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Discussion Starter #1
Hello, after many weeks of scouring forums, taking my bike to local pro’s etc i am posting here to see if there is some wisdom or “ out of the box “ solution for my dilema, what i have is a 98 m600, at 184000kms i decided to reward her with a new heart, a low milage 800 from a 2005 s2r, the swap was essentially a no brainer, still running on carbs with an ignitech 48-2 for ignition, after a series of different pilot and needle options tested on the road i took it to the local independant duc guys and ran it on dyno to check AFR’s, was pretty close to right however it clearly showed a lean “hole” in the transition from pilots to mains, the advice given was to bump up a pilot size and lift needle another notch, upon doing so things went pear shaped, tried various options, any richer than where i currently am bike becomes a big bag of poop, it currently has 42.5 pilots, 140 mains, y2 emulsion tube and dyno jet needles, regardless of what jets are in the thing when snapped to wide open throttle the lean hole remains, if just rolled on you would not know there is a problem, leaning towards thinking possibly the slides are not working quick enough and the lean hole shown on dyno is not just lack of fuel but a lack of fuel and air and it recovers once the slides catchup, is there a sensible way to increase the speed at which the slides respond to vacum ? Or another solution at hand ?
 

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First, sometimes there are holes on the dyno that are not there in real life. You can have a fluctuation for a few hundred rpm that you blow through in . 2 seconds and will never actually feel.
How can you try to reduce the effect, real or imagined ? You can drill the port to the diaphragm larger to cause it to rise quicker . You can cut a few coils from the diaphragm spring and the slide rises quicker. Since the needle rises with the slide it affects how quickly the fuel ratio changes.
I found it was better to use a 40 pilot jet and adjust the screw outward than to run a 42.5 and Adjust the screw inward . I ended up best , smoothest transition, at about 5 1/2 turns ccw. The adjustment screws will affect mixture at least that far out.
Synchronize your carbs by whatever method you like. It will help to have them close.
I use FactoryPro Titanium needles and Nickel plated emulsion tubes because they wear less than the others. Whatever the needle taper is, it works well with my engine.
I‘ve never had the bike on a Dyno. It will putt along at 1200 rpm. It will pull like a freight train from 3500rpm up to redline. It runs smoothly between idle and 3500 rpm. If I put it on a Dyno there is likely a hole between 2500 and 3500 rpm, but I do not lug the engine so I don’t care.
 

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Discussion Starter #4
Ducvet, apparantley from what is explained to me the yellow circled area is the problem, it is not imagined and just visible on the dyno, it is well and truly felt on the road, it is the reason i took the bike in as the general rules of thumb were not working, it can be jetted rich through all areas and it exists, or jetted lean all the way through and it exists. That was three runs at 1/3 2/3 3/3 throttle, if that was done at cruise in 4th then throttled up that hole would appear at a higher rpm then recover, carbs are balanced, ignition is verified, basically the “pro’s” said all they could do would be pull carbs over and over till they got it right and suggested i keep doing the same at way less cost.
 

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If the area circled is air fuel, then you are looking at Rich not lean as I read it. ( higher than 14:1 is lean and lower is rich) From the roll-on to full throttle there is a enriching of the mixture causing the dip you see this often with accelerator pumps but I doubt you have them with stock carbs. There will be time from the run being at steady throttle to full throttle so at what rpm were they are full throttle by?
i would guess this is somewhere in between and that's why when they snap it open too fast you are getting too much fuel.

Not too uncommon to see so first I would start the roll ons at a lower rpm , I use 3000rpm for most Ducati's but if the motor tolerate it I use lower ( have a Guzzi 1100 on the dyno that starts at 2000). See if the dip moves down the rpm band to determine if it is simply the transition. this is a different dyno than mine so reading the screen appears a bit different but outside of that it looks not unlike one I have done similar (750 monster to 800 motor).

Did you use a dynojet kit?
Which needles as there are 2 different base diameters ( open or closed airbox) well at least for 900 Check to see what ones you have .
Did you use the softer dynojet springs?
Is the bike running closed airbox or drilled/open top or pods?
 

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79 sounds like a lot for an 800 with long manifolds. certainly different to eric's carbed 800 chart. does it have the 600 inlet manifolds? second eric's questions re jetting specs. shouldn't be that hard to make one of these work ok.
 

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Discussion Starter #7
Hi Belter, yes has the 600 inlets, currently has a dynojet needle and presumably dynojet springs, upon ducvets explanation it seems i have a rich hole and then leanish through the midrange but A ok at wide open throttle, any richening of the system from this point turns it into a bag of poop, especially when fully warmed up,
 

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Discussion Starter #8
All jets and emulsuon tubes are mikuni, float height has been left alone and sits at 14mm
 

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I would put the lid back on the air box, drop the needle one clip and one size smaller main. Then you might have a better idea of where you’re going.


Sent from my iPhone using Tapatalk Pro
 

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Run the engine with no filter and watch the slides open. Is there a correlation between slide movement and the flat spot ?
 

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I have an off topic question. I was sinking the carbs yesterday since I've been chasing a carb problem but anyway at idle I was shining a flashlight in the bores of the carbs and noticed a slight amount of fuel was bypassing the needle on one carb. Any ideas why. Just put new needles and emulsion tubes.
 

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The seal between the carb body and float seat casting could be leaking fuel . If that happens you have fuel leaking into the float bowl instead of going through the orifice in the float seat.
The seal between the main jet holder and carb body may leak and then fuel can get pulled around that
by vacuum rather than going through the main jet.
The needle and seat may leak.
The float adjustment might be off.
 

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Thanks don't know exactly what I'm going to do. I'm contemplating putting individual mikuni tdm flat slides. Fcr's are a little out of my range.
 

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Ok pulled down the carbs tonight these are my findings. ALWAYS BUY ORIGINAL MIKUNI PARTS. Turns out I switched needles and it turns out the spring pressure in the new ones were much weaker than the originals and under normal circumstances probably wouldn't be a problem but I've installed an electric fuel pump and now with an adjustable fpr set to 1psi and the floats are now set to 12mm idle screw is 2 turns out. The bike now starts great idles very well and runs smooth as I can tell got to late for a road trip post later. I'm thinking now if runs great I'm going to buy new floats and slides just because.
 
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