Topic: Plug wire resistance?

[lowriderbud]

While trying to diagnose running issues on my '78 750F I measured the temperature of each exhaust header about 6" from the head.  #4 - 247*,  #3 - 250*,  #2 - 185*, and  #1 - 87*.  Pulled the plugs and found #1 to be a bit wet, #2 black, #3 and 4 very dark grey.  Rather than go into the carbs again I decided to check the resistance of the plug wires.  The wires are 8mm silicon core and cover, connected to a Dyna ignition and coils.  They were on the bike when I bought it.
#1 - 5.49 ohms
#2 - 4.76 ohms
#3 and 4 both read like a capacitor; make connection and reading jumps to about 19 then drops back to infinity.

I would have expected just the opposite since 3 and 4 appear to be running correctly.  What am I missing here?  Anybody seen anything like this? 

I ordered new plug wires.

Later, Bud...

[Flyin900]

I would also check the plug caps and the coils for the correct resistance specs too. The least likely suspect is what seems like modern new wiring. You should get a very low if next to no resistance from 2 ft of spark plug wire, yet I would also check that they are a metal core wire conductor and not a car style wire that uses a no metal conductor as the voltage carrier.

[Don R]

 I think he says they are silicone core and cover. That sounds like carbon core wires.

[Bodi]

Something is wrong, obviously. I see no point in chasing it, get new wire and caps. You want metal core wire (not suppression wire with a non metallic core) and resistor caps, 5K or 10K depdnding on model although either should be ok on any model, and non redistor plugs. You can use direct caps if you only use resistor plugs. 5K caps should be ok with 5K plugs. 10K caps and resistor plugs is likely to cause weak spark.

[calj737]

Is there any chance that you have the coils wired to the incorrect plugs? One coil serves plugs 1/4 and the other coil serves plugs 2/3. If you have them paired 1/2&3/4 you’ll have endless issues.

[Bodi]

Reading it again, you don't say how you're measuring these cables - the only way would be to remove them from the plug cap and the coil so I assume that's what you've done?
5 ohms or so could not be suppression cable, this will have 3K or more per foot. The ones with odd readings... there isn't any way a capacitance could be introduced to a cable, and it is odd that the ones that seem to fire OK read weird - so I have no clue.
Measure the coil output resistance. You should have a pretty high resistance between the "screw" cable contacts and no connection from either to ground. I have not heard of a Dyna coil failing but anything is possible.
The plug caps are known to sometimes fail. There are sometimes-removable resistors inside, removable ones you unscrew the plug contact, it will have a slot for a screwdriver. Sometimes POs replace that resistor with something to eliminate the resistor (sometimes really poorly), sometimes the resistors fail. The cap should measure close to it's rated resistance, for NGK caps this is part of the part number marked on the cap - a VD05FP is 5K, a VB10FP is 10K.

[Deltarider]

I have not heard of a Dyna coil failing but anything is possible.

Mine did. I had the 1+4 failing after only 50.000 kms.
As far as plug wires, carbon core wires are a no no for our bikes.

[lowriderbud]

While waiting for my new spark plug wires I pulled the carbs to check float levels and replace a couple screws that were stripped out.  When I pulled the float bowl for #1 the pilot jet was loose (just laying there on its side).  So, I pulled the other bowls and #2 had a piece of trash (something like grass or straw) in it.  Completely cleaned all the passages and jets for all carbs - real good cleaning this time.

The plug wires came in and I put everything back together, and it starts right up, idles without the choke, revs nice and smooth.  Had to turn the pilots jets in to 1/2 turn on each one.  Now all exhaust header temps are about 275* .

Thanks for the help and suggestion.  BTW - I also bought a new digital voltmeter to correct the ohms reading on the plug wires.

Later, Bud...