79 CB750F Plug Fouling

[mrmach5]

79 CB 750F , Soot black carbon fouled plugs after 20 minutes of running .Compression test all four range from 155-160 PSI. Adjusted the valves. Stock exhaust and factory air box. Complete carb (VB42A) rebuild , ultra sonic clean all new o rings, air cut off valves and accel pump. Used all factory brass parts 68 mid jet 102 main jet, Mixture screws 1 1/4 turns out. Confirmed choke blades fully open, Carb sync and dynamic timed. New plug wire caps. Denso
X22EP-U9 plugs.
I'm about out of ideas??

 

[mrtwowheel]

I'll bet big money that you used a carb kit with new needles that don't resemble the originals in any way.

 

[mrmach5]

No , I never use junk kit jets, needles , etc. I never use anything but the original brass parts unless there is significant wear, then only genuine parts.

Oh forgot to mention float height 15MM.

 

[Oscar Meyer]

No , I never use junk kit jets, needles , etc.

What brand of kits did you use? Was there a problem before the carb work? How does it run now (besides fouling the plugs)? Have you considered deleting the air cut off function? Have you eliminated the vacuum petcock?

 

[mrmach5]

I think the kits where K&L , but again didn't replace any brass parts. The bike was fouling plugs when I got it. Coils tested good, plug caps all tested bad so replaced with new NGK caps (thought I had found the problem---- Not!) Vacuum petcock eliminated. With fresh plugs the bike runs great, till the plugs foul , about 20 minutes of run time.

 

[Oscar Meyer]

What condition were the fuel cut off diaphragms?

 

[mrtwowheel]

Still think you have the wrong needles in there.

 

[mrmach5]

The air valve diaphragm's are new.
As far as the needles go, I did not change or replace the needles during the rebuild. However certainly not impossible that the previous owner made a change.
Bottom line is carbs are coming back off for further inspection.

 

[MP-x]

Try hotter plugs. Denso recommends X24ES-U for your bike, with 0,7 gap. If your carbs are fine, and timing is correct, and plugs still fouling, then your plugs are probably running cold, and can't keep themselves clean. Iridium plugs run well too, I hear, haven't used them myself though... IX24 according to Denso catalogue.
https://assets.denso-am.eu/production/attachments/Spark-Plug-Motorcycle-Catalogue-Multilingual.pdf

Useful reading about plugs (among other things): https://www.motorcycleproject.com/text/spark_plug_facts.html