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Discussion Starter #1
My 1999 Adventurer stumbles upon acceleration at low RPM. I suspect fuel/air mixture. Am I lean or rich? Should I adjust the mixture screws in or out? Is this possibly an indication of a weak or bad coil? Would fouled spark plugs cause this?

Thanks for the help,
Bonavent
 

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Not sure of what carbs you have but if they are Keihin's you could try shimming the needles 1/2 mm. If it improves you are slightly lean on the pilot jet and/or fuel screw if it gets worse too rich. What is your jetting? Have you made any airbox changes?

If you have Mikuni's the emulsion tubes could be worn causing a rich condition...
 

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Discussion Starter #4
B-B, Thanks for the response,

The reason I asked whether coils would cause the low RPM stumble is because the stumble was present before and after I changed jets and airflow. I had 38/98 jets with stock airbox set-up and changed to 42/138 & K&N pods, but the stumble remained.

I did not adjust the A/F screws which are 3.5 turns. I plan to run the screws in to 1.5 turns out. If the 99' Adventurer still stumbles, I plan to chnage to 40 pilots.

How do I add a shim?

Thanks,
Bonavent
 

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I have not had the joy of experiencing a coil failure so no idea what it feels like but it is possible you have this problem. I doubt you would get lucky enough to completely change airflow and jetting and end up with (what I presume is) the exact same stumble!:)

3.5 turns out is a lot and most will suggest that if you need to go more than 3 turns out to go one up on the pilot jet although you seem somwhat convinced that it is too rich. Maybe turning it in will help?

The lean flat spot I have the bike feels like it runs out of fuel, does yours feel this way or is it "soggy" like it needs to clear out extra fuel then takes off? At what RPM does it start to run well?

Shimming the needle will allow the bike to pull extra fuel at all throttle positions except for 3/4 position and up. To shim them you need to pull the tops of the carbs off, pull the slide out being very carefull with the rubber diaphram and the needle will be in plastic slide. Take the needle out and put one shim under the needle base to raise it up and reassemble, make sure you don't kink the spring. This is a good time also to make sure that your diaphrams are sealing properly...

One other item is try to make your changes one at a time then if it improves or gets worse you know what affected it. Making too many adjustments at once does not allow you to isolate an issue, don't ask me how I know this...:)
 

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Discussion Starter #6
Thanks, BRG-Bird, for the explanation. I'll add the shim assuming a lean condition at 2500-3000 rpms, especially since I added the K&N pods. The engine just won't take the throttle. If I ease the throttle it will rev & at 5000 or so becomes strong. (It's not sluggish nor misfiring, which I think is a rich condition.)

Thanks again,
Bonavent
 

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You're welcome, hopefully this helps you and post your findings please...:)
 

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Discussion Starter #8
'99 Advent Stumble Update

I added two .5mm shims. 2500-3000 RPM range much improved. There is still a slight drag on the throttle at that range, but the bike can be ridden. I have K&N pods, 42/138 pilot/main jets with the a/f screw out 3.5 turns.

The bike backfires upon deceleration which I'd like to eliminate or minimize.

I plan to run it with seafoam and see if there is an improvement.

Thanks again for the help,
Bonavent
 
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