SOHC.co.uk Forums > CB500/550
Sooty plugs.
K2-K6:
--- Quote from: Johnny4428 on October 02, 2024, 10:27:01 AM ---Now I am sort of giving myself a kick up the arse here. I had a nice run out yesterday on the k3 but quickly realised only 3 cylinders were firing. This is the second time this has happened to me this year on this bike. I got back home and checked cyl 2 and the plug was just sooted up then checked the rest and they were all sooty. Then the penny dropped that I had replaced the pilots with 45’s instead of 42’s but had made no compensating adjustment to mixer screws. As most of my running around here is in the lower to mid range rpm it stands to reason that it is getting slightly too much fuel! The reason I changed the pilots was to improve cold starting which it did do. So today I will make an adjustment to the carbs and take for another run, since it is a nice day.
--- End quote ---
Some thoughts on original posting/question and to look at it from different perspective.
The change to pilot Jets 42 to 45 gives a plus 7% shift in the idle circuit flow potential. If that's paired with the main jet as being roughly half the size of main, then at idle to approximately half throttle operation will give from 7 diminishing to 3.5% additional fuelling in that range. Ultimately still 2% at full throttle too. Effectively its richer right across the board by a diminishing percentage as rpm /throttle is increased.
The experienced "sooty plugs" evidence that this is in place.
To just raise mixture at base of idle, you'd be better in considering std pilot jet size and evaluation of more fuel set on adjustment screw, say 1/2 turn out further than the existing. This should just bring up the initial mixture fuellling level at "closed " throttle position, while leaving general running mixture alone.
Johnny4428:
Nigel I understand what you are saying and makes complete sense. The only reason I upped the pilot jet size was because of poor starting, certainly sorted that out. Even after cleaning out the old pilot jets about three times I wasn’t making headway. The bike ran very lean evidence of that was in the plugs and midrange hesitation cured temporarily by pulling the choke. I was going to buy new pilot jets anyway to try but thought I would increase slightly the size. I haven’t been out with the bike again since my last adjustment on the carbs but I think I’m going in the right direction with No 4 plug looking like we have a good fuel air mixture. If I can’t stop the fouling I will of course replace pilots with the correct 42’s.
Johnny4428:
Managed a nice run out this afternoon 60mls or so, bike was running great. Blew the main fuse again but thankfully had a spare not sure yet what is causing that. Checked the plugs when I returned and was pleased to see an improvement with uniform colour across the 4. Maybe a little tweak yet.
Sent from my iPad using Tapatalk
McCabe-Thiele (Ted):
Thats not a bad colour imho - depends a tad on how hard it was ridden.
Johnny4428:
Not ridden hard Ted. Just up in the mid range mostly. Sticking mostly to the legal limit!😜
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