Topic: CB500 too lean on fuel

[Teddyhoeg1982]

Been really struggling with this bike.

It is a CB500 K3 from 1977 Europe, where the carbs are replaced with Keihin 649A.

When i apply full throttle it bogs and dies until i release the throttle. Otherwise it starts and idles fine. Also with 25% throttle it runs fine.

I replaced the float needle and float housing with new spares from Keyster. Also replaced the gasket for the fuel bowl and slides. Jets are #100 from Keyster, also tried the #78 included in the kit. Carbs were Ultra sonic cleaned. I checked spark plugs and timing.

Note that the cover on the Air filter housing I think is not original for these carbs. I tried restricting the Air intake area which helps a bit, bit then the engine feels weak.

Any advice how to move on? Please see the video for how it runs.

https://youtube.com/shorts/OM0rp2LuXUU?feature=share

[Stev-o]

First off, looks like a nice bike.  How do you know it is lean, what do the plugs look like?

And I would not run aftermarket carb parts, have been know to cause issues!  Get your hands on genuine Keihin parts.

Here is more info that may help...

http://forums.sohc4.net/index.php?topic=138929.0

[Little_Phil]

The 500K3 with 649A carbs did have 78s jets I think. May also have low rise cam. Deltarider knows this model, maybe he will be along here soon.

[Deltarider]

Not bad. Picks up nicely and rpms drop quickly like they should. Caution: an all too brisk opening of the throttle, will make the engine choke.
As fas as the issue you have, the air intake may play a role here, aftermarket exhaust not so much. Yours probably still has the K3 air intake. You can keep it, but then may have to change the main jets to say #90. If the air filter case and filter happen to be the old style, the models CB500, CB500K1 and CB500K2 had, you have a choice: either run #90 (or #100) main jets, or have the #78 main jets continental Europe had, in combination with the AIR DUCT (part number: 17340-323-610) on top of the airfilter box. See pics below.