SOHC.co.uk Forums > CB500/550
First Start Highs and Low!
deltarider:
Black smoke usually indicates a (partially) blocked air intake. Could be a forgotten cloth, an airfilter element gone wet or choke flaps stuck closed.
I'd first temporarily remove the airfilter element, see if you can peek at the carbs mouths to see if the choke flaps are up. Are you sure the tiny O-rings around the main jets are Ok and seal well?
McCabe-Thiele (Ted):
I don't know if it's possible to check the needle clip position without removing the sliders.
What did you clean with wire wool, was it just cosmetic cleaning of none metering parts.
Personally I think 1/8" is too big a gap as it does not leave room for a lower adjustment of the idle screw.
On my 400 iirc I used 3/32, on my 500 I used the point when the slider just started to move from the fully closed position effectively zero. Snag I hit was the adjuster screw was was so undone the spring was not compressed, I built a raised stop to fix this.
Neilw:
--- Quote from: deltarider on September 28, 2026, 07:53:07 PM ---Black smoke usually indicates a (partially) blocked air intake. Could be a forgotten cloth, an airfilter element gone wet or choke flaps stuck closed.
I'd first temporarily remove the airfilter element, see if you can peek at the carbs mouths to see if the choke flaps are up. Are you sure the tiny O-rings around the main jets are Ok and seal well?
--- End quote ---
I installed new O-rings, and the flaps were operating correctly before i re-installed the carbs, but I will check. No other blockages.
Bryanj:
1/8 is too big you need a bew 1/16 or smaller, unscrew the tickover screw all the way and make sure ALL the slides bottom out, adjust the tickover screw untill the snank of the new drill(if its new its smooth!) Just slides under the engine side of the first slde that moves then adjust other 3 to same.
Unscrew the tickove screw again to check that all the slides still bottom out, adjust tickover again till slides only just open and try to start, hopefully this gets you good enough to run
Neilw:
--- Quote from: Bryanj on September 28, 2026, 08:19:10 PM ---1/8 is too big you need a bew 1/16 or smaller, unscrew the tickover screw all the way and make sure ALL the slides bottom out, adjust the tickover screw untill the snank of the new drill(if its new its smooth!) Just slides under the engine side of the first slde that moves then adjust other 3 to same.
Unscrew the tickove screw again to check that all the slides still bottom out, adjust tickover again till slides only just open and try to start, hopefully this gets you good enough to run
--- End quote ---
I've followed the above, and checked and rechecked. I didn't manage to get the slider needles out as the screws were in very tight, and i didn't want to force anything and strip them. From what I could see, there was only a small amount of the needle sticking above the circlip, so it would suggest position 2.
I put the carbs back on and double-checked all seals and, frankly, everything. I set the air screws at 1.5 turns to begin with.
Bike started on choke, but still raced to 5000 rpm. With choke off, I had to hold the throttle open to keep it running, or it died. Lots of black smoke.
I adjusted the air screws to 2 turns and then 2.25 turns and retried. Each time, something similar. I then left it for 10 mins went back and tried again, and it seemed to run ok, with no smoke, i even got it to idle for a while off choke using the throttle screw. Then the bike died; I realised I had left the inline fuel switch off!. Turned it back on tried again with similar results as before.
I've taken a break to write this, have some lunch and go back and try again.
It feels like there is too much fuel getting through or not enough air or both lol.
Just got back from lunch, tried a restart, ran out of fuel lol. Put more in the external tank i'm using and now i have an over flow leak on bowl 2, that no amount of tapping seems to stem the flow!.
I sorted the leak, i removed the bowl, blew in some carb cleaner and compressed air, jiggled the float about and put it all back.
Bike starts on choke, races to 5-6000rpm with some black smoke, take choke off, have to hold throttle open to keep it running, and then it stalls on own accord after about a 30 sec - a minute.
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