Valve Stem Seals, or maybe throw in the towel

[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]

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]

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.

[Rob S.]

Towelie says:

"Don't forget to bring a towel!"

[Rich82GS750TZ]

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

[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 will measure coils this evening. Thanks.

[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.

[Redman]

Rich,
I suggest when checking the exhaust header pipe temperatures, do it about 3/4 minute after cold start up, and again after a couple minutes, before the head and cylinders heat up.
Oh, and just tap the pipes with back of fingers (skin is thinner on back of fingers, if use finger tips the skin is thicker there and delays your detection and then skin is already heated up oww oww. Old boiler operator guy taught me that decades ago.).

That is all I got to help.

(I was expecting new plugs to clear up the symptoms.)

[Rich82GS750TZ]

Interesting. Hmm, tomorrow evening I’ll take the lid off # 4 carb and inspect the diaphragm.

[KiwiAlfa156]

Check the carb diaphragm on number 4. I'm having something similar on the 750 Kat and I've discovered some holes in mine. I was doing a WFO main jet plug chop and it suddenly felt like something 'gave' and the performance degraded somewhat, which sounds very much what you describe in your OP. Anyway the story is all here https://www.thegsresources.com/_foru...on-the-kat-750

[Rich82GS750TZ]

Ed, before installing the new plugs, I did check spark on each against the head. All 4 sparked.

After my first cold startup with the fresh new plugs shown in the video above, only running for the 3 minutes of the video, I touched each header pipe at the top bend. # 4 was hot but I could keep my thinly gloved hand on it for a few seconds at least. 1-3 were much hotter in comparison. I could touch them but but only very briefly.

Here’s the new plugs after 1/2 hour ride.


Close-up of # 4

[Nessism]

Check from a cold start. Very carefully touch the headers right after a cold start. You should be able to quickly figure out if number 4 isn't firing... If it isn't, visually check spark next, with the plug held against the engine.

[Rich82GS750TZ]

Sounds like it’s running on three cylinders. Check the temp of the header pipes.

Agreed. I have no way currently to measure the actual temperature. # 4 is hot, but not as hot as the other 3.

[KiwiAlfa156]

Sounds like its running on three cylinders. Check the temp of the header pipes.