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Old 09-08-2012, 05:00 PM   #1
Juancho
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Default Sputtering, hesitating at a certain throttle input

I recently had a mechanic disassemble, synch and clean my carbs, among a whole bunch of other things ( 1+ tooth front sprocket!!!) and when I picked up the bike, it had this new symptom that it had never had before. Under throttle, let's say from 8%-12% I get a constant, consisten hesitation that won't let the motor rev out of it, it will stay stuck at that specific RPM point in which I held the throttle at that point. It's not RPM specific, but throttle input specific.

He said the carbs were VERY dirty ( blame the PO )
The motorcycle currently has K&N air filter, Harley Dyna mufflers with the center thing punched out, new NGK spark plugs, and that's about it. The miss was before AND after the new plugs. He said the carbs have stock jets, and that the adjustment screw. . . the one that has a cover that you need to break in order to access it? The one you guys always talk about 2 and a half turns? Forgot the name. . well, it's still untouched.


I'm thinking that the carb is running really lean or rich or something at that specific throttle input, but I know SO LITTLE about how a carb works. . . :/

This is very frustrating. Example situation;
I'm doing 55 mph, with traffic, and I accelerate just a teeny bit as traffic naturally fluctuates, and it starts sputtering, so I have to accelerate even MORE to get out of that SPUTTER ZONE, or roll of a little throttle, which is already at very little throttle input, so I have a hard time keeping a steady distance from other vehicles comfortably.

Thanks in advanced!
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Old 09-08-2012, 05:50 PM   #2
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I think the sputtering /hesitation is cause by too little fuel, its running lean, and the part on the carb under the plug is called pilot air screw, that plug needs to be removed and the screw opened up counter clockwise another turn, does your bike have 1 carb or 2 carbs, if it has 2 carbs you need info from someone else cause I have no idea about 2 carb bikes
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Old 09-08-2012, 05:52 PM   #3
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why is probably hesitates as your going 55mph and open up the throttle to go faster, it lets in more air and becomes lean, too much air and not enough fuel, i suspect the pilot air screw needs to be turned out more
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Old 09-08-2012, 06:16 PM   #4
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It's a Vulcan 500, parallel twin engine. I HOPE that this is the problem. Now I just need to figure out if there are any other possibilities, and how to access the pilot air screw.

Thanks for your prompt reply! This is very depressing since it's my main method of transportation to school, makes the ride to school a bit hectic..
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Old 09-12-2012, 11:02 PM   #5
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The problem ended up being that the o-rings in the pilot screws were damaged. I don't know how they got damaged, if by human action or just by old age and crap, since the pilot screws were capped when I reached for them. I bought the o-rings and inserted the screws and proceded to do a lot of riding, stopping, turning 1/4 turn both carbs. But I didn't realize that my idle was going out of wack and that I needed to set it to fully test out the new setting on each ride. So I discovered that, and ended up finding a sweet spot on an unknown number of turns ( stopped so many times to turn out a 1/4 turn that I lost count and decided to just keep going. Later I'll turn them in, counting turns, and return it to it's position.)

Anyways, just in case someone else has the same situation, there you go. If you can't tune it with the pilot screw, before suspecting that you need a rejet, verify that the three parts of the screw are in there and in proper condition. That is, screw, spring, washer, o-ring. In that order. After that, of course, check vacuum lines, etc. But that's what solved it for me.

It DID end up working properly at or around 3 turns out, which makes me curious. Might I need a rejet?

Thanks to all that helped, I hope this thread is useful for some future searcher. Pilot screw threads are few and not much help for this bike. Got most of my info by searching for the Ninja 500.
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Old 11-18-2012, 03:00 PM   #6
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Problem completely solved. Harley Dyna mufflers + K&N air filter indeed REQUIRES a rejet. It's way past what pilot air screw adjustment could fix. Installing a new OEM air filter completely fixed this problem. Thanks for all your help!
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Old 11-18-2012, 03:58 PM   #7
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Quote:
Originally Posted by Juancho View Post
Problem completely solved. Harley Dyna mufflers + K&N air filter indeed REQUIRES a rejet. It's way past what pilot air screw adjustment could fix. Installing a new OEM air filter completely fixed this problem. Thanks for all your help!
yeah I had a heckuva time getting my carb figured out after replacing the broken air filter box with a new one and putting on new mufflers.

adjust adjust adjust adjust adjust ... never worked out. finally rejetted, sync'd carbs, and then some mech dude did something with a little air machine and then "tuned" the exhaust or some crap.

Now it runs like a dream. Like a completely new bike.

I still get the infrequent burp backfire on downthrottle but he told me that would happen. I can live with that for the way she purrs when I throttle. Such a sweet sound and feeling.
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Old 11-18-2012, 04:23 PM   #8
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Originally Posted by coolmesser View Post
yeah I had a heckuva time getting my carb figured out after replacing the broken air filter box with a new one and putting on new mufflers.

adjust adjust adjust adjust adjust ... never worked out. finally rejetted, sync'd carbs, and then some mech dude did something with a little air machine and then "tuned" the exhaust or some crap.

Now it runs like a dream. Like a completely new bike.

I still get the infrequent burp backfire on downthrottle but he told me that would happen. I can live with that for the way she purrs when I throttle. Such a sweet sound and feeling.
I wanted to rejet. . . but it seemed like such a costly way to go versus just installing an OEM filter and adjusting pilots. I do get those burps or backfires on decceleration too, I don't mind it at all. These bikes, when running properly, are so wonderful. What mpg's are you observing after your rejet? What mufflers are you running?
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Old 11-18-2012, 05:53 PM   #9
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not sure about the mileage. I havent had a working speedometer or odometer on this bike in 2 years. I know I am getting a lot more now than I was before the carb was redone.

The exhaust is just a couple of universal mufflers I got from Python. I've been waiting on a set of Scorpion pipes but they have never told me I could buy a set. Oh well.

My bike is still a work in progress.
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Old 11-18-2012, 06:12 PM   #10
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I wanted to rejet. . . but it seemed like such a costly way to go versus just installing an OEM filter and adjusting pilots. I do get those burps or backfires on decceleration too, I don't mind it at all. These bikes, when running properly, are so wonderful. What mpg's are you observing after your rejet? What mufflers are you running?
Jets are only a few bucks apiece.??? Mine always backfired slightly on deceleration.
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