Could it be valve stem oil seals (1980 XS400 4G5)

[Fentible]

After exhausting all the possibilities regarding a misfire on my 1980 XS400SE (4G5) with regard to ignition and carbs. I am still left with a bike that stutters and misfires from around 4K revs and struggles to rev beyond when on the move. Still starts on the button and ticks over at exactly 12K revs without a stumble, revs on the stand and on the move pulls quite strongly until the 4K barrier.

The Right plug shows a perfect mixture while the left looks black and sooty. Decided to try a hotter plug today so went and bought a BP6ES, new plug in and 400 yards onto a straight I opened her up to the now annoying 4K stutter, took it about half a mile and with the new plug in, it did drive through the 4k barrier, also pulled better in the higher gears but still misfiring. Once back on the drive I took the new plug out to see what looked like wet oily deposits around the end and nose of the plug.

That got me thinking, wet oil on the plug would surely cause tracking and a misfire?? Source of the oil ingestion can only be damaged oil rings, worn valve guides or the valve stem oil seals aren't sealing. This bike had stood for 18 years prior to me getting it running and has an indicated 18K miles on the clocks.

Any thoughts from the collective??

 

[Jtoroli]

I know this is contrary to your oil hypothesis but is it possible the resistor caps on the ends of the wires are bad? Mine had a very similar condition and when I took the plug wires apart I found the ceramic resistor and the spring virtually disintegrated inside the cap. Just a thought and they are cheap enough to swap out. Again just a thought.

Joe

 

[Fentible]

Hi Joe, thanks for your suggestion and believe me any are welcome at this stage of the game. As the R/H cylinder is behaving faultlessly I initially swapped the caps over to no avail. I did then replace them for new items unfortunately with the same negative result. All ignition components, aside from the main loom wiring (checked out okay) have been replaced for new. I've even just spent a day doing the full range of diagnostic testing of all the new components fitted and swapping between cylinders where possible. Carbs the same, all jets needles etc renewed and as far as I can tell they are performing as intended.

Its rarely I am unable to eventually diagnose and fix a motorcycle problem. This one has me absolutely stumped. Thanks again for the input.

 

[Jtoroli]

Did you do the resistance check on both coils? Do both primary and secondary coils give appropriate readings..like 3 ohms and 9K ohms (or in that neighborhood)? Just curious. I assume they are fine if they are giving a good fat spark.

So it fires right up and idles well so it sounds like the idle circuit is working OK. The plug is sooty though which indicates rich condition. Is the plug wet and gassy or wet and oily. You stated oily but again just asking.

I am assuming valves are adjusted, everything is timed, good compression etc. Although these would cause hard starting and yours starts right up.

I also assume that in the recent past it was running well and would pull through 4K although this may NOT be the case based on your post.

And of course the carbs are clean etc...nothing weird going on there fuel flow etc...I could be totally off base but I am putting money on left carb not being clean or something is fouled up or assembled incorrectly (by accident). For instance I once tore one of the o-rings on the float needle valve seat...caused headache... I also forgot to install the main jet retainer clip on one side once, another headache. Pilot jets are clean and free, bowls are venting. etc. I would look again at carbs. If you are getting good healthy spark (timed correctly) and you have decent compression it should run even if the guide seals were weeping some oil. Idle mixture screws turned out 3 turns.

I am betting on something simple.

Joe

 

[Jtoroli]

Also a couple of things.
The rubber diaphragms are intact and seated so no leaks?
Choke is closing completely?
Clean air filter (assuming stock)?
Clean petcock screen ?

 

[Fentible]

Yes carb diaphragms checked and swapped over, choke operates as it should, air filter material replaced with proprietary air-filter foam (have also run bike without filters) and good flow on petrol tap.

 

[Fentible]

Well turns out it was the valve stem oil seals, replaced them and misfire gone. Clearly nearly 2 decades of standing didn't do them any favours and in their deteriorated state, were allowing the engine to ingest oil causing tracking on the spark plug . However all now replaced and running as sweetly as it did from new.

Thanks to all who contributed.

Fent

 

[Jtoroli]

Great News! Glad to hear it was solved. Will note this for my future reference. Now you can enjoy the ride!