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post #1 of 14 (permalink) Old 06-15-2014, 07:58 PM Thread Starter
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Backfires

Newbie here.
I know ya'll have seen this 1k times.
VN750 2006 2k miles. Vance & Hines exhaust, rejetted. Just marbled and now starts and idles way better. but still backfiring at steady throttle 3000rpm and up and on deceleration on rear cylinder. And a slight stumble or hesitation on acceleration. Found an intake leak earlier by spraying Berryman. Tightened clamps but not sure that solved it I used up all my Berrymans. It has set long periods. Also at the exhaust rear cylinder is not 1/2 as strong as front. Thinking may have to pull carbs.

Last edited by my207; 06-15-2014 at 08:10 PM.
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post #2 of 14 (permalink) Old 06-16-2014, 01:47 AM
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Check the air cutoff valves on the carbs. Those valves could be checked without taking out the carbs. Mine never stopped backfiring and I know these are the culprits, I just haven't replaced them. You can find them for about $35 each set on eBay, including the spring and the tiny o-ring. You could also find them on other motorcycle parts sites, but if you're only buying one or two parts, it might be cheaper on eBay.

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #3 of 14 (permalink) Old 06-16-2014, 10:31 AM Thread Starter
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Now leaning toward electrical. I pulled the spark plug wires and rear cylinder runs strong. So that tells me I'm not getting enough voltage to run both coils at the same time.
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post #4 of 14 (permalink) Old 06-17-2014, 04:58 PM
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You could measure the voltage at the coils with a voltmeter or multimeter.

Look into the coil relay mod. Basically, you disconnect the coils from the wiring harness and add their own circuit. Just going straight from the battery, into a relay, and then into the coils. The relay should be activated by the ignition switch so you don't drain your battery while the bike is parked. This helps a LOT with low coil voltages. You basically get the same voltage as the battery, at the coils.

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #5 of 14 (permalink) Old 06-17-2014, 08:07 PM
..have a vulcan good day!
 
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Quote:
Originally Posted by Ceal View Post
You could measure the voltage at the coils with a voltmeter or multimeter.

Look into the coil relay mod. Basically, you disconnect the coils from the wiring harness and add their own circuit. Just going straight from the battery, into a relay, and then into the coils. The relay should be activated by the ignition switch so you don't drain your battery while the bike is parked. This helps a LOT with low coil voltages. You basically get the same voltage as the battery, at the coils.
x2

Relay $2 - $3.
I activate my Relay off (original) rear cylinder Coil switched 12 VDC !
Wonders of difference ! ....Relay strapped to right frame rail forward of battery box.
Do it !


'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!

Last edited by WilliamTech; 06-17-2014 at 10:15 PM.
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post #6 of 14 (permalink) Old 06-17-2014, 09:23 PM Thread Starter
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Quote:
Originally Posted by Ceal View Post
You could measure the voltage at the coils with a voltmeter or multimeter.

Look into the coil relay mod. Basically, you disconnect the coils from the wiring harness and add their own circuit. Just going straight from the battery, into a relay, and then into the coils. The relay should be activated by the ignition switch so you don't drain your battery while the bike is parked. This helps a LOT with low coil voltages. You basically get the same voltage as the battery, at the coils.

I did check the resistance primary and secondary.

primary 2.0 book says 1.8 - 2.2
secondary 33.3k book says 19 - 29k

A bit out of range. But both coils read the same. maybe difference in my meters and Kawasaki's.

I like the idea of putting the on their own circuit. may give that a try.

I've gone ahead and pulled the carbs and clean them just to be sure. Also gonna do ear shave and coaster. My filters are rotten and I don't want all that stuff in the way when I try and reinstall carbs.
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post #7 of 14 (permalink) Old 06-17-2014, 09:26 PM Thread Starter
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Quote:
Originally Posted by Ceal View Post
Check the air cutoff valves on the carbs. Those valves could be checked without taking out the carbs. Mine never stopped backfiring and I know these are the culprits, I just haven't replaced them. You can find them for about $35 each set on eBay, including the spring and the tiny o-ring. You could also find them on other motorcycle parts sites, but if you're only buying one or two parts, it might be cheaper on eBay.
I actually swapped them around and no change.
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post #8 of 14 (permalink) Old 06-17-2014, 10:16 PM
..have a vulcan good day!
 
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Where are you located ?


'86 VN750 13,700 mi -Stock
-Purchased 2008 w/8800mi
Replaced ALL Cables . Kuryakyn LED Voltmeter
SilverStar Ultra 9003/HB2 H4 Headlamp, Jardine Fwd Controls,
Iridiums DPR7EIX-9 & Wires, Tuxedo Mod, Coil Mod, P/U Sensor Mod, Fork Seals,
Splines Lubed - 11/4/2012 - MF AGM Battery
Additional Flashing LED Brake Light on Trunk
Dampers went out @ 13+K !
After TOC MCCT's..... so amazingly quiet I discovered a rattling heat shield on my stock exhaust !
...have a vulcan good day!
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post #9 of 14 (permalink) Old 06-17-2014, 10:32 PM
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Quote:
Originally Posted by my207 View Post
I did check the resistance primary and secondary.

primary 2.0 book says 1.8 - 2.2
secondary 33.3k book says 19 - 29k

A bit out of range. But both coils read the same. maybe difference in my meters and Kawasaki's.

I like the idea of putting the on their own circuit. may give that a try.

I've gone ahead and pulled the carbs and clean them just to be sure. Also gonna do ear shave and coaster. My filters are rotten and I don't want all that stuff in the way when I try and reinstall carbs.
You don't mention the voltage measurement though. The relay mod will help you with voltage drops, if there are any.

For some reason, the voltage at the coils dropped a lot on my bike out of the blue. I did the coil relay mod and it fixed it right up.

Easy to check with a voltmeter at the coils. If the voltage is considerably lower than the battery's, that might be the problem. If not, gotta keep looking, I guess.

1985 VN700 unknown mileage (over 33K)

AGM Battery
Iridium spark plugs
NGK wires and caps
Relocated R/R
Marbled
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post #10 of 14 (permalink) Old 06-17-2014, 11:33 PM
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you say the rear cylinder does most of the farting?
did you block off the vacuum port on the right side carb?
have you synchronized the carbs?
did you make sure to block off the big hose coming from the air box?

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i got my helmet but it sure makes licking the windows a chore

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