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Stumbling, Carb fart...please help

Old 08-15-2010, 08:43 PM
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Stumbling, Carb fart...please help

Bought 98 SH w/ 14K miles about a month ago. Bike was stumbling around 3K rpms, so I put new filters (stock), plugs, fluids, etc. and bike still ran the same. Noticed I had exhaust leak, so got new gasket and the bike seemed to run and sound better but still had 3K rpm stumble under steady state throttle position (city driving). Pulled the carbs and cleaned them well, put everything back together and bike ran better yet, but still has stumble although not as bad. Checked vacuum lines and no leaks and carbs are synched. Has Jardine highmount exhaust and has 175 front main jet and 180 rear main jet with 45s pilot jet. Stock slides and springs. I have read many posts on here to get to this point so thanks to all the posts. I did not pull the needle to see if they are stock or not. My question is where should I go from here? I was thinking of going to 48 pilots and seeing if that would help since it only stumbles around there and is lean I assume. Also, I could check to see if there is aftermarket needle in the rear carb since it has the 180 main and maybe adjust the needle? I also read about adjusting the TPS, but is that necessary since I have very close to stock setup? Thanks.
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Old 08-15-2010, 11:26 PM
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When you cleaned the carbs, did you completely disassemble them, including removing the pilot (slow) jets, main jets and the pilot screws? Did you then shoot carb spray through all orifices and blow out with compressed air? If not, something may still be gunked up.

The first time I cleaned my carbs, 5 months ago, one of the pilot jets was completely plugged, causing a misfire, and the other was barely open. I had to soak them in carb spray for about 1/2 hour then blow out with more carb spray and compressed air. I repeated this, as well as pulling the pilot screws, a few weeks ago, since my bike was inactive for 8 weeks after a rear ender.

Clean carbs are happy carbs. You may need to play around with the jetting, needles and pilot screw adjustment though.

Oh, and welcome to the Forum!
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Old 08-16-2010, 06:18 AM
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Thanks for your replies. I took out all the jets and soaked them in carb cleaner, then boiled them, carb cleaner again, and boiled once more, so I'm sure they are clean as a whistle (I could see light through them). I also cleaned out the body of the carbs and all holes I could find with carb cleaner and compressed air many times as well as cleaning the needles and slides. However, I did not pull the needles from the slides. There is no drill holes on the slides and the springs are very long (about 12 inches), so from what I read, that would be stock springs. I'm sure the PO used dynojet, as there are decals on the fairings. I did not mess with the pilot screw when apart, but maybe I should? It sounds like most ppl have them turned 2.5 out.
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Old 08-16-2010, 11:52 AM
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Also, should I turn out both pilot screws 2.5 turns or just the rear carb with the bigger jet?
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Old 08-16-2010, 04:33 PM
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Just a heads up (of course this could vary somewhat, bike to bike). After installing 48 pilots I adjusted the pilot screws to 2.75 out, and it wouldn't idle above 2500 feet. So when I pulled the carbs about 3 weeks ago, I cleaned and readjusted the pilot screws to 2.5 out...Bingo. An occasional carb fart at sea level, but it idles great at up to 3000 feet, and pulls like a mad bull. In the fall I'll give it the 6000 foot test.
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Old 08-28-2010, 12:55 PM
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So I got ambitious and took carbs off, front needle clip was position number 2 and the rear needle was a position number 3, if starting to count from the pointed part of the needle. Should I move both needles to position 5 or just the rear since that is the only one with a bigger jet?
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Old 08-28-2010, 02:50 PM
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Both clips should be in the same groove. I do run 1 more shim on the rear than the front as that is what my bike likes but that is about it.

Also I've never had to pull the needles that high up. Try dropping them both down to the second grove from the top (or flat end of the needle)
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Old 08-28-2010, 03:39 PM
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You never mentioned what type of air filter you are using. Stock? K&N?
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Old 08-28-2010, 04:16 PM
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Thanks for the reply. I have a stock air filter. I put the clip for both needles in the 5th notch from the top (2nd notch if counting tapered end of needle) and the bike ran like crap. It would not rev over 5k rpm and smelled very rich. So took apart and moved both needles to 4th notch from the top ( 3rd notch if counting from tapered end of needle) and the bike runs strong and I got rid of the 3k stumble, but bike idles erratically and the bike pops excessively at idle and on deceleration. Lost now, please help.
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Old 08-28-2010, 04:18 PM
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Also there is a small shim, the size of the washer in thickness on the rear needle
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Old 08-28-2010, 04:33 PM
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And I should also state that I broke the plastic part of the choke that screws into the front carb, the rear one is fine and the choke still works though. Could this be causing my problems? I have a new cable on order.
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Old 08-28-2010, 05:44 PM
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Ordered Pair removal plates, maybe that will help with the backfire?
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Old 08-28-2010, 06:13 PM
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Will only help on decel if you have removed the PAIR system.
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Old 08-28-2010, 06:16 PM
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Ok, so what would cause this trade off. No stumble but excessive backfire or 3k rpm stumble but little to no backfire?
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