mikuni running rich..

discussions specific to the 750 Paso

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paso750
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Re: mikuni running rich..

Post by paso750 »

we`re all missing some info here. The carb is from Phil so I`m assuming it originally came from a 900. Did it come with a Dynojet kit or not ? Another difference between the 750 & the 900 jetting (except to the pilot jet) is the needle jet. Y-6 vs. Y-2.
What happens if you turn the pilot screws in. Does the engine stall, does it not or doesn`t it react at all ?

G.
birdduck
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Re: mikuni running rich..

Post by birdduck »

I have the needle jet written down, but can't remember what one.. yeah it will stall out or choke up if it's under 2 turns... I am ritchening the mixture by turning the screw out correct? Dunkin jet needle is the only part not mikuni
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paso750
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Re: mikuni running rich..

Post by paso750 »

Base set up is 2,5 turns out. 4,5 is a lot.
birdduck
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Re: mikuni running rich..

Post by birdduck »

Needle jet is a y-2
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paso750
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Re: mikuni running rich..

Post by paso750 »

that`s the one for the 900
birdduck
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Re: mikuni running rich..

Post by birdduck »

Gotcha, I did drop back down to the 130 jet that was in it, and it didn't like the 35's so went back up to the 37.5 and am 2.5 turns out and it seems to be running smoothly. Still sputtering as soon as my needle starts to play in... This could be my needed jet being to large? I am at 4500 feet so my jetting will be leaner than most
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paso750
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Re: mikuni running rich..

Post by paso750 »

I don`t know why the 900 uses a Y-2 and the 750 a Y-6.
The Y-2 is leaner as the Y-6.

The later in the alphabet the larger the diameter of the needle jet.
From one letter to the next (with the same number; i.e. Q-2 to R-2) is 0,05mm difference.
From one number to the next (with the same letter; i.e. R2 to R3) is 0,005mm difference.

I found this german file which shows it, too. Based on the table at the bottom of p.1 the Y-2 is 3,11mm diameter, the Y-6 is 3,130mm.
https://app.box.com/s/a0aktvn8cj2nmp059sa8

Generally I would always start with a known/standard setup i.e. the one of the 750SS and go from there.
But that`s me. I personally find it more difficult to start off with something already modifyed (in this case a Mikuni with a pilot jet from a 750, the needle jet from a 900 and an unknown aftermarket needle). Of course you would need a leaner jetting for the altitude you`re living at but on the other side you don`t have an airbox, but you`re using UNI pod filters and have (what seems to be) a Two Brothers exhaust which will make carburation leaner. The effect of the altitude could be equalized by filters and exhaust or maybe their effect is stronger than the thin air.
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Re: mikuni running rich..

Post by birdduck »

I am not sure on the exhaust brand, I have looked at it but can't remember.. it seems to be running good, still not idling as well as I would like.. I still have symptoms of a vacuum leak but there is none..
When it is warmed up completely the idle will hang about 1000 rpms higher but I can't find the middle ground. It's either to lean or to rich
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paso750
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Re: mikuni running rich..

Post by paso750 »

if the idle is high, you can`t lower it and you`ve verified there are no airleaks I`d say the carb is running lean. Noone else have an opinion ? :)
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Derek
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Re: mikuni running rich..

Post by Derek »

birdduck wrote:Gotcha, I did drop back down to the 130 jet that was in it, and it didn't like the 35's so went back up to the 37.5 and am 2.5 turns out and it seems to be running smoothly. Still sputtering as soon as my needle starts to play in... This could be my needed jet being to large? I am at 4500 feet so my jetting will be leaner than most
The needle jets (emulsion tubes) in the Mikuni carbs are notorious for wearing oval. It doesn't need much to affect the carburation. When I had a 900ss it developed a flat spot with a misfire on opening the throttle as I exited corners. This was at about 21,000 miles. Leaning the pilot mixture a little alleviated this but caused problems with the idle. The answer was to replace both needle jets. I had to examine the old jets with a magnifier to see the wear, it was so slight but had a marked affect.
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birdduck
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Re: mikuni running rich..

Post by birdduck »

That is a possibility, the needle jets do show some wear..
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Re: mikuni running rich..

Post by giocast »

as said somewhere above it's worth not only confirming that you have original needles (operated via diaphragms) but that you also have original return springs (not the weaker Dynojet versions) plus the equivalent jets for above needles aren't worn (usually becoming oval instead of round on inspection) as any/all of these can contribute to the initial stumble and splutter on pick-up that you mention on a 'sensitive' engine.
birdduck
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Re: mikuni running rich..

Post by birdduck »

the needles and springs are new dyno jet.. i think new needle jet( e tube) will smooth it out if them being worn can cause rich spots. everything else on these carbs has been meticulously inspected multiple times for possible damage or wear..
the bike is pretty happy at its current setup, running just a tad lean when cold but just right when the engine has heated up.
birdduck
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Re: mikuni running rich..

Post by birdduck »

With my fuel line, I just have a tee fitting to the carb then to the tank. I almost feel its not getting enough fuel into the carbs under heavy load situations.. input?
birdduck
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Re: mikuni running rich..

Post by birdduck »

150 dyno jet installed, carbs vacuum synced... Status... MONSTER!.... I think one more step up on the main jets will be perfect
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