SOHC.co.uk Forums > CB500/550
idle jet differences , 069's
fogrider:
That's helpful, mine are the ones in the lower photo. Don't know what model ref . It would be great to check the size of that air drilling in the carb face before the carbs go back on again. probably correct, but worth checking if they are part of that massive weakness bottom end.
The way it went more and more rich when well up the rev range is strange, especially with that thin foam filter in. That would normally weaken it off . I have read on here that it could be fuel being pulled past inadequate O rings on the main jets. They are new on the aftermarket jets, I will get some better ones tomorrow even though they are are a reasonably firm push fit.
The jets do check out as 98 though.
fogrider:
After being side-tracked by other things, finally had another run with the original needles back in. That's all the original internals now back in, clip 2, float levels correct. Ignition back to standard. Screws 1.1/2 out.
Started fine , sounded good, had to take the choke off very early (opposite to before) as it was showing very rich on the Lambda reader. Even after 10 miles it was showing way over rich the full range of throttle and yet STILL had a small stumble just after opening up.
Back home, took the plugs out, jet black.
All I've done is put the original needles back in !
The aftermarket needles had made it vastly weak right to the middle range , then too rich top end.
I tried some new main jet O rings, 1.2x4 (on stock, 20p for 4 ), but I'm waiting for some 1.2x4.3 which calculate as the right size (80p for 4 ). Maybe that's the top end richness issue, can but try........
K2-K6:
Interesting there's so much difference and echoes that which most find, that current replacement are highly suspect.
Be interesting with the replacement O-rings to see if it clears out the top end jetting.
Have you got paper filter in yet? as I'd perceive that to make difference to that just off idle characteristics of slightly lean. If it's with a new filter, then it looks like perhaps 1/4 turn in on airscrew would assist.
fogrider:
1.2x4 should do, as the originals AND the new aftermarkets were 1x4. But, the stretch on 4 may be just a little too much and not enough crush. Cheap enough to try some 1.2x4.2.
I've tried it with and without an air filter, both paper and the foam that was in it. No difference. All I can do is try something else and see what happens. The difference on needles is incredible. I'm going to try the aftermarkets, which made it too weak bottom end, but in one clip richer. At least with the Lambda in I can see the results clearly and it stops me making wrong assumptions.
These carbs seem very sensitive to small changes, I suppose that's a product of a small choke size.
I know it's said these carbs need the original brass bits in, but I suspect there is wear in the needle jet, that would explain the issue somewhat, not entirely sure what size it should be, more research req'd. Must find Deltariders chart again.
K2-K6:
"These carbs seem very sensitive to small changes, I suppose that's a product of a small choke size."
Looking at it logically, creating the carburettor bore size for maximum rpm of each cylinder and aiming for an ideal airspeed at that point will give a proscribed venturi size they felt was optimum.
Going to the other end of the range, at idle, then some very small metering is required to get a linear hiccup free response. Quite a balancing act, along with tiny changes shifting the AFR in fairly large percentage terms.
Always learn more about something when it's not working properly though as it makes us think harder.
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