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Old 01-02-2005, 03:23 PM   #1 (permalink)
IronButt
 
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Carb-Fart Theory

I've been reading a lot of posts regarding carb-farts. When I brought home my 2005 XL200C, the only way I could make a carb-fart was when it was cold. Is it just a lean mixture like I've been reading about or could it just be unburned fuel or a combination that caused carb-farts. That got me thinking about the "whole picture", intake through ignition through exhaust. What is different about my Sporty and why are most of the carb-fart threads about pre-04 Sportys?

With the exception of the new Twin-Cam 88 and the 2004 / 2005 Sportster 1200, all carbureted Harley Davidson engines have dual-fire ignitions. A single coil winding with two high voltage output terminals fires the spark plugs on both cylinders simultaneously. Each plug is fired twice during each four-stroke cycle. This approach was used to cut costs because only one coil and pick-up are required. The single-fire ignition uses separate coil windings and electronic pick-ups to fire each spark plug. With a single-fire ignition, spark plugs only fire during the compression stroke.

With dual-fire ignition, the rear cylinder is fired on the compression stroke, the front cylinder is on the exhaust stroke, but when the front cylinder is fired on the compression stroke, the rear cylinder is already on the intake stroke. Because of this condition, a combustible mixture may exist in the rear cylinder and the wasted spark causes a backfire through the carburetor. Long duration camshafts and lean carburetor jetting can contribute to the problem, especially when abruptly opening the throttle from idle where the ignition timing is advanced due to high vacuum. Adding a high-flow air filter and less restrictive exhausts leans the combustion mixture due to improved scavenging. The combination of dual-fire, spark advance, increased air-fuel mixture velocity due to scavenging and the partially open intake valve result in a carb-fart.

Harley Davidson changed to a single-fire ignition on the 2004 / 2005 Sportster, which coincides with the higher compression, high flow heads and hotter cams added for those years. I'm sure they made the change out of necessity.

By the way, I paid the Harley Tax. What a difference on the bottom end and warm-up.
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Old 01-02-2005, 03:37 PM   #2 (permalink)
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Quote:
Originally Posted by honus402
I've been reading a lot of posts regarding carb-farts. When I brought home my 2005 XL200C, the only way I could make a carb-fart was when it was cold. Is it just a lean mixture like I've been reading about or could it just be unburned fuel or a combination that caused carb-farts. That got me thinking about the "whole picture", intake through ignition through exhaust. What is different about my Sporty and why are most of the carb-fart threads about pre-04 Sportys?

With the exception of the new Twin-Cam 88 and the 2004 / 2005 Sportster 1200, all carbureted Harley Davidson engines have dual-fire ignitions. A single coil winding with two high voltage output terminals fires the spark plugs on both cylinders simultaneously. Each plug is fired twice during each four-stroke cycle. This approach was used to cut costs because only one coil and pick-up are required. The single-fire ignition uses separate coil windings and electronic pick-ups to fire each spark plug. With a single-fire ignition, spark plugs only fire during the compression stroke.

With dual-fire ignition, the rear cylinder is fired on the compression stroke, the front cylinder is on the exhaust stroke, but when the front cylinder is fired on the compression stroke, the rear cylinder is already on the intake stroke. Because of this condition, a combustible mixture may exist in the rear cylinder and the wasted spark causes a backfire through the carburetor. Long duration camshafts and lean carburetor jetting can contribute to the problem, especially when abruptly opening the throttle from idle where the ignition timing is advanced due to high vacuum. Adding a high-flow air filter and less restrictive exhausts leans the combustion mixture due to improved scavenging. The combination of dual-fire, spark advance, increased air-fuel mixture velocity due to scavenging and the partially open intake valve result in a carb-fart.

Harley Davidson changed to a single-fire ignition on the 2004 / 2005 Sportster, which coincides with the higher compression, high flow heads and hotter cams added for those years. I'm sure they made the change out of necessity.

By the way, I paid the Harley Tax. What a difference on the bottom end and warm-up.
Well thought theroy but not all true reasons HD went to single fire, nor is your reasoning for carb spiting back. But you are very close to the reason the 04 and 05 Sporty is spitting through the carb. The timing is off on the new Sporty, check with the dealer....they got a service bulletin on this problem. People were riching up the carbs with a larger pilot jet....that didn't work, then they were changing the accleartor pump nozzle and it didn't work. Then HD sent out the bulletin on the timing at low rpms.
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Old 01-02-2005, 03:48 PM   #3 (permalink)
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Interesting.

Quote:
Originally Posted by honus402
I've been reading a lot of posts regarding carb-farts. When I brought home my 2005 XL200C, the only way I could make a carb-fart was when it was cold. Is it just a lean mixture like I've been reading about or could it just be unburned fuel or a combination that caused carb-farts. That got me thinking about the "whole picture", intake through ignition through exhaust. What is different about my Sporty and why are most of the carb-fart threads about pre-04 Sportys?

With the exception of the new Twin-Cam 88 and the 2004 / 2005 Sportster 1200, all carbureted Harley Davidson engines have dual-fire ignitions. A single coil winding with two high voltage output terminals fires the spark plugs on both cylinders simultaneously. Each plug is fired twice during each four-stroke cycle. This approach was used to cut costs because only one coil and pick-up are required. The single-fire ignition uses separate coil windings and electronic pick-ups to fire each spark plug. With a single-fire ignition, spark plugs only fire during the compression stroke.

With dual-fire ignition, the rear cylinder is fired on the compression stroke, the front cylinder is on the exhaust stroke, but when the front cylinder is fired on the compression stroke, the rear cylinder is already on the intake stroke. Because of this condition, a combustible mixture may exist in the rear cylinder and the wasted spark causes a backfire through the carburetor. Long duration camshafts and lean carburetor jetting can contribute to the problem, especially when abruptly opening the throttle from idle where the ignition timing is advanced due to high vacuum. Adding a high-flow air filter and less restrictive exhausts leans the combustion mixture due to improved scavenging. The combination of dual-fire, spark advance, increased air-fuel mixture velocity due to scavenging and the partially open intake valve result in a carb-fart.

Harley Davidson changed to a single-fire ignition on the 2004 / 2005 Sportster, which coincides with the higher compression, high flow heads and hotter cams added for those years. I'm sure they made the change out of necessity.

By the way, I paid the Harley Tax. What a difference on the bottom end and warm-up.
==============================

Nice material. I agree with all except your conclusion that the dual fire system was cheaper and that, therefore, HD chose that route. Fact is, it was very dependable and worked very well for the better part of a century. It enabled the shade-tree mechanic owners to set one set of points and run their bike without too many problems. Remember, it was designed prior to the advent of solid state electronics and, really, was quite a practical setup for a long period of time. There are many HD's still running very well using that system. The alternative would have been to go to a distributor cap setup like Indian used. These were not completely trouble free as I recall.

As to firing through the carburetor, due specifically to the extra exhaust spark, I sorta doubt this. It was more likely to ignite the mixture in the exhaust pipe on the plug that was fired.

What you call "carb fart" is also called "chuffing". I've found that it is often due to an excessively lean mixture or a cold engine. Prior to my changing to a single fire ignition, my shovelhead used to do that occasionally when cold but it was never fire exiting the carburetor. We had a stock 1995 Sportster for several years that never did exhibit that condition.

You are correct about single-fire being the way to go for long duration cams and excessive overlap. It's certainly helps.

My 2 cents

Last edited by newultraclassic; 01-02-2005 at 05:06 PM.
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Old 01-02-2005, 03:56 PM   #4 (permalink)
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Another band-aid. Timing at low idle is advanced 30 degrees BTDC. With that much advance, the intake valve on the rear is already opening. There is no "one-fix" cause. What I'm attempting to get out is that carb-farting has a combination of causes.
About the issue of "cold engine" farts or backfires, any four-stroke wil fart when cold if you force it.
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Old 01-04-2005, 07:16 AM   #5 (permalink)
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Hey Doc,
Any more info you may have on this "Service bulletin"? I ran my wife's VIN for her '04' 883 through the HD site and it showed nothing. Maybe Fog Hollow has it or do you think only the dealer will have the info? I already went one up on her slow jet to try and humble the farts and now you have to "force" it to do it....still does it though. Haven't drilled and adjusted the mixture but would be interested in how the HD bubbas are told to remedy this.....didn't think you could "adjust" the timing on the newer carb bikes without a different module. Don't hurt me on this one......I love to wrench and have for years.......just new to bikes. Learn something new every evening here.

thanks, George
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Old 01-04-2005, 07:33 AM   #6 (permalink)
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Quote:
Originally Posted by fatboyskip
Hey Doc,
Any more info you may have on this "Service bulletin"? I ran my wife's VIN for her '04' 883 through the HD site and it showed nothing. Maybe Fog Hollow has it or do you think only the dealer will have the info? I already went one up on her slow jet to try and humble the farts and now you have to "force" it to do it....still does it though. Haven't drilled and adjusted the mixture but would be interested in how the HD bubbas are told to remedy this.....didn't think you could "adjust" the timing on the newer carb bikes without a different module. Don't hurt me on this one......I love to wrench and have for years.......just new to bikes. Learn something new every evening here.

thanks, George
George,
The dealer has the fix unless they didn't read the service bulletin, they have to program the idle timing which is way advanced right now. Its not a recall, its maintence bulletin that the techs are to read and keep in their minds when they hear a complaint on the carb spitting on the 04 and 05 sporty only. Check it out...se how up to date yourservice dept. is..... :p
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Old 01-21-2005, 11:14 PM   #7 (permalink)
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Talking

Don't know anything about the timing change, but just finished with my Harley Tax payment on an 04 883. SE air cleaner, Cycle Shack taper cut mufflers, and 45 low speed jets.

No more farts, sounds like a real Harley, and the low end torque is wonderful.

First tank was 51 MPG.

Totally pleased with it now.

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