Question about 2003 Harley Davidson XL 1200 C Sportster Custom

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My 03 sportster back fires through the carb

Runs perfect, through all gears, no hesitation or backfire. When I stop at a light and roll on the throttle it coughs through the carb. My Indy shop said it needed a re jet. Had carbs cleaned plugs replaced and reject. It still coughs through the carb!

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The cough or pop back through the car is from a lean condition, likely the accelerator pump system is not giving you the correct extra shot of fuel to overcome this, you can diagnose this by removing the air cleaner and then rolling the throttle open either running or stopped engine and you should see the extra shot of fuel exit the accelerator orifice when the throttle is rolled open. If there is none or not enough you will need to disassemble the carb and clean it and check out the pump and one way valve for the accelerator pump and may as well clean the whole carb at this time. As the engine is otherwise running okay the float level which is critical is likely okay so DO NOT mess with it or change it or you may have more problems. WEAR safety glasses and do not get too close to the carb throat opening when you try the accelerator pump test in case of a spit back etc. If you want to see something VERY COOL hook up a timing light and shine its light into the carb while the engine is running and watch the fuel mist dance everytime the strobe light flashes on it.

Posted on Oct 06, 2014

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Check your timing, sounds a bit too far advanced.
Check your valve adjustment.
Check for a cracked valve seat, worn valves will cause this too,( it's an early warning before the edge of a valve breaks off .)
If not one of those, check for a carbon buildup on the heads, might as well since it's off anyway.
Be sure your valves aren't hammered too thin(worn), same for the seats.
Ride safe!
Good Luck

Posted on Oct 06, 2014

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May need to clean or rebuild carb--or jets need to be ajusted by a pro-its runnin to rich---------they have to trac-run bike to get it rite cause to lean a condition can fry the top end

Posted on Jan 01, 2013

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Posted on Jan 02, 2017

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SOURCE: My 97 1200 Sportster backfires back through the

Its probable simple it very often is with Harley's I think you may have an engine exhause that may have an air leek, even though you hear no sound from the exhaust that may give you clue, just make sure all carbs/injectors, air box and exhaust's are on tightly go can be as carfull as you like it happens to the best of us even me after 42yrs of bikeing I hope this solves you problem its due to old idears on bikes *** bikes have different methods but its impossble work on them at leat you can repair a Harly, Good luch chum some time you need an artificial sealer dont ask me what.
Regards, Chas

Posted on Mar 05, 2009

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SOURCE: 2003 dyna lowrider,sputters spits and coughs.clears up when you p

Check HT leads, connection and the spark at the plug to see if it nice and bright.

Is the battery charged fully or is it old? They don't last very long on Harleys.

See if the regulator is any good ( dealer or friend swap if possible to check).

Posted on Feb 18, 2009

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SOURCE: i have a 1994 1200 sportster that back fires

try to adjust the air of the carburator to obtain a equal mixture of gas and air try to clean your spark plug as well.

Posted on Apr 16, 2009

  • 1 Answer

SOURCE: when it warms up it miss fires

I had the same problem with my 04 1200C. It was a defective crank position sensor. replaced the sensor and no problems since.
Good Luck

Posted on Aug 30, 2009

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SOURCE: carb trouble on harley Sportster

Not saying this will be your solution, but certainly worth considering. We had some Onan generators that sat too long at out RV dealership and were basically told carb would need replacing, as they do not have rebuild kits. At $500 a shot we decided to dump some Lucas fuel treatment and fresh gas in it. We basically tripled recommended dosage on bottle, and when engine ran we had enough lucas in there that it smoked like crazy but ran. We would run a little more through it, shut it down, let it sit for a few hours, start again, run a little, shut it down. Then let all that cleaner sit in overnight, and filled it with fresh gas again, and started it up, and after an hour or so of running steady, quit smokin, and ran like a charm. $10 worth of Lucas vs $500 carb. Did same to 3 other generators with same problem, and 2 out of 3 were cured. As it says on their site, too much will not damage the engine, so we didn't worry about that. After they were running good again, we replaced spark plugs and they were good to go, and owners were thrilled. Maybe we got lucky, but for $10-$15, I'd try it.

Posted on Apr 18, 2010

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Sounds like the throttle control motor is not adjusting the engine speed to compensate for the load. Does the generator rev up to full speed if you turn OFF eco with NO load?
Funny thing, I have one on my workbench that is doing pretty much the same thing. Runs on eco, revs up with eco off with no load, coughs, sputters and dies with a load in either mode. I will update with what I find when I figure it out in the next week or so. Have a new carb on order.

I swapped the carb and it made no difference. Still backfiring (rather dramatically) and won't stay running. I see on some other posts that Valve lash will cause this. It makes total sense as the exhaust valve gap actually tightens as the engine wears which would keep the exhaust valve slightly open when it ain't supposed to be allowing raw fuel to enter the muffler.
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As for the backfiring, I would question if you have carbs or fuel injections. If the later, you might be leaving your throttle open slightly and giving the bike fuel which pops off while changing gears.

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Well, this depends on the degree of the hesitation. An S&S carb will have a small hesitation no matter how well you tune it. On those, I suggest that you roll the throttle on rather than snap the throttle wide open. I won't get technical as to why this happens.

On your SE carb, it's designed more like the CV carb that came on your bike from the factory. Since it's a Mikuni carb, there are usually an infinate number of variables that you can change on the carb. If you can find a supplier of the parts that will fit the carb, you can change the accelerator pump nozzle to a smaller nozzle. This will allow the same amount of fuel to be delivered to the carb when you open the throttle but over a slightly longer period of time. Or, you could go the other way. A larger nozzle delivering the pump shot over a shorter period of time. Fine tuning is an art and it can get time consuming and expensive buying all the variable parts.

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A word of advice. You can tune a carb for the rest of your life trying for perfection and never achieve it. Get the carb as close as you can get and ride the bike. Do not become a "rolling mechanic". That's a person that instead of enjoying his ride, he's working on his bike in his head the whole time he's on it. Although I work on Harleys all the time, when I ride mine, I disregard every noise and thing that it does unless it just quits. If it breaks, I'll fix it when I get it home. Until then, I'm enjoying my ride. The newest bike I own is a '94 FXDWG and I love that bike.

Ride Safe.

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