle/light throttle with the Mikuni: While it is true the main jet kicks in at max acceleration and that is the concern for main jet size selection, often you read that the main does not affect idle and cruise at all. But as with the Weber, so too the main on the Mikuni does affect the idle and light acceleration. This is mentioned on the Factory Pro tuning guide web page, "...since the main jet WILL affect low speed operation...":
http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,cv,high_rpm_engines.html

Some referencing: I ordered the Factory Pro Mikuni jet kit for the Ducati M900 93-96. This seemed a close enough match for the 906. Why? http://www.postdiluvian.org/~mason/moto/jetting.html shows some jetting comparisons of the Weber having the same main jet for the 906 and 900SS and for the Mikuni the 900SS and 900M also have the same Main of 140.

The Factory Pro kit (CRB-D12-1.1-ti ProKit), apart from better needles and springs has Main 135, 145, 155 and Idle jets 42.5 with suggested options of 35, 37.5 or 40 for LEANER PART THROTTLE. The kit is designated for M900 93-96 and 97-99 including for the option without airbox lid or w/o snorkels and with stock or aftermarket pipes. As with Mikuni setups on the 906, we mostly have pod filters and often aftermarket pipes (as I do). I also ordered some extra 45 and 47.5 Pilot jets.

I have ended up with a 135 Main, because the 145 main, while best for WOT left it too rich at part throttle. I also used my larger 47.5 idle jet again to work on the part throttle, as with dropping the float level 0.5mm from the 14mm initial recommended setting and setting the idle (higher than recommended for the Weber) at 1100-1200rpm. The weather is hot at the moment here in Brisbane, so I dare say when the weather cools the 140 Main would probably be right. With this setup I get a drop to low 11s AFR on light throttle and on hard acceleration I get low 13s. Otherwise everything flows around low to high 12s. That is as good as I think I can get it. My Idle is at 12.5-12.8 AFR.

My guess is then that the trend with the Mikuni is that they run a bit rich if you do a lot of rounds town and/or light riding, but probably not so as to cough and splutter and so most are very much happier with a swap from a Weber to Mikuni (especially if the Weber was running on a factory setup). I am on a 906, but this is more apparent it seems for 750 conversions.

This tendency to run a bit rich is also noted on the Factory Pro website
http://www.factorypro.com/tech_tuning_procedures/tuning_float_height_adj_procedure.html where they said, "CV Carbs: Too RICH at full throttle / low rpm (that's the most common in CV carbs)"

SETUP: Of course in this process I have wondered in the back of my mind about the FCRs but then I didn't want to spend that much. I have not really sorted the throttle cable yet because the positioning of the Mikuni forces a hard curve on the cables where they connect to the carb. As with lane1feathers post I had to drop the coolant piping where the cables connect by about 5mm. If I had an FCR setup I could get the Motion Pro Push/Pull Throttle Cable Set for Ducati with Keihin FCR
http://www.power-barn.com/servlet/the-755/Motion-Pro-Push-fdsh-Pull-Throttle/Detail with a Yamaha throttle grip. I tried those but it's still not an exact match for the Mikuni, so I will still still be working on that solution.

I hope this is helpful to someone and I would be interested if you agree or not with the trends I have been finding. Well, I have maybe unwisely :beer: made a few comments about the Weber - Mikuni differences too.

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Last edited by romus on Fri Jan 17, 2014 11:38 am, edited 5 times in total.

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sat Dec 21, 2013 10:47 am 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
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Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Don't worry Romus I can't bitch slap from here :lol: :lol: :beer: :beer: :beer:

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sat Dec 21, 2013 10:40 pm 
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Joined: Sun Feb 24, 2013 11:14 pm
Posts: 70
Location: Melbourne Australia
model: other
year: 1995
Just some thoughts.

On an M900 with open airbox, I tested the smaller pilot/slow jets (35-37.5) and they worked well in summer and crap in winter. The 40 slow jets worked well in Autumn/Spring, but rough,doughy down low in Summer. The 42.5s seemed to work well in Winter, but fouled plugs in Summer :banghead: :lol:

I suppose you have to select your compromise settings. I went on the rich side for acceleration with the softer jet kit springs, but I would now use stock springs and a more moderate amount of fuel. The softer kit springs made me want an accelerator pump. I would now try to help acceleration by keeping the air speed up with stock, stiffer springs rather than copious amounts of fuel and soft springs.

Those bloody Mikunis ice up in Winter! I thought of making manifold "cock socks" out of insulation material that could Velcro on in Winter. I did notice that cleaning up the manifolds helped a little with reducing fuel pooling and freezing.
I "radiused" and texture "ported" the manifolds. The manifolds had a disturbing combination of both shiny mirror sections, rough areas and casting ridges! Nice work Ducati! I cleaned them up and left a satin, machined, cross hatch finish.

In the end my wife bought me FCR 41s. It's nice to run a smaller slow jet for economical, smooth, cruising when you have water pistols squirting fuel in your carbs. A few years ago, a mate picked up a grey import, final edition TRX 850 and I noticed it had Mikunis CVs with accelerator pumps. He sold it and I never got to study them.

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sun Dec 22, 2013 4:55 am 
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paso grand pooh-bah
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Posts: 1633
Location: Newzealand
model: 906 Paso
year: 1990
My mikunis ( not on the bike ) had plumbing to the float bowls to allow engine oil to be pumped there to stop icing pic if you want

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sun Dec 22, 2013 10:29 am 
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paso grand pooh-bah
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When I had the 900SS I araldited a 15Ω aluminium cased resistor to each float bowl with an illuminated switch up by the dash. I could turn them on when needed.

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Wed Jan 01, 2014 2:56 am 
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Joined: Wed Mar 25, 2009 12:06 pm
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Location: Brisbane, Australia
model: 906 Paso
year: 1990
higgy wrote:
Don't worry Romus I can't bitch slap from here :lol: :lol: :beer: :beer: :beer:

Yeah, well, you know I couldn't resist trying the Mikuni out. :idea: Lucky I have the mentality to keep fiddling with things since having one of these bikes suits that temperament, aye :beer: :beer: . Now I want to try the FCRs but I will have to wait for the bank balance to balance :thumbup:

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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Sun Jan 05, 2014 7:16 pm 
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Joined: Sun Jan 05, 2014 4:30 pm
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Location: Denmark east
model: 906 Paso
year: 1990
Hi Romus, I have been going through the same exercise as you last year with mikunies and a LC1 inovate wideband sensor.

When the Paso came in to my hands it already had open Cans, mikuni carbs and with small pod filters, that made a hell of a noise, so I made up an filterbox with a big pipercross filter.
And I thought It would be realy rich because of the box, but no, these small pods must produce some kind of flow distorsion or harmonics, because I went from 13,5 afr at about 110 kmh to an almost unrideable 17-18 afr at the same speed.

I think I pulled the carbs of some 7 or 8 times doing all kind of adjustments especially the needles and in the end I used 1/4mm shims from a softgun gearbox, to get where I wanted.

My afr is roughly as follows

idle 13 afr
1/8 throttle 11,5 :dunno:
1/4 14,5
1/2 14,5
3/4 13,5
wot 12,8 well I think it is it was a bit difficult to monitor since it goes pretty fast at that setting, ha ha

air screws 3,3/4 turns out
idle jet are 42,5
can't remember which groove on the needle
mains are 145 and maybe a little rich

And now where it goes really great, I pulled the carbs and put them in a box and installed an injection system instead, controled by a Megasquirt system.
It has been started a few times, but thats is another story

regards
Michael


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 Post subject: Re: Tuning a Mikuni with Wideband sensor for AFR
PostPosted: Fri Jan 17, 2014 10:26 am 
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Joined: Wed Mar 25, 2009 12:06 pm
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Location: Brisbane, Australia
model: 906 Paso
year: 1990
Hey yeah, good comparison
Micpaso wrote:
...so I made up an filterbox with a big pipercross filter.
And I thought It would be realy rich because of the box, but no, these small pods must produce some kind of flow distorsion or harmonics, because I went from 13,5 afr at about 110 kmh to an almost unrideable 17-18 afr at the same speed.
I had the bellmouths from a M900 setup - I used some rubber rings so I could snug the pod filters over the bell mouths. Then just recently I made an airbox by cutting and solder-welding a 906 box with a base of a M900 box, leaving a fully open lid with K&N filter. The pod snugging the Bellmouths idea must have been okay because I cannot tell any difference in the AFR.
Micpaso wrote:
My afr is roughly as follows...

Those 14.5 readings seem high, but I suppose okay if it's taken in winter (thick air). I'm aiming 12.5-13.5.
Micpaso wrote:
...air screws 3,3/4 turns out, idle jet are 42,5..
That seems the M900 900SS standard. I tried it but It was too lean even with smaller Mains (135). I have a 47.5 Pilot jet in at the moment with about air screws 3, 1/2 turns out, but I still go rich (low 11s, even sometimes dipping to high 10s) on very light accel up to 1/8. It's fine on idle and after 1/8 but a lot of riding is done between just off idle and 1/8 throttle.

I am going to try and increase the Pilot air jet and one way or another get that off idle into 12s.

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Ducati Paso - Soul of a thoroughbred :P


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