Rear cylinder backfire, doesn't fire on idle. - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 9 (permalink) Old 09-08-2013, 12:55 PM Thread Starter
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Rear cylinder backfire, doesn't fire on idle.

So my problem with my 1997 Vulcan 750 is that the rear cylinder doesn't fire all of the time on idle. It will fire when on the throttle but backfires once u let off. Any ideas as to why this is occurring would be greatly appreciated. Just bought it and other than this, a small oil leak is my only concern.
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post #2 of 9 (permalink) Old 09-08-2013, 01:54 PM
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Seems like a clogged pilot jet to me. Could be combined with the coasting enricher not working too.

Seafoam treatments might help, but a teardown and cleaning may be needed. In order for the Seafoam to work, there has to be at least some flow through the dirty jets and circuits. The Seafoam has to get into contact with the dirt.

Backblow is often recommended here (there's a sticky post), but I envision a volcano of dirt, a mushroom cloud of crud, rising out of the drain well in the carb. And all that has to go through the jets if it's not completely drained out of the bowl. People do report positive results.

Have been Seafoaming a problem myself. It clears up in ten miles, runs fantastic for that tank, then it's right back to being limited to 5000 rpm if there's no Seafoam in it. I must be cleaning the carb out one molecule at a time. Need to just bite the bullet and rip them off there.

If you have the factory air intake system in place, it's no easy job to R&R the carbs.

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post #3 of 9 (permalink) Old 09-12-2013, 02:46 PM
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float needle could also be sticking,causing it to flood at low rpms.

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post #4 of 9 (permalink) Old 09-12-2013, 09:13 PM
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have you adjusted the mixture screw on the rear carb yet? I'd start there.




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post #5 of 9 (permalink) Old 09-22-2013, 05:51 PM Thread Starter
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The only thing I can possibly think is that because it is straight pipes from headers, the rear cylinder doesn't have enough pressure and isn't firing at idle. Everything works fine once I get out of the 800-1500 rpm range and it rides just fine. I am going to but a goat back on it and I will see how it does. Also the backfire on decel was driving me crazy.
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post #6 of 9 (permalink) Old 09-22-2013, 06:15 PM
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If it's backfiring on decel, you should adjust the screws out a little. What do you have your idle set at? Should be 1100-1200. Also you should check the voltage at the coils and compare it to battery voltage with it idling. If there's drop off, there's a sticky on the coil relay mod that can cure it. The mod also helps with hot start problems.

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post #7 of 9 (permalink) Old 09-23-2013, 01:02 PM
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Quote:
Originally Posted by Jakzen View Post
If it's backfiring on decel, you should adjust the screws out a little. What do you have your idle set at? Should be 1100-1200. Also you should check the voltage at the coils and compare it to battery voltage with it idling. If there's drop off, there's a sticky on the coil relay mod that can cure it. The mod also helps with hot start problems.

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Just to clear things up there are two coil mods one for the ignition coils that helps with lost voltage to the coils which will remedy a weak spark the other mod is for the pickup coils which will help with hot starts

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post #8 of 9 (permalink) Old 09-26-2013, 10:01 AM
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He lives close to me. I believe it is a petcock issue. Reserve is not working either. I told him how to check it out.
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post #9 of 9 (permalink) Old 10-23-2013, 09:34 AM
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Backfiring just off idle and under free rev. Thought it was leaning out. Put a new petcock on because it needed it anyways. New plug wires, needed those too.
I noticed a tear in the left carb boot. Need to fix that. Exhaust pressure on left side, short pipe, seems lower than right side at idle.
Voltage to the coils good 12.4 and rises while revving, but we are going to do the relay mod anyway.
Rear jug temperature was 75-100 degrees colder than front with a laser thermometer?
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