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post #1 of 10 (permalink) Old 03-25-2007, 07:34 PM Thread Starter
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I Think I am Stumped!!!

I have tried dozens of combinations of jets and needles setting and no matter what, it breaks up under full throttle at about 6K rpm. I can accelerate past 6K rpm under part throttle, but not WOT.

I am wondering now if one or both coils is bad. It's $ 150 for a couple of new coils so I need to be sure. From what I understand, just checking the resistance is not the tell all. Anyone know a good way to check a coil other than with a DMM?

I am almost ready for the C4.

Jon

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post #2 of 10 (permalink) Old 03-25-2007, 07:46 PM
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Jon, the book I quoted out of for Lootux, Motorcycle Electrical Systems has a whole section on ignition coil testing. A couple of things:

a. "With the ohmmeter connected to the coil's primary or secondary windings, try tapping on the coil with a screwdriver handle, or try heating it with a hair dryer. If the ohmmeter's readigns chamge, the internal coil windings are broken.

2. YOu can make an inexpensive ignition coil tester with an old condenser and some jumper wires. Make sure the condenser is good by testing it on a known good coil. To use the tester, disconnect the negative side of the coil. Connect teh condenser wire to the negative coil terminal and ground the condenser's mounting tab using a jumper wire. Conncet a second jumper wire to ground and turn the ignition on. Now tap the grounded jumper wire to the negative side of the coil. It should produce a spark between the high-voltage terminal and ground on a conventional coil, or between high-voltage terminals on a dual-plug couil. If oyu don't get a spark, check for battery voltage at the coil and good connections on all jumper wires. Be careful doing this on a dual-plug or stick type of coil - the spark prodced can really zap you if you get in the way.

Conventional ignition coild with threaded or spade terminals can be mistakenl connected backward causing reverse polarity and a weak spark. A coil with reverse polarity will start and keep an engine running, but could misfire under load because the coil produces about 15 percent less voltage when connected backward..."

Didn't you have to guess at some point about connecting those terminals??

C
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post #3 of 10 (permalink) Old 03-25-2007, 08:32 PM Thread Starter
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Quote:
Originally Posted by Crobins365 View Post
Jon, the book I quoted out of for Lootux, Motorcycle Electrical Systems has a whole section on ignition coil testing. A couple of things:

a. "With the ohmmeter connected to the coil's primary or secondary windings, try tapping on the coil with a screwdriver handle, or try heating it with a hair dryer. If the ohmmeter's readigns chamge, the internal coil windings are broken.

2. YOu can make an inexpensive ignition coil tester with an old condenser and some jumper wires. Make sure the condenser is good by testing it on a known good coil. To use the tester, disconnect the negative side of the coil. Connect teh condenser wire to the negative coil terminal and ground the condenser's mounting tab using a jumper wire. Conncet a second jumper wire to ground and turn the ignition on. Now tap the grounded jumper wire to the negative side of the coil. It should produce a spark between the high-voltage terminal and ground on a conventional coil, or between high-voltage terminals on a dual-plug couil. If oyu don't get a spark, check for battery voltage at the coil and good connections on all jumper wires. Be careful doing this on a dual-plug or stick type of coil - the spark prodced can really zap you if you get in the way.

Conventional ignition coild with threaded or spade terminals can be mistakenl connected backward causing reverse polarity and a weak spark. A coil with reverse polarity will start and keep an engine running, but could misfire under load because the coil produces about 15 percent less voltage when connected backward..."

Didn't you have to guess at some point about connecting those terminals??

Cindy -

Not only did I connect them not know which wire went where, we also broke off one of the terminals which we repaired.

So, two areas of concern on the rear coil. Can you take a look at one of your rear coils and let me know whichi color wire you have where?

Thanks,

Jon

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post #4 of 10 (permalink) Old 03-25-2007, 09:25 PM
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Red is on the top/foremost. From what I'm reading, if you just switch 'em out, you can't do any damage - there is a polarity test involving a "lead" pencil...but I don't own one, and I didn't know if you did or not. Let me know, and I can tell you how to test polarity with an antiquated device.

C
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post #5 of 10 (permalink) Old 03-26-2007, 07:23 AM
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Any luck?

Hey, Jon -
Just checking in to see if you had any luck with the coil last night. Keep me posted - I can fax you the write-ups from this manual, if it would help.

C

C
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post #6 of 10 (permalink) Old 03-26-2007, 08:02 AM Thread Starter
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Quote:
Originally Posted by Crobins365 View Post
Hey, Jon -
Just checking in to see if you had any luck with the coil last night. Keep me posted - I can fax you the write-ups from this manual, if it would help.

C
Cindy -

I think my only choice is to find a known good coil and swap it for my rear one.

I'll watch ebay for one and try that.

At this point, I can't imagine that with all the different fuel changes I made that the same problem happening is caused by the fuel. By accident one setup should have worked.

Jon

93VN750, under re-construction
vn750.com Member # 828

Rick's Stator and R/R
MF AGM Battery
Coastered
Degoated
Shaved & Jetted
Iridium's
MAC Tapered Staggered Pipes
Battery/Temp/Clock Gauge
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Dunlop D-404's
23,XXX miles and holding pending gasket change (underway!!!)
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post #7 of 10 (permalink) Old 03-26-2007, 08:11 AM
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Did ya get a chance to try switching the leads? Not that a new coil wouldn't be preferred (I do remember now you going through all that with having to solder on a new lead)... I'll keep my eyes posted on eBay for ya - there have been coils in the last few weeks; bound to be some more with spring coming.

C
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post #8 of 10 (permalink) Old 03-26-2007, 08:19 AM
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Jon swap the front and rear coils and see if the problem moves to the front cyl.. You should also remove and check the wires and screw caps for any water etc.. make sure that the wires are making good contact with the pins inside the coil. You can actually trim the wire ends about a 1/8 - 1/4 inch and re-insert them. This worked for me when I had your identical problem, by now you have to realize it's not the carbs. By the way it does not matter where you put the green and red wires on the coil.
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post #9 of 10 (permalink) Old 03-26-2007, 09:40 AM
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Quote:
Originally Posted by artman View Post
Jon swap the front and rear coils and see if the problem moves to the front cyl.. You should also remove and check the wires and screw caps for any water etc.. make sure that the wires are making good contact with the pins inside the coil. You can actually trim the wire ends about a 1/8 - 1/4 inch and re-insert them. This worked for me when I had your identical problem, by now you have to realize it's not the carbs. By the way it does not matter where you put the green and red wires on the coil.
Okay, now I'm stumped - if the wire placement doesn't matter, then how can one switch polarity on the coils? Is there something else we should be looking at (given that I always forget which wire goes where on my front coil?). Or did that reference not refer to the kinds of coils on the 750s?
Thanks.

C
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post #10 of 10 (permalink) Old 03-26-2007, 09:54 AM Thread Starter
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Quote:
Originally Posted by artman View Post
Jon swap the front and rear coils and see if the problem moves to the front cyl.. You should also remove and check the wires and screw caps for any water etc.. make sure that the wires are making good contact with the pins inside the coil. You can actually trim the wire ends about a 1/8 - 1/4 inch and re-insert them. This worked for me when I had your identical problem, by now you have to realize it's not the carbs. By the way it does not matter where you put the green and red wires on the coil.
Problem is that I can not tell which cylinder has the problem. I assume the rear because of what I went through with that coil. Have to replace it, moving it will not tell me anything.

Jon

93VN750, under re-construction
vn750.com Member # 828

Rick's Stator and R/R
MF AGM Battery
Coastered
Degoated
Shaved & Jetted
Iridium's
MAC Tapered Staggered Pipes
Battery/Temp/Clock Gauge
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Dunlop D-404's
23,XXX miles and holding pending gasket change (underway!!!)
93VN750 is offline  
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