Wire diameter

[redglbx]

What is the diameter of the stock spark plug wires where they go into the caps. 7mm is what I’m thinking but am too lazy to walk up to the shop to actually check. I have 7mm solid core bulk wires currently on my 76 but am thinking that they are a little loose.

Normally I add a little smear of RTV to the OD before screwing them into the caps.
What say ye.

[ritalz]

Not positive but I think 7mm was stock. I like to use 8mm.

[low-side]

Stock is 7 mm stranded copper core.

[Rat]

You never struck me as the lazy type …

Gord(would relate)Jones

[redglbx]

Thanks Gord, but with the multiple back surgeries walking has become a bit of a chore !

So on the plug wire thing an 8mm wire will fit into the plug caps w/o to much hassle ? Up to this point I’ve used a 7mm stranded solid core wire that I put a thin swipe of RTV around the OD to help waterproof things but since I’m thinking of replacing the wires I’m thinking of using 8mm solid core to replace the 7mm piece’s that are currently there.

[Sidecar Bob]

It also depends on what the coils are set up for. I have no idea what the various aftermarket coils will accept but the compression fittings on GL1100 coils (and the GL1500 coils I'm using) are a good, tight fit on 7mm wire and cramming 8mm wires into the compression rings (olives/ferrules) probably wouldn't be easy.

As for the boots on the spark plug caps being loose, they were designed to seal well on 7mm wire when new but how old are they now? A wise man once said that rubber ages like dogs so 10 year old rubber parts are like a 70 year old human and 20 year old rubber is long past its useful life.
Don't even think about using 45 year old rubber if you can possibly avoid it.

Adding silicon sealant between the boot and the wire is a common bodge (I've done it myself, especially on winter machines) and won't hurt anything but it should be considered a temporary fix at best.
Why? Because if the boots on the wires are loose the ones that go over the spark plugs surely must be too. Do you use silicone sealant there or do you wait until you have problems on a rainy day, have it towed home and then replace the caps?

FWIW, when I changed to the GL1500 coils in 2016 I also replaced the wires & caps. The rubber boots on the new caps were much tighter than the ones on the caps they replaced.....

BTW, a quick check shows that the aftermarket has indeed stepped up to fill the gap left when NGK stopped making spark plug caps. These ones are listed as equivalent to the XD05F
https://www.randakks.com/spark-plug-cap ... dwing/1979

[redglbx]

Thanks Bob, good insights ! I currently use Dyna coils and have been for many years and have replaced several that failed in various ways.

My thoughts driving this is that my 76 has had a “chuff” on the left side cylinders (2 & 4) at idle and has had this for a while. This has been the case since probably around 2020 or so. I haven’t really ridden it much or been able to work on it due to my back surgeries and problems but I have replaced the plugs multiple times and done multiple compression and leak down tests all with good results , so that says a leaking valve seat is not probable. By the way it runs very good down the road.

‘So being that the “chuff/miss” remains constant on the left side logic says that it has to be a plug, plug cap or wire. And being that I have changed plugs multiple times as well as the caps I’m down to the wires which are currently solid core 7mm that I bought off a bulk roll, so I’m thinking that maybe they have a break or some other defect internally ,, I have also done resistance tests on the wires and they did check ok BUT they are probably 10yrs or more old., so that’s my thoughts driving this.

My back issues are getting to a point where I’m hoping to get on the 76 so I can ride her again (my wife and Dr just cringe) once the weather gets good again. So I’m thinking new wires and normally I rtv the od’s going into the plug caps and use dielectric grease on all the other contact points like the plug caps to plugs and the wires into the coils.

The only other thing I have in my mind on this is a possibly bad anti-backfire diaphragm in one of the left carbs since I use 1100 carbs and they have those on each carb.

So since my first surgery in 2021 and being fairly debilitated since I’ve had plenty of time to just sit and noodle on this. Since I’ve bought new coils ,a new Dyna module although I had at one point replaced the current Dyna with the Prestolite module (I never liked it) I installed back in 78 with the exact same results so I don’t think that’s the problem.

So with the improving back issues I’m hoping to maybe get this thing sorted so I can ride it again,,,,, I do need to change the belts before that happens as it’s been a while since I changed them last. It’s just funny to me on how this thing just calls to me since I bought it new in October of 76, I still just enjoy riding it. So we’ll see…..

[Sidecar Bob]

The wires should last at least 10 years but on the other hand they don't owe you anything. And I've often repeated the story of limping home in the rain on 1 cylinder on my '77 because the water was shorting out the spark pulses the other 3 plug wires and that bike was only 16 years old at the time so new wires probably aren't a bad idea.

I just had a thought: If you suspect the "chuff" is caused by the plug wires and it only occurs on the left side it might be a good idea to try swapping the wires from left to right to see if the problem moves to the right. (You shouldn't need to move them at the coils, just switch the cap ends from left to right)

[redglbx]

Good idea Bob !