lean miss, or? - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 7 (permalink) Old 09-01-2013, 10:52 PM Thread Starter
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Question lean miss, or?

I bought my 2003 Vulcan not running. I read lots of threads on here about carbs, fuel, etc. I disassembled and cleaned the carbs, put them back on and adjusted per the forum. The bike runs well below 4000rpm and over 5500rpm. But it stumbles and backfires in that rpm range whether cruising down the highway or accelerating. I'm wondering if I have a lean miss. I'm near Santa Fe, running between 6000 and 7000 feet altitude. The carbs are box stock, jet-wise. I've been using sea foam and sta-bil. The problem is there with a full tank of gas. I've had it up to 90 mph and no miss. I'm wondering if I need to go larger on the jets or possibly shim the needles. Does anyone have any suggestions or experience with a similar problem? Any help would be appreciated. oh, yeah, I also trimmed the vent hose at a 45 degree angle. Thanks

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post #2 of 7 (permalink) Old 09-02-2013, 10:55 AM
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Does it stumble at that rpm or at a certain place in the throttle opening?
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post #3 of 7 (permalink) Old 09-02-2013, 11:45 AM Thread Starter
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throttle opening?

Well that's hard to say, but its probably throttle opening. I'll check today. Thanks for direction.
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post #4 of 7 (permalink) Old 09-02-2013, 01:26 PM
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Are you absolutely sure every little passage and drilling in both carbs is clear and clean? They are the most fussy carbs I have ever come across. When they work they are great, but....... Do the slides go up and down smoothly and do they both feel the same? I have no surge tank, pod filters on the carbs, no emissions equipment and blanked off exhaust reed valves. I have standard jets and she pulls like a train with good plug colour. I would suspect those carbs.

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post #5 of 7 (permalink) Old 09-02-2013, 07:34 PM
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Originally Posted by clemww View Post
I bought my 2003 Vulcan not running. I read lots of threads on here about carbs, fuel, etc. I disassembled and cleaned the carbs, put them back on and adjusted per the forum. The bike runs well below 4000rpm and over 5500rpm. But it stumbles and backfires in that rpm range whether cruising down the highway or accelerating. I'm wondering if I have a lean miss. I'm near Santa Fe, running between 6000 and 7000 feet altitude. The carbs are box stock, jet-wise. I've been using sea foam and sta-bil. The problem is there with a full tank of gas. I've had it up to 90 mph and no miss. I'm wondering if I need to go larger on the jets or possibly shim the needles. Does anyone have any suggestions or experience with a similar problem? Any help would be appreciated. oh, yeah, I also trimmed the vent hose at a 45 degree angle. Thanks
This sounds like a problem transitioning from the slow jets to the main jets. Try putting a #4 washer under each of the needles. (the nail looking things). If one washer drops the stutter but doesn't cure it, then go with a second washer. When riding up in Colorado, I needed two washers on each needle to get rid of the same problem.

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post #6 of 7 (permalink) Old 09-03-2013, 09:09 PM Thread Starter
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it's a lean miss

I emailed back and forth with an expert carb rebuilder. He suggested I try the more air/less air test, which I had never heard of. So, following his instructions to cut off half the air, I taped over one air filter. Imagine my surprise when the bike ran much stronger and with no miss at all, but now it was too rich and had afterburn when decelerating or blipping the throttle. So as the carb guy and gibbo said, I probably still have some partially plugged passages. Because I really don't want to remove the carbs again right now, I am probably going to partially untape the air cleaner until it runs without afterburn and keep riding it occasionally until winter sets in. Hopefully I won't damage it. I guess I'm just an okie at heart!
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post #7 of 7 (permalink) Old 09-04-2013, 04:26 AM
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Oh.... go on, take the carbs off again - you know you want to.

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