Valve Stem Seals, or maybe throw in the towel

[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.

[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

[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

[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.

[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.

[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.

[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.

[pdqford]

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

[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.

[Rich82GS750TZ]

Cipher,
All in good time. I know the top end all needs redone. I'll get to it. If you've followed from the beginning, you know I'm going to rebuild the top end for the '82 engine that was originally in this bike. In the course of looking at things, coils/wires/caps came up to investigate. Investigation lead to where I am now - needing new caps, Then I'll get to building the other engine to put in this frame. When I do that, I'll have reliable coils/caps. This is a long-term project. I'm in no hurry now. As you saw and commented on my other thread, I will likely have another bike to ride before too long. Sorry if my story, the way I choose to work on my bike, or the order in which I choose to work on it doesn't agree with you. I have seen you rarely contribute helpful advice in threads such as these. If can do so here and give me link to the correct non-resistor or resistor cap (I don't really care at this point), then please do. Or you could just scroll on.

rphillps, yes both of those of the links I posted above from Z1 are resistor type. only this one is available. I had thought one of them was a non-resistor, but as you saw, it was in the description.

https://www.z1enterprises.com/spark-plug-cap-10mm-12mm-102-degree-terminal-stud-5k-ohm-black.html

If I can get someone here to tell me this will definitely work, I'll buy it. Right now I'm second guessing myself, because I have never done this before. Trying to learn.

[rphillips]

I never remember seeing this size caps without resistors. You got me nosey, I just went out in the garage and took one of my new XD05F's apart, they are made exactly the same as the OEM, not a clue about how the K & S brand is made, NGK can easily be changed to non resistor, just as the OEM can... Did you find non resistor caps? both of these say 5K ohm, one in title, the other in description

[Cipher]

Update:


Have disassemble, cleaned and reassembled all plug caps. 2,3, 4 read low to high 9K ohms. So they’re no better. # 1 cap I can’t get a reading at all no matter how many times I take it apart and put it back together. So I now have a new problem I didn’t have before my bigger problem of a mostly dead cylinder.
I will not be modifying these plug caps.
These caps are done.
Moving on.
I will be buying new caps.

Non- resistor caps and switching to resistor plugs seems like a good idea.
Known solutions, specific links for what I need would be appreciated. Parts that are known to work with the 16 valve 750 engine. I don’t know what to look for now.
Would like to put the coils and plugs to bed so I can get into rebuilding one of my other heads.

Endoscopic camera arrived today. So played with that a little. All pistons have quite a bit of carbon buildup on top. This is the offending cylinder # 4. You can see oil pooling in the indents.

Mirror polished bores. Pull the head ffs. Swanning about with changing caps is a ludicrous sidetrack.

[Rich82GS750TZ]

Big T, et.al.
Z1 does have the resistor caps for $6.95 - I think this is the right one - If I want to keep with the resistor caps.

https://www.z1enterprises.com/spark-plug-cap-10mm-12mm-102-degree-terminal-stud-5k-ohm-black.html

I think maybe I should just get these?

They appear to be out of stock of these other resistor caps.

https://www.z1enterprises.com/ngk-spark-plug-cap-12mm-10mm-102-degree-xd05f.html

[Suzukian]

That carbon deposits on your pistons could cause detonation or ignite the fuel mixture too soon. This oil leaking into the pistons means in the very least you need valve stem seals, but you may also need new valve stems. Catching it now could prevent touching the lower end of the bike. Polishing the top of those pistons could only lead to good things. IMHO

[Rich82GS750TZ]

Todd, thanks. I’m sure I can get these just about anywhere. If you have a part number that would help. I don’t want to get the wrong thing. I think now I want to go with non-resistor caps.