Intermittent misfire

[Eidollan]

I have a 1983 GS750ES. The issue just suddenly happened, I was warming up the bike, started normally, with no issues whatsoever, when the bike was warm suddenly the idle became rough and inconsistent, I had my idle set to 1.2k rpm, I notice it will drop to 1000rpm or below occasionally the bike will shut off, I didn't think about it too much and went riding, at first she was running smooth and I was cruising around 6000 rpm, rode for about 50 miles I think, had the bike rest for 3hrs before going back, when it was time to go back the bike is a bit hard to start and it doesn't fire on all 4 but after a few minutes it will fire all 4, then idle will be rough and inconsistent again, I also notice sometime cylinder 3 and 4 will not fire especially below 4000rpm or after letting go of the throttle for a while then getting back on the throttle it will take some time for cylinder 3 and 4 to fire again like its not getting gas.


thinking it was the spark plug since I have not replaced it for a very long time, so I replaced all 4 and cleaned all 4 carbs, put everything back, and start the bike again, still does the same thing, and this time after high revs like 6000rpm and up after getting off the throttle then back on the throttle cylinder 3 and 4 will not fire but after some time it will again.

[Rich82GS750TZ]

*idle*
How long have you had the bike? Did it run well up until this recent episode?

Describe "cleaned all 4 carbs"
Did you follow this?:
https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

[Eidollan]

*idle*
How long have you had the bike? Did it run well up until this recent episode?

Describe "cleaned all 4 carbs"
Did you follow this?:
https://gsarchive.bwringer.com/mtsac...d_Tutorial.pdf

Had the bike for over 5 years now, had the engine refreshed 2 years ago, yes, it was running well until recently.

Yup followed the guide, all jets are not clogge, floater is leveled, made sure all small holes are not clogged as well.

Addition to that I checked for vacuum leaks by spraying Carn cleaner almost everywhere that might have a vacuum leak and checking for rpm changes and found none, also check each spark plug if it will give spark and switch the spark plug of 1and 2 to 3 and 4 but still the same thing

[tobaccorancher]

I would take an impedance reading from spark plug cap to spark plug cap 1 to 4, and 2 to 3. I had a condition on my GS750 where the reading was essentially infinite from 1-4, but the bike ran sometimes.

[Eidollan]

found the culprit, the spark plug set I bought 2 are defective, bought a new set and just replaced the one for cylinders 3 and 4 and she ran great no dead cylinder.

[Grimly]

Were they genuine NGK or fakes?

[Eidollan]

Were they genuine NGK or fakes?

actually don't know, I got them online, I'm assuming they are fake, never had an issue with NGK before this is the first time. I notice the difference when I spark check the new ones and you can definitely see the spark strength difference, the new ones' spark is deep blue color the old one is kinda bluish orange, I guess that's the issue, it's not igniting the fuel-air mixture

[Grimly]

The world is awash with fake plugs. You never know what you're getting if you buy online, unless it's a trusted source.
Even at that, I once had a box of ten dud genuine Champions.
That caused some head scratching when the car wouldn't fire, and the next four didn't work either.