True Results of Jetting Changes

jaguar

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#1
I'm pretty sure that every other graphic you have seen showing the amount of jetting effect each adjustable part has was only some persons idea coming from their own experience, not from a computerized jetting calculator. And none of it was "specific", only generalized. This graphic comes from my jetting calculator and is specific in that the amount of change is due to +/- 20% change. For example, if the main jet is a #100 than the graphic shows the change from 80 to 120. 20% change for the needle was from 1.2 degrees to 1.8 degrees taper.
How a main jet change affects idle richness: At closed throttle the flow are around the needle (between it and the needle jet) is about half of that of the main jet. So it is an open flow area that is being acted upon by the suction in the carburetor. Its gas flow adds to the idle jet flow. Although the main jet area at closed throttle is twice the flow area around the needle, it still affects the flow by presenting some flow resistance. So any main jet change will somewhat affect the idle richness. My calculator says a 20% main jet change creates an 11% idle richness change which is a 2:1 ratio. So if you increase the main jet by 2% (a typical change) then you affect the idle richness by 1%. Of course that is hardly anything which is why they drew the graphic the way they did. But still it is erroneous. I need to recalculate everything to show typical changes.
 

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#3
Say, Jaguar, I changed my jetting on my kehin carb to clean it up a little. I went from a 178 main, and a 58 pilot to a 175 main to a 55 pilot, should I raise or lower my clip on my needle? The engine seems a little crisper. Do you think it will ok to run it like this?
 

jaguar

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#4
Well my full write-up on jetting is at Needle Selection
Reducing the main jet size also affects mid throttle jetting as you can see on my graphic above.
If you feel the mid throttle jetting is still a bit rich then you can lower the needle although the right way to do it is to buy a needle with a lesser taper angle.
From using my calculator I now believe the clip position shouldn't be used to change mid throttle jetting but to set when the needle taper kicks in which for most PWK carbs is at 25% of the slide height. If you use a felt marker to put a black dot on the needle right where the taper begins and then lower the slide in the carb till the dot is right at the top of the narrow part of the needle jet then you should measure that slide height. Change the clip so that it's at 25% of car bore. And then select a different needle if you want to change how it is jetted at mid throttle.
 
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#5
Well my full write-up on jetting is at Needle Selection
Reducing the main jet size also affects mid throttle jetting as you can see on my graphic above.
If you feel the mid throttle jetting is still a bit rich then you can lower the needle although the right way to do it is to buy a needle with a lesser taper angle.
From using my calculator I now believe the clip position shouldn't be used to change mid throttle jetting but to set when the needle taper kicks in which for most PWK carbs is at 25% of the slide height. If you use a felt marker to put a black dot on the needle right where the taper begins and then lower the slide in the carb till the dot is right at the top of the narrow part of the needle jet then you should measure that slide height. Change the clip so that it's at 25% of car bore. And then select a different needle if you want to change how it is jetted at mid throttle.
I pulled the plug and it was carbon fouled, and sooty. Thanks for the info, ill check with my cycle dealer if he has some more needles...
 
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#6
Ok jaguar, I set the needle clip to 3rd position, jetting is nice and crisp, I have to tweek the air screw, and put in some fresh pre-mix. It had mundo smoking before this jetting change, and now to my amazement its cleaner than ever! I purchased it from a dude from Penn state, and it was wrongly jetted. There still is a small flat spot, so ill try to clean it up with the air screw...If this works good, then I will proceed to clean up my 1985 cr-250! Stay tuned!
 
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#7
well its getting an off idle bog, just blowing smoke. If I turn the air screw all the way in it clears it up, it if screw it out 2 turns it pings and runs badly. It idles perfectly, but as I said early, off idle up it bogs badly. I had 4 month premix in it, and I think I might have put in too much stabil to protect it, should I dump the fuel and put in some fresh gas and oil? What cha recommend...
 
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#9
dump it!
off idle is the domain of the carb slide and the clip position.
Jaguar, I ordered a new set of reed valves, boyseen, and im down on the pilot from a 58, to a 55. Is a 160 on the pilot safe? I bought the bike with a 160 main jet and the engine never burnt up. 2nd piston in it looks good. It calls for a 178 main, and a 58 pilot, third position on the needle. I changed oil to sikolene from maxima 927 and it seems to run a bit cleaner. I just rode it a couple of hours ago and it still bogs off idle, but giving it the gas, it screams pretty fast! LOL! Thank goodness for breaks!
 
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#11
Move the nedle clip up and try it.
then move it down and try it.
You can go as thin as 50:1 on that Silkolene.
Ok, great, ill experiment, I am glad I have a fresh piston and rings, I wonder if the ethynol in the gas is causing all these bikes to run poorly? I remember back in the day, you mix your fuel, splash it in your fuel tank, and enjoy a nice day of riding. Now all i do is try to keep my bikes running, Thanks again Jaguar!
 

jaguar

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#12
It is advised to stay away from ethanol laced gasoline but if you have to use it then you have to jet richer for ethanol. I was using ethanol gas unknowingly on a bike with the old style seals that arent ethanol-proof and soon enough I had to split the cases to replace the seals.
 
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#13
It is advised to stay away from ethanol laced gasoline but if you have to use it then you have to jet richer for ethanol. I was using ethanol gas unknowingly on a bike with the old style seals that arent ethanol-proof and soon enough I had to split the cases to replace the seals.
Wow! We have 10% ethynol here in DFW, I mix it with staybil fuel treatment, 1 oz per gallon, and it "seems" to help. I miss the old days when fuel was cheap, and not too many additive were added to it. Just put my boyseen reeds in, and I noticed something, the original owner forgot to put a gasket on the reed block? I had a big task, but its now running great. I will try to find some ethynol remover at auto zone. You had to split your cases for seal replacement, thats no fun! :$