Valve Stem Seals, or maybe throw in the towel

[Rich82GS750TZ]

Dave, I believe I'll take your recommendation. I found the XB05F on Amazon for 12.88/pr. Look good to you?

All 4 caps on my bike seem to be of the longer variety XB05F. But I will measure before I order.

https://www.amazon.com/marddpair-Resistor-Replacement-Degree-Motorcycle/dp/B09FG4QSXT/ref=sr_1_2?crid=30ZT3JRQ4992O&keywords=XB05F&qid=1 695228575&sprefix=xb05f%2Caps%2C184&sr=8-2

[Redman]

Rich,

some comments:

- I do recall discussion/situation about something NLA, either plugs without resistor or caps with resistor. You want one to have the 5k, and not both have 5k, and not both have none.

- yours caps are well above 5k, and the worse one is in the 1-4 circuit, which is where is an apparent problem….. so….. I think you are about to become more familiar with the situation. Look for XB05F and VB05F (2 each) . These NGK caps are used by lots of bikes, so maybe can find them at local multi brand dealer. Longer ones are for center cylinders.
My experince is that if at 2 or 3 or 4 times the 5k they will still spark when test them, but present running problems, worse when engine hot, and are on way to completly failing. I can’t say anything about taking apart and cleaning, I replaced them at $5 apiece when ordering other stuff.

-yah, trim off just a little, 1/4th inch or 3/8th, from end of wire only if strands of conductors are damaged at the end (compare end to what maybe can see up in there a little bit). Don’t just do that for no reason cuz isn’t much extra length on those wires to do that more than a few times.

- my stated spec of 35k ohm cap to cap (5+25+5) is from my memory of 850g and 1100g. Have now verified that in my 1100G FSM says 30k-40k. I don’t know if your T (4 valve per cylinder) uses a different coil or not being more of a higher performance engine.
FSM doesn’t acknowledge anything about the caps as a separate item.

- some folks may say the resistor in the circuit (either cap or plug) is just to reduce radio interference on AM radios, which it does. But I have reason to suspect is also needed for best operation, but I don’t really know.

- when measuring something less than 20k, I don’t think it much matters if use the 20k scale or the 200k scale, other than how many decimal digits (assuming good batterys in meter). But before someone suggests you need to use the lower scale, you can try both and see if much difference.

- your caps are something to be aware of, and replace because are easy and cheap (and before completly fail)…. But your preliminary compression check (thumb) seems like a more critical situation.

[Rich82GS750TZ]

Jim, thank you. Looks like I’m in the ball park. Will just clip fresh ends on the leads, reinstall the caps and call the coils/caps good/ not involved in my recent engine failure.

Ron, thanks. Will will look more closely when I get home.

[pdqford]

Dave. I believe the caps are resistors (think I read that somewhere about my bike). So I took them off.
meter set so 200K
probes in the 1& 4 leads 11.6
probes in the 2 & 3 leads 11.8

I then measured just each cap by itself at, meter at 200K:
1- 11.5
2- 8.7
3- 9.5
4- 8.5

Just for clarification, at the 200k setting, all your measurements should have a “k” on the end of them.
So your resistance measurement for the 1 & 4 wires and coil would be 11.6k ohms, or 11,600 ohms.

Using my handy dandy little calculator your 11.6k ohms plus the resistance of the 1 & 4 caps comes to 31.6k ohms.
The Manual on BikeCliff’s web site specs 30k-35k ohms.

[1978GS750E]

Rich,

Check the plug caps and see if there isn't a brass fitting that goes on the plug with a screwdriver slot in it. You can just unscrew the brass fitting and the resistor will be under it. Normally there is a spring in there also. I use a short piece of heavy copper wire to replace the resisitor.

[Rich82GS750TZ]

I will do my best to clean up the screw end and the plug cap ends of the caps, see if I can get closer to 5k ohms.
If there is a write up or pictures somewhere of removing/cleaning the resistors, I haven't seen it. It's not apparent to me how that can be done.
Then, again, I didn't know you could just unscrew the caps. I will also clip about 1/4" to 1/2" off of each lead before reinstalling the caps. Have read that that's a good idea.

So if each caps should give me 5k ohms resistance, and I'm getting 11.7 average in the leads, I should get 5+5+11.7 = 21.7k ohms when measuring 1-4, and 2-3? Is that how the math works?
Redman posted above a numbers 5cap+5cap+25leads = 35k ohms. Is that where I should be for the secondary resistance?

Let me clean things up when I get home today, put the caps back on, measure again and see what I have.

This all has to be looked at part of the whole, I know I'm still looking at rebuilding the head of one of my engines, rings, etc.

Figured I'd just try to nail down and potential coil issues now, while I have these parts apart.

I guess I could just buy something like (2) pairs of these. Unless I need to replace the leads as well.
https://www.amazon.com/labwork-Spark-Resistor-Degree-Motorcycle/dp/B08JSFFV1V/ref=sr_1_2?crid=3S5ZEJUTYIFPP&keywords=ngk+spark+p lug+resistor+boot+motorcycle&qid=1695221755&sprefi x=ngk+spark+plug+resistor+boot+motorcycle%2Caps%2C 68&sr=8-2

Well, not those boots exactly, Most of the reviews say they're garbage.

This one looks like the real deal:

https://www.amazon.com/Spark-Resistor-Cover-Degree-XD05F/dp/B07GFQWQ4V/ref=sr_1_1?crid=2WNH88I1CY58A&keywords=ngk+xd05f&qid=1695222052&sr=8-1

[Nessism]

The caps should be 5k ohms. You may have corrosion inside the cap. I've read here, but never verified, that the small resistor element can be removed and cleaned. That may be worth a try before trying to chase down new caps. I've also read that NGK has discontinued selling old style plug caps like our bikes use, so finding some may take a little work.

Edit: found this thread showing the cap taken apart, with the resistor element exposed...

https://www.thegsresources.com/_forum/forum/technical-forums/electrical-ignition/39513-spark-plug-cap-resistance

[Cipher]

Caps are dead or corroded. They corrode internally get some ngk equivalents.

[Rich82GS750TZ]

Ok I was watching videos. Getting more confused

Jim, thank you for the details.

I got 3.7 ohms on the primary side of both coils.

Dave. I believe the caps are resistors (think I read that somewhere about my bike). So I took them off.
meter set so 200K
probes in the 1& 4 leads 11.6
probes in the 2 & 3 leads 11.8

I then measured just each cap by itself at, meter at 200K:
1- 11.5
2- 8.7
3- 9.5
4- 8.5

Someone tell me if this is what I should expect. Or is there something wrong here. Either with the numbers, my testing method, or both.

I also got the lid off carb # 4 and pulled the plunger. Diaphragm is in perfect condition.

[Redman]

One probe in 1 cap, other probe in 4 cap (doesn’t matter which where). Should be about 35k ohms. Your meter has a 20k range (means goes up to 19.99k) so use the 200k range.

if it reads high, say, like 80k or 150k, you can twist/pull off the spark plug caps from the wires, and measure from 1 wire to 4 wire, should be about 25k. Can also measure each spark plug cap, should be 5k each……
oh, wait, I am assuming stock caps that have a 5 in the number for 5k. The caps with resistors in them can go bad by going to higher resistance (but still spark some) and then go completly open.

maybe you can hope for plug cap(s) being the problem, since they are cheap and easy to replace.

but your preliminary compression check (thumb) seems like is ominous.

[rphillips]

If you can feel a noticeable difference in suction on # 4, that doesn't sound good. Not say'in anything just talk'in, Possibly valve not closing or burnt, or compression ring.. but don't give up, still possibly more minor than that.

[pdqford]

And here is more than you asked for, but the magic of electricity is:

V=IR. Or volts equals current(In amps) time resistance (ohms).
To solve for current divide each side of the equation by R and you get:

V/R = I

So when you were hustling up that big hill with the HDs trying to keep up,
and your charging system was cranking out 14 volts,

14V / 3.7ohms = 3.78 amps is all your coil was sucking up!

[pdqford]

To measure the resistance on the secondary side of your coil, put a probe in each sparkplug cap.
That will read the total resistance of the coil’s secondary circuit, including the spark plug wires and caps.
Since your meter is not auto ranging, you need to select the scale that would show, maybe 30k (30,000) ohms.

[pdqford]

Let’s measure the resistance (ohms) of the primary circuit of the ignition coils.

To properly measure the resistance of a circuit the circuit must be off, as in no current flowing through it.
Unplug the O/W (power) wire from the coil and the white (ground) wire from the coil.

Set your meter to the ohms setting. For this circuit probably the lowest setting.
Put the red meter probe one the power connector of the coil and the black lead on the ground connector of the coil.
(When the meter is in the ohms mode it puts a very tiny amount of current to test the resistance of the circuit.
Thats why the circuit needs to off, as in no current flowing through it.)

The meter will display the resistance in ohms between the meter probes, I.e., the coil primary circuit resistance.

How many ohms resistance are your coils?

[Rich82GS750TZ]

OK, watched a video on measuring resistance. I got 3.7 ohms on the primary side of both coils. Can’t get a reading on the secondary sides. I’m sure I’m doing setting something wrong.

edit:
Figured it out.