Valve Stem Seals, or maybe throw in the towel

[2BRacing]

Just my 2c here in the hope that there may be an easier solution than remove/replace cylinder head to replace seals: plug number 1 also looks fouled, but certainly not as much as number 4.

Have you closely inspected electrical connectors and measured voltages and resistance values at the coil feeding 1 and 4? Also plug wire and cap of number 4. Plugs 2 and 3 look OK, but if valve stem seals are the culprit, I would expect all the seals to be similarly worn (causing all plugs to be fouled), and not just seals badly worn on 4 and slightly worn on 1.

My first guess is therefore an electrical issue on 1 / 4 side.

[Rich82GS750TZ]

I will measure coils this evening. Thanks.

[Suzukian]

Maybe you missed my post, you can drill through a spark plug and make your own adapter for a pressure gauge. Not difficult to do, easier if you have a lathe, but I've done it by hand.

[Rich82GS750TZ]

I didn’t miss it. Thank you for the tip.

[Rob S.]

Towelie says:

"Don't forget to bring a towel!"

[93Bandit]

Thanks, Bob. I’ll consider it. Harbor freight has a few that are reasonably priced. But how do I know if they have the right adapter to fit the threaded hole in my intake port?

I think there might be some confusion here. The compression tester shouldn't be connected to the vacuum port on the intake. It is threaded into the spark plug hole. I believe most compression testers come with a few adapters for most standard plug thread sizes, 14mm 12mm and maybe 10mm. My 850 uses 14mm, not sure what your 750 uses but just verify plug thread diameter and check compression gauge specs. The Bosch tool I recommended covers most common sizes, and has extensions for plug holes that are recessed in the head like on our bikes.

Sounds like you have something else going on though, since new plugs didn't help. I agree with others, ensure adequate voltage at the coils and check the carb for # 4.

[Rich82GS750TZ]

93Bandit,
I'm sure you're right. Forgive me. I was thinking of the ports on the intake bosses on the back of the head that are used for carb tuning. My mistake. I've never done this before. Thanks for clarifying.

[93Bandit]

93Bandit,
I'm sure you're right. Forgive me. I was thinking of the ports on the intake bosses on the back of the head that are used for carb tuning. My mistake. I've never done this before. Thanks for clarifying.

No worries, glad I could help clarify! Not to worry about never this before, compression test is easy to do. There are tons of threads discussing the do's and don'ts of a compression test. Everyone starts somewhere. I'm fairly experienced with engines but there are plenty of things I've never done before. Learning every day.

[Rich82GS750TZ]

Not a lot done tonight. Have a meeting in a bit so could only start to take things apart. Got 3 of the 4 screw off number 4 carb lid. Stripped the last one. Yes, I Know, JIS, impact driver. Still stripped. I’ll cut a slot with the dremel and get it out. Have to quit for tonight though.

I’ve ordered a compression tester but will probably not get to testing till next week, Sunday evening at the earliest. I’ll be away all weekend. I did the only thing I could do: put my finger over each plug hole and run the starter, yes, throttle wide open. 1-3 audibly “chuffed” and felt strong, felt like it wanted to suck my finger into the hole. 4 was weak by comparison. Hardly empirical evidence, but I’ll have to wait on the gauge to have any numbers.

I’ve also ordered a cheap endoscopic camera for my phone. Always wanted one anyhow.

Measured 12v+ at the O/W + lead on both coils. I have no clue how to read ohms or what I’m doing here. Searched a bit but no luck. I have a multimeter. Would someone mind a brief instruction on how to set my meter, where to put the red and black leads. Sorry, not my forte. Would be greatly appreciated.

[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.

[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?

[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]

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!

[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.

[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.