Tuning a Mikuni with Wideband sensor for AFR
Posted: Sat Dec 21, 2013 9:30 am
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_p ... gines.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_p ... edure.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-7 ... tle/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 made a few comments about the Weber - Mikuni differences too.
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_p ... gines.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_p ... edure.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-7 ... tle/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 made a few comments about the Weber - Mikuni differences too.