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  #1  
Old 2 Weeks Ago
MarlboroMan MarlboroMan is offline
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Sportster/Buell Model: 1100 XLH
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Default 1987 Sportster 1100 Help (Sputters And Backfires)

I am currently trying to diagnose a problem with my bike. It starts up easy and idles smoothly, however when I get to about 3500-4000 rpm (3rd and 4th gear) it begins to sputter and backfire through the carb bad. The weird part is that when it is sputtering my rpm gauge rapidly climbs to 8000 rpm even though the engine couldn't be spinning at more than 4000. Could this be a wiring or ignition coil problem? I thought that if the spark was shorting somewhere it would cause lose of power but not a tach that reads 8000. This also leads me to believe that it is not a fuel issue. Any advice would help a lot, Thanks.
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IXL2Relax IXL2Relax is offline
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Sportster/Buell Model: Sportster 883►1200
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WELCOME TO THE XLFORUM!

When you're having issues with the electrical system, use this Sportsterpedia section about
checking the battery cables, voltages and grounds - DO THIS BEFORE ANYTHING ELSE:
http://sportsterpedia.com/doku.php/t...place_to_start

On the model years before 1994, the grounds were distributed all over the bike.
Many devices were grounded thru the frame right next to the devices.

Your battery ground is located on the upper rear motor mounts. This provides the ground point thru the frame.

All over the bike, loose connections or broken wires can cause issues. Be sure to use a good multimeter to check that your grounded wires (on the frame) have good continuity back to the battery ground or use the Sportsterpedia described way of checking grounds using the voltage setting on the multimeter.


The tach going upward crazy would indicate that the coil trigger from the ignition module (which also goes to the tach) is becoming erratic. Check the wiring connections at the coil to be sure they are secure.

Also be sure the ignition module BLACK Wire is grounded properly to the frame.

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Last edited by IXL2Relax; 2 Weeks Ago at 03:20..
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MarlboroMan MarlboroMan is offline
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Ok, I'll have to check the connections tomorrow. It just seems weird that a grounding issue would let it run fine and then only effect the bike above 3500.
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IXL2Relax IXL2Relax is offline
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Vibrations on the frame-mount era bikes WILL definitely cause issue when you have loose connection - even more so than with the later rubbermount versions.

Sometimes there's a 'sweet spot' for vibration frequency that will cause the sometimes erratic connection to become intensely present.

If everything externally turns out to be solid, there is a chance that the internal electronics of the 30+ year old ignition module (assuming it's the OEM one) has become intermittent also - but don't assume that until you've eliminated all the possible external loose connections.

Ignition Module & Coil section:
http://sportsterpedia.com/doku.php/t...k:evo:engctl01

You can also check the Cam Position Sensor in the nosecone & it's wiring to the Ignition Module:
http://sportsterpedia.com/doku.php/t...osition_sensor


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Tustan Tustan is offline
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Quote:
Originally Posted by MarlboroMan View Post
I am currently trying to diagnose a problem with my bike. It starts up easy and idles smoothly, however when I get to about 3500-4000 rpm (3rd and 4th gear) it begins to sputter and backfire through the carb bad. The weird part is that when it is sputtering my rpm gauge rapidly climbs to 8000 rpm even though the engine couldn't be spinning at more than 4000. Could this be a wiring or ignition coil problem? I thought that if the spark was shorting somewhere it would cause lose of power but not a tach that reads 8000. This also leads me to believe that it is not a fuel issue. Any advice would help a lot, Thanks.

Sounds, to me, like it’s the ignition module and/or coil.


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