SOHC.co.uk Forums > CB500/550

CB550 Tuning Advice

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Oddjob:
I think it was a good suggestion either way. We seem to forget the tap uses not only the doughnut seal but orings as well. All could be failing after this amount of time. I’ve already upgraded my 500 petrol tap to all Viton fittings.

Whose to say the next failure could well be the fuel tank cap seal.

deltarider:

--- Quote from: Oddjob on October 20, 2026, 08:50:26 PM ---[...]
Whose to say the next failure could well be the fuel tank cap seal.

--- End quote ---
Mine has been failing for the last 46 years. :o I don't mind and do not worry about blocked ventilation holes. During hibernation I put some plastic foil in between cap and tank to prevent evaporation.

Neilw:
Back to tuning, and here's where I need help!

Carbs are back in, connected and good to go. Have the air box carb side fitted, but not connected to the air filter, etc., so air is just passing through unhindered. I have the air screw at 1.5 turns, and the carbs are synced using Bryan's method with the smallest drill bit i had. Less than 1/16th.

Starts on choke and revs up to about 4000rpm.

I start decreasing the choke and revs come down; when I put the choke off, the bike dies on its own.

I repeat the sequence and hold the throttle open as I turn the choke off, and the bike continues to rev at where I'm holding it (but not below about 2000 rpm). The smoke is white for a while, then becomes black when I'm holding it with the choke off.

So in terms of moving forward i can presently keep the bike running by holding the throttle open. I tried adjusting the throttle stop and it wont keep the bike running

Tappets were set, and I have dynamically set the points using a strobe light, so i know that side is as good as it gets.

What are the next steps?

deltarider:
Just to make sure: when the little chokelever is horizontal the chokeflaps are down (to facilitate cold start), when the lever is vertical the chokeflaps are up (engine warmed up).

Neilw:

--- Quote from: deltarider on October 21, 2026, 03:59:50 PM ---Just to make sure: when the little chokelever is horizontal the chokeflaps are down (to facilitate cold start), when the lever is vertical the chokeflaps are up (engine warmed up).

--- End quote ---

Good point - i'm calling the choke off when the lever is horizontal ( in this position the butterflies in the carbs are closed) and when it's pressed down i'm calling that on/engaged ( and the butterflies are open/vertical). Kehlin 069a Carbs.

Hmmm sounds like i may have my words back to front - so the bike starts when the choke lever is down i.e off, and dies when it's horizontal i.e on?

Guess that makes a huge difference!

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