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post #1 of 8 (permalink) Old 09-12-2011, 09:47 PM Thread Starter
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low spark on rear cylinder

I recently found that the reason my bike was backfiring was because the rear coil was producing a low spark. At idle the rear cyclinder will not run when I checked spark it was very low. When I rev it up it kicks in and runs great. When I pulled the plugs it is obvious there is unburnt gas on them. I have replaced the coil and is a little better but the rear cyclinder still won't fire at idle. The battery seems to be OK although I will be getting a AGM very soon. If there is anyone out there with some advice it would be greatly appreciated. What should I check next. Thanks in advance
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post #2 of 8 (permalink) Old 09-12-2011, 10:17 PM
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If you have some old plugs lying around replce them and give it a try. If that helps get some new plugs.

02 honda sabre 1100
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post #3 of 8 (permalink) Old 09-12-2011, 10:29 PM
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Quote:
Originally Posted by jscarb View Post
I recently found that the reason my bike was backfiring was because the rear coil was producing a low spark. At idle the rear cyclinder will not run when I checked spark it was very low. When I rev it up it kicks in and runs great. When I pulled the plugs it is obvious there is unburnt gas on them. I have replaced the coil and is a little better but the rear cyclinder still won't fire at idle. The battery seems to be OK although I will be getting a AGM very soon. If there is anyone out there with some advice it would be greatly appreciated. What should I check next. Thanks in advance
First things first. Welcome to the Vulcan madness.
Now we can get down to business.

Did you replace the plug wires too? or just the coil itself?

The impedance, or resistance, of the coil secondary circuit measured through the spark plug wires at each cap should be from 19K to 29K ohms.

In a recent thread a problem similar to yours was being discussed. I suggested to take the plug wires off and measure the resistance of the coil and each wire separetely. Each of his coils measured about 20K ohms.

One set of wires measured about 5K ohms each, which is about right because plugs wires are often designed with ~5k Ohms resistance per foot.

His other set of wires had infinite resistance, meaning there was a break in them. He replaced that set of plug wires on one coil, and put the original used good set back on the other coil and the bike runs well now.

That is a quick, easy no cost test (assuming you have a multimeter already ).
Be sure to test the front coil and wires too so that you can compare a known good coil wire combination against the rear set that are not working properly.

If the coils are in spec and the wires are not, new wires may cure the problem.

If you still need more spark, look at fergy`s coil mod supplying 12v directly to the coils from the battery through a relay:
https://www.vn750.com/forum/showthread.php?t=17276

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/11-vn750-general-discussion/9127-top-ten-items-you-would-suggest-new-owner-do-his-new-ride.html
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post #4 of 8 (permalink) Old 09-12-2011, 10:39 PM
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Quote:
Originally Posted by niterider View Post
If you have some old plugs lying around replce them and give it a try. If that helps get some new plugs.
Good point niterider. Sometimes the obvious escapes me!

When you buy new plugs get NGK Iridium DPR7EIX-9.
They will last virtually forever and cure several starting and running problems.

I believe Advance Auto has been mentioned recently as an inexpensive supplier with prices around $7 each sometimes.

Gordon

1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15


TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
https://www.vn750.com/forum/showthread.php?t=9127
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post #5 of 8 (permalink) Old 09-12-2011, 10:54 PM Thread Starter
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yes I checked the resistence in the primary, secondary and plug wires and everthing looks to be in spec. I also ran a hot wire from the battery to the positive side of the coil with the same results. The voltage from the battery is 13.0 and the voltage from the postive wire to the coli is about 12.5. I cleaned all my electrical plugs also. Thanks for the help you guys are keeping me sane.
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post #6 of 8 (permalink) Old 09-12-2011, 10:57 PM
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I meant spark plugs, jscarb.

02 honda sabre 1100
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post #7 of 8 (permalink) Old 09-13-2011, 02:51 AM
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Quote:
Originally Posted by jscarb View Post
yes I checked the resistence in the primary, secondary and plug wires and everthing looks to be in spec. I also ran a hot wire from the battery to the positive side of the coil with the same results. The voltage from the battery is 13.0 and the voltage from the postive wire to the coli is about 12.5. I cleaned all my electrical plugs also. Thanks for the help you guys are keeping me sane.
so you still have the old plug wires on it? I would take the rears and swap them with the fronts and see if that makes a difference. your spark plug caps might have hair line cracks and could be arcing against the engine at idle or low rpm so maybe before swapping wires tape the caps up with a few wraps of electrical tape, especially the long one. theres more area on that one that is in close proximity to the cylinder head. good luck.

Dave

Mods:
Jetted, shaved & coastered
Home made under-tank single filter
Converted ACCT's to manual (thanks wib714)
Dual vac lines on petcock (my cure for ticking)
LED tail/plate lights
Relocated helmet lock to license plate

Added stuff:
Customized hard bags w/ lights
Chrome levers
Hellcat windscreen
LED running lights
Home made 'Vita-Volt' meter
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Tunes

Hmm....she was on the back when we left.... ??

Last edited by DavesVulster; 09-13-2011 at 02:57 AM.
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post #8 of 8 (permalink) Old 09-17-2011, 03:41 PM Thread Starter
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Hello guys it is me again, So I changed the pluggs (as niterider suggested - great advice ) and it seemed to get a little better, but as I was experimenting with the vacuum lines I noticed when I pulled the line off the rear cylinder just above the fuel air mixture screw and plugged it up the idle problem ceased. This line goes to the emmisions valve under tank. Is there a problem if I plug this up? because I noticed on the ear shave process that valve is removed completely and if so what do you do with the line that comes out of the front cylinder carb that goes to the tank?
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