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Coil test - weak spark?

17K views 13 replies 4 participants last post by  EdK 
#1 ·
I have a 1987 VT700C that starts and idles great, just feels like there is a lack of power at high speeds, and it especially bogs down in overdrive. I went through the carbs because I had to replace a float bowl gasket and everything looked very good, and the jet needle was shimmed by a PO.

I just put everything back together, and it fired right up like it always does. I went out for a short ride just around town and I didn't get it up to overdrive speed or go out on the highway. When I came back, I noticed the front cylinder seemed cooler than the rear. So I pulled the front left plug and checked for spark. I had what i think is a weak spark - rather small, and orange not blue. Didn't look robust. I ended up checking for spark on all 4 plugs, and I'm getting the same (weak?) spark in all four.

I did a lot of reading, and decided to check the battery and coils. The battery is supposed to be about a year or two old, and it reads at 12.4 volts. I did not check it at charging rpms. I have a manual and decided to check the coils. This is my first time with multimeter, but here is what I found:

Primary resistance 2.2 - 2.4 but my meter reads .2 before I test
Circuit check reads 11.40 on both coils, checking black & white wire with ignition on and kill switch on Run.
Secondary resistance, I could get no reading on either coil. Is there a trick to it?
Plug wires I could get no reading. I pulled the caps off the plugs belonging to the same coil, then put the leads into the caps where they connect to the plugs and could get no reading on my meter. Am I doing something wrong?

As always, any help is appreciated, I am a novice when it comes to electrical problems.
 

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#2 ·
Sounds like you are going in the right direction at least. The 2 ohm reading is fine. The other readings for the secondary and wires and end cap need a higher scale on your ohm meter. Put it on a 50,000 ohm scale or higher to get the secondary reading. It should be about 30,000 to 40,000 ohms with all of the caps and wires assembled. They are solid wires and the end caps have about 5000 ohms each.

On your spark testing, since both plugs are in series and the coil power goes through both of them in series, to get the best output reading from the coil you should test the spark jumping from one wire to the other. You can get sparks from only one side but it is not total output since the other side will be jumping it's gap.
See the diagram below of the dual fire system.
 

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#4 · (Edited)
If you have a very strong battery while cranking you probably will get good strong sparks. But if the battery drops down to about 10 volts while it is under load the spark will be weaker. As long as it is good on both cylinders it should always start .

A good test to see if the old wires and caps are still insulating well is to have it running and spray water on all the wires and caps and see if it arcs over any where.
 
#6 ·
Yes both plugs fire a TDC, one in a forward current direction and the other in reverse direction. Sounds weird thinking of one plug firing from the side, or ground of the head, to the center electrode, but that's what it does.
 
#7 · (Edited)
Ok, quick recap, 87 VT700C, thought I was getting weak spark. Bike bogs down sometimes when under load.

Battery is newish, reads 12.6v cold
Coils show 11.4v
Primary coil resistance tests 2.2 - 2.4 (meter starts at .2)
Plug wire resistance 30.9 front - 32 rear (Thanks Swifty)
Pulse generator resistance around 425 front - 435 rear
Spark unit shows 11.48 v

Bike starts and idles very well.

Do you think this means ignition sytem is OK?
 
#8 ·
I would say the ignition is fine. My bike always starts very hard if under 45 degrees. So I tried an experiment since I had some voltage drop through the ignition to the coils also. I tried a wire direct from battery, through a switch, to the coils that I could give good direct battery voltage to the coils. It didn't seem to help anything. So I guess it is as good as it will be.
Maybe the coils get weak as they age and I have been thinking about Dyna coils but haven't spent the $$$ yet.

Your bogging problem may still be fuel supply. Do you still have the factory intake tubing set up or aftermarket filter? Are the jets still factory sizes? You said the needle has been shimmed.Did you inspect the rubber slide diaphrams? What color are the spark plugs after a ride. It is possible that it is running too rich at high speeds and bogging.
 
#9 ·
Thanks, I haven't tried to start it in 45 degree weather, but as it starts up on 3 plugs when I'm trying to check spark. Really can't complain!

I have stock intake and exhaust, with what I think is a high flow filter that looks new. I'm almost positive the jets are stock - I ordered a new stock jet just so I could compare (see attached). The diaphragms looked good, did not even see a pinhole.

Once I get it back together I'll try to get it bogging and come back and pull the plugs.

Thanks for all your help.
 

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#11 ·
Check your stator wires... I know my 1100 had to have the connectors replaced... An easy way to test it is to see of it runs better at idle/unloaded RPM with the battery connected to a non-running car or truck, with jumper cables... Mine ran better just connecting it, and even better with the vehicle running. You should be seeing over 13.5 - 14 volts at the battery with the bike idling.
 
#12 ·
He may not have that high of voltage at idle depending on the normal loads of his system. But at least revved to 2 or 3000 it should get up to 13.5 to 14.5.
 
#14 ·
Just wanted to follow up in case someone else is experiencing the same problem. It looks like this must have been a carb problem all along. Although the carbs seemed very clean, I did clean them again because I had to replace a float bowl gasket. That seemed to have solved the problem of bogging down in overdrive especially. I finally got a chance to really air it out, and it feels like I'm getting more power as well. Nothing really apparent when i opened up the carbs, must have been a small clog, or maybe the slides weren't as free as they should be.

Thanks for all the help!
 
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