Valve Stem Seals, or maybe throw in the towel

[KiwiAlfa156]

Don't discount the coil. Mine measured perfect and when I ran it with one lead disconnected from the plug (the cylinder I was having issues with) I got this.

[pdqford]

Rich, IIRC, the inner cylinder’s caps are different from the outer cylinder’s caps on the 16-valver.
Can someone confirm?

[Rijko]

Don't discount the coil. Mine measured perfect and when I ran it with one lead disconnected from the plug (the cylinder I was having issues with) I got this.

don't really understand what you mean.
But i can't quite make out what i am looking at, is the plug cap still on ?

Cool picture, that looks like a powerful coil/spark.

Made me think of the Suzuki service bulletin for GT750, how to do a rudimentary test of a coil without the Suzuki testing equipment.
Clip off the whole arm of the spark plug and check out how it sparks.
Spark should at least reach the side of the plug, preferrably a strong blue spark.
Yellow spark is already considered weak and a weak yellow spark means bad coil.

[Rich82GS750TZ]

Jim, that's what others have said. As you can see in pic where I have them all apart, all 4 caps on my bike are exactly the same, right or wrong. I have no fitment issues. And I have every reason to believe the the coils/wires/caps on the bike are original from 1982.

[Rob S.]

Rich, IIRC, the inner cylinder’s caps are different from the outer cylinder’s caps on the 16-valver.
Can someone confirm?

They all appear identical on my '82 1100 16 valve.

[rphillips]

Suzuki parts diagram shows the 750TZ uses 4 cap, spark plug... part # 33510-45010... no other description. NGK says XD05F::X= 102 degree, D=fits 10 or 12 mm plug, 05= 5K resistance, F= pushes onto plug, no nut or stud. P=special water proof cover, it shows main body is 70mm long and neck is 24mm.... I'd say they will definitely fit, but "I won't guarantee it".... Decisions decisions.... I'm thinking the 8V Suzuki's had different caps, inner and outer.

[KiwiAlfa156]

don't really understand what you mean.
But i can't quite make out what i am looking at, is the plug cap still on ?

Cool picture, that looks like a powerful coil/spark.

Made me think of the Suzuki service bulletin for GT750, how to do a rudimentary test of a coil without the Suzuki testing equipment.
Clip off the whole arm of the spark plug and check out how it sparks.
Spark should at least reach the side of the plug, preferrably a strong blue spark.
Yellow spark is already considered weak and a weak yellow spark means bad coil.

This pic is of arching between the coil itself and the frame. The resistance across the coil terminals were all good when measured, and there is no external damage visible on the coil. The new NGK caps and copper leads measured 5Ω each. I've have the same issue and symptoms as the OP, I experienced a sudden drop in power when riding hard (doing jetting plug chops). And since then the bike idles a little rough and runs poorly in the mid-range and pulling the plugs shows just one cylinder (No. 1) being very (sooty) rich. Because I'm in the process of jetting the carbs, I assumed it was a carb issue, thinking that a bad coil would effect both the cylinders it served as its wasted spark. But nothing seemed to be making a difference, so to eliminate the coils I decided to swap the 1 and 4 leads. As a further test I pulled the lead that was feeding the problem cylinder off the plug and got this serous arching at the coil. I think pulling the plug lead increased the load on the coil and it discharged through the fault (path of least resistance). This is probably happening to a much lesser degree at idle and isn't bad enough to be noticeable at lower rpm where compression load is lower, but as the rpm and cylinder BMEP rise, so does the effective resistance across the plug gap and it arcs like this at the coil and the spark to cylinder 1 fails. The fuel in cylinder 1 is being sporadically ignited and the plug goes sooty. As I said, the fact that it was only effecting one cylinder put me down the wrong path.

Here's the video

https://youtube.com/shorts/04nz3nAFnbg?si=_WInNVxf4S6F9rEu

[Rijko]

This pic is of arching between the coil itself and the frame.

oh, wauw, never seen that before.
Thought at first i was looking at the bare spark plug lead without cap but after seeing the video it's very clear
what i was looking at. Funny how that works.
I've heard about leaking coils but never thought it could be anything like this.

Thanks for that, SO cool you got that on picture and video.
Very good to know, another piece of troubleshooting knowledge.
Learning something new here every day

[Grimly]

I've heard about leaking coils but never thought it could be anything like this.

The original coils on the zuks were notorious for arcing out in the rain*, which is why I got rid of them as soon as I could. Time hasn't done them any favours, either.

*Talking about riding 8 to 10 hours in the torrential rain, or the endless spray of a wet motorway - that will soon reveal weaknesses.

[KiwiAlfa156]

oh, wauw, never seen that before.
Thought at first i was looking at the bare spark plug lead without cap but after seeing the video it's very clear
what i was looking at. Funny how that works.
I've heard about leaking coils but never thought it could be anything like this.

Thanks for that, SO cool you got that on picture and video.
Very good to know, another piece of troubleshooting knowledge.
Learning something new here every day

We're all learning. Just thought this might help to OP. It really easy to go down a troubleshooting rabbit hole.

[Rich82GS750TZ]

OK, just bought 4 of these. Hopefully thats the right thing and will take care of the caps and I can move on to something else.

https://www.z1enterprises.com/search?q=Spark%20Plug%20Cap%20-%2010mm-12mm%20-%20102%20Degree%20-%20Terminal%20Stud%20-%205k%20Ohm%20-%20Black

Now, for all the morbidly curious, the compression gauge showed up today, so here’s the results:

cyl. 1- 165 psi
cyl. 2- 155 psi
cyl. 3. 160 psi























and what we all knew all along.

cyl. 4- A whopping 15 psi.

So, when the plug caps get here I’ll take care of that. I really hope I don’t have to bug you nice people anymore about something that should be so simple. I’m learning a lot. So thanks, everyone.

I’ll work on getting the head off this engine, and laying it out alongside the 2 ‘82 heads. Stripping everything down and making three heads into one. Ugg, this sounds tedious already. I expect not to have anything to report for a while. I may have some questions about valve spring compressor tools, but not just yet.

[KiwiAlfa156]

Burnt valve? Guess we'll find out.

[pdqford]

A couple of squirts of oil in that cylinder will help decide if it is a piston ring seal or a valve issue...
Could it also be a head gasket issue?

[Rich82GS750TZ]

Head gasket doesn’t leak. At least outside the engine. I suppose it could leak on the inside to the top of that piston?

OK, Jim. I’m game.
So, I squirt some oil on top of piston 4, re ran the compression test and I still get 15 psi. What does that tell me?

[Redman]

No4 compression 15, yikes, that is low, and shows that your thumb test was valid.

So, ah, lets not be too concerned with plug caps. Yes, you need some, but that is not the big problem.

.
.
So, I squirt some oil on top of piston 4, re ran the compression test and I still get 15 psi. What does that tell me?

likley the low compression not related to rings, as the oil would help seal up worn rings.
And your camera thingy doesnt show any hole or crack in piston.
So suspect problem is more related to valves.
Valve hanging up not closing, or valve (exhaust) with burnt edge so not sealing, or valve with edge chipped off (but dont see any dings in piston top).

Hey, i wonder if your camera probe thingy have ability to be turned and look back up at the valves.