SOHC.co.uk Forums > CB500/550
First Start Highs and Low!
Neilw:
Well, I've finally arrived at the point where I can attempt a first start, having fixed the start button issue and the timing plate. The neutral switch is a little project to resolve, but I've managed a first start, which I'm elated about and the bike just about runs.
But i could do with some wise heads/advice, please.
The carbs, I cleaned thoroughly and reused all the original brass wear. The only thing I didn't check was the position of the circlip on the needles. I did a bench sync and set the float height at 22mm (it's a 069A set of carbs). The air/mixture screw is set at 1.5 turns.
I've been through lots of starts; most of the time, when I start with choke, it races to 5-6000 rpm immediately. I can temper the choke back and hold it just on so the bike idles at 2000rpm, but when i release the choke fully, it cuts out. I've adjusted the throttle stop to keep the throttle open, and that works to a point, but it eventually dies. I'm getting black smoke from the exhaust and the plugs are black and sooty.
I've tried adjusting the mixture both in and out half a turn; not much difference.
I can't see anything holding the throttle open, everything moves as it should. I've taken the tops off each carb to see if the sliders are stuck, and they are not.
I'm at a loss as to where to go now. At one point the bike was idling and i didn't have black smoke, so something si changing to cause the issue. Also, the racing to 6000rpm disappeared for a while, which makes me think something is getting stuck open/close, idk.
I've had the plugs out a few times to clean them while going through this process. The neighbours love me when i start it lol.
Any help, ideas or wisdom is greatly appreciated.
Cheers
Neil
Bryanj:
You may have your bench sync wrong, its easily done
Neilw:
--- Quote from: Bryanj on September 28, 2026, 04:38:19 PM ---You may have your bench sync wrong, its easily done
--- End quote ---
Can i check them while the carbs are still on the bike - i assume you mean the slider height and float height.?
McCabe-Thiele (Ted):
You state you have set the float heights, that's fine.
A bench sync is where you set the sliders with the carbs off the bike, it's pretty much impossible to do it accurately on the bike.
You need to set the gaps so all four are exactly the same.
I have only bench synced my 400 & 500 with good results.
You can later fine tune the bench sync with a vacuum guage if you have one.
Done correctly ensures the engine (when warm) only idles on the idle circuit not by drawing fuel from the main jet, the air screw on each carb should respond to the air screw adjustment.
Did you clean all the airways when cleaning the carbs?
If you are using the original air filter it makes sense to check the needle height position, it may have been altered by a PO especially if its had pods fitted in the past. You also need to check the needles are the correct ones.
Neilw:
--- Quote from: McCabe-Thiele (Ted) on September 28, 2026, 06:14:33 PM ---You state you have set the float heights, that's fine.
A bench sync is where you set the sliders with the carbs off the bike, it's pretty much impossible to do it accurately on the bike.
You need to set the gaps so all four are exactly the same.
I have only bench synced my 400 & 500 with good results.
You can later fine tune the bench sync with a vacuum guage if you have one.
Done correctly ensures the engine (when warm) only idles on the idle circuit not by drawing fuel from the main jet, the air screw on each carb should respond to the air screw adjustment.
Did you clean all the airways when cleaning the carbs?
If you are using the original air filter it makes sense to check the needle height position, it may have been altered by a PO especially if its had pods fitted in the past. You also need to check the needles are the correct ones.
--- End quote ---
I did the bench sync using a 1/8th drill bit as shown on a couple of youtube videos. I followed the rebuilds to the letter, cleaning all the paths with wire/brushes, sonic cleaner, carb cleaner, and compressed air; I did everything. Some of the brass bits were corroded, but cleaned up with a bit of fine wire wool.
I have a vacuum gauge, which i'll use when i can get the bike to idle. The airbox is the original, with a new filter fitted. I've replaced all the rubber, so there are no air leaks.
I guess i can check the needles while the carbs are on the bike, that way i'll know if the circlip is in the correct position and the sliders are functioning correctly.
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