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Carbs and Fuel System
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post #1 of 10 (permalink) Old 09-09-2010, 01:31 PM Thread Starter
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Gas problems

Ok I put seafoam in my gas and the back cylinder fires no problem but if I dont have seafoam in the gas the back cylinder will not fire at low rpms just at high rpms. I have done the mod where the ignition coil is wired directly to the battery and it did make the spark stronger because I was only seeing 11.5 volts at the coil now i am seeing 12.5. I just want to make sure it is the coil before I replace it or if the carbs need to be rebuilt and synced. I have two of these bikes and both my 2002 and my 1993 are both acting the same way.
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post #2 of 10 (permalink) Old 09-09-2010, 02:32 PM
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Since the problem goes away when you're running Sea Foam in the gas, I'd think it would be carb related. Have you tried running 1/2 a can per tank for a tank or two? Might get rid of whatever is gunking things up.

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post #3 of 10 (permalink) Old 09-09-2010, 02:48 PM
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Welcome to the Vulcan madness. Here is a link to a free service manual if you need it.
http://www.tocmanufacturing.com/File...nd%20Parts.pdf

Have you done an impedance/resistance test on both circuits in the ignition coil?
Check both coils on both bikes to get a good base line for what is normal.
Swap one of the front coils to the rear of the other bike to see if that works, before buying new coils.

The primary coils specs are 1.8-2.2 ohms. Specs are not as important as the fact there is continuity between the two terminals.

The secondary resistance between the two spark plug leads should be 19K-29K ohms.

The coil needs to be warmed to a minimum of 68*F for accurate readings.

Have you checked the condition and gap on all the plugs, or replaced them with new ones yet?

Gordon

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

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TOP TEN THINGS A NEW RIDER/OWNER SHOULD DO. Click on link.
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post #4 of 10 (permalink) Old 09-09-2010, 03:14 PM Thread Starter
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Ok I moved the front coil to the back and the back coil to the front and it is doing a little bit better. I have replaced and regapped the plugs all the way around. I did not switch them from one bike to the other. I think it was just that the rear coil is getting weak seeing as it is in the worst spot of the two now with the two reversed it seems to be running ok. Are the rear coils a big problem on these bikes? On my 93 it was flooding the plugs but not so much on my 02 maybe because my 02s coils havent gotten that weak yet. This really didnt start happening till I got some bad gas but when that happened I drained the tanks and the fuel bowls on both bikes and I replaced the plugs and it ran fine for a few days then started acting up. So I am wondering if it was the coils all along and not bad gas.
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post #5 of 10 (permalink) Old 09-09-2010, 03:34 PM
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Quote:
Originally Posted by Loganwolf05 View Post
Ok I put seafoam in my gas and the back cylinder fires no problem but if I dont have seafoam in the gas the back cylinder will not fire at low rpms just at high rpms. I have done the mod where the ignition coil is wired directly to the battery and it did make the spark stronger because I was only seeing 11.5 volts at the coil now i am seeing 12.5. I just want to make sure it is the coil before I replace it or if the carbs need to be rebuilt and synced. I have two of these bikes and both my 2002 and my 1993 are both acting the same way.
you seem to have answered your own question here .it seems to be fuel related since just adding Seafoam causes the problem to go away.Although I would ,if you haven't already, install Iridium plugs.I had the same problem on my 750 and the Iridium Plugs cured it.I don't know what it is, but mine had a much bluer ,fatter spark with them than regular NGK plugs,may be the thin center electrode and maybe they will fire hotter with marginal voltage.Also look at regapping your ignitors to.018,there is a thread on here about how to do it.




If you see it on my bike I did it
VROC#30324
92 vn750(sold)
Current ride 05 1500 Classic FI
lovin' the new scoot



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post #6 of 10 (permalink) Old 09-09-2010, 04:49 PM
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Quote:
Originally Posted by denny6006 View Post
you seem to have answered your own question here .it seems to be fuel related since just adding Seafoam causes the problem to go away.Although I would ,if you haven't already, install Iridium plugs.I had the same problem on my 750 and the Iridium Plugs cured it.I don't know what it is, but mine had a much bluer ,fatter spark with them than regular NGK plugs,may be the thin center electrode and maybe they will fire hotter with marginal voltage.Also look at regapping your ignitors to.018,there is a thread on here about how to do it.
I think denny means regapping the ignition pick-up coils, located down behind the alternator cover.

Link to procedure:
http://www.scribd.com/mobile/documents/34298778

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 #7 of 10 (permalink) Old 09-09-2010, 07:01 PM
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Quote:
Originally Posted by OlHossCanada View Post
I think denny means regapping the ignition pick-up coils, located down behind the alternator cover.

Link to procedure:
http://www.scribd.com/mobile/documents/34298778
See OlHoss does speak fluent hillbilly .Excellent translation and thanks for looking up the link




If you see it on my bike I did it
VROC#30324
92 vn750(sold)
Current ride 05 1500 Classic FI
lovin' the new scoot



Quote:
"When all is said and done,usually more is said than done" UNK
Click on one x and drag to the other to read between them.

Psalm 40:1...
XI waited patiently for the Lord; and he turned unto me, and heard my cry. X
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post #8 of 10 (permalink) Old 09-11-2010, 10:15 PM Thread Starter
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Ok on the 93 the back cylinder is flooding the plugs. On the 02 the back cylinder isn't getting enough gas or isn't getting any at all. I am wondering if the carbs are out of tune but what I don't understand is why the front carb on both bikes is acting right while the back carb is messing up. Is it a normal problem on these bikes for the carbs to mess up?
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post #9 of 10 (permalink) Old 09-11-2010, 10:18 PM Thread Starter
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Oh and I have already reduced the gap on the pickups as well as direct wired the coils so I know it is nothing to do with the spark.
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post #10 of 10 (permalink) Old 09-11-2010, 11:02 PM
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Quote:
Originally Posted by Loganwolf05 View Post
Ok on the 93 the back cylinder is flooding the plugs. On the 02 the back cylinder isn't getting enough gas or isn't getting any at all. I am wondering if the carbs are out of tune but what I don't understand is why the front carb on both bikes is acting right while the back carb is messing up. Is it a normal problem on these bikes for the carbs to mess up?
The flooding carb sounds like trash holding the float needle open, or the float level adjustment too high. Sounds like it`s time for a carb cleaning and refurbishing. I suggest doing one bike at a time.

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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