Top reasons a carb runs best with air screw closed?

Jlovvorn · Post by **Jlovvorn** » Sun Dec 01, 2013 8:54 pm

Here is a stumper,

My left carb on my cb77 runs best with the air screw all the way closed, and still has a white plug.

At more than 1/2 turn out, it stops firing at idle.

Right carb is at 1.5 turns and a brown plug.

Have checked the insulator for cracks, sprayed WD40, checked my carb caps, etc.

I think there must be an air leak here somewhere, but I can't chase it down.

Timing is spot on with bulb and gun

What am I missing?

G-Man · Post by **G-Man** » Mon Dec 02, 2013 3:48 am

The air screw really only affects mixture at low throttle openings. Try comparing parts such as slide, needle and jets from carb to carb. You might have an odd slide or needle.

Try swapping things from carb to carb in a methodical way to see when the problem changes from side to side.

Check the carb flange with a straight edge to see if it is flat.

http://amalcarb.co.uk/downloadfiles/ama ... _guide.pdf

The above is for Amal carbs but they all work pretty much the same..... :-)

G

Jlovvorn · Post by **Jlovvorn** » Mon Dec 02, 2013 7:45 am

Thanks! I am running amals, so this is good.
I have not tried swapping parts. I have been chasing this problem since I got the bike and have never found the issue.

Snakeoil · Post by **Snakeoil** » Sun Apr 06, 2014 9:23 pm

Assume you have Concentrics. Directly across from the idle screw is the idle circuit. It is probably plugged. Take the screw out and insert a needle across the air passage and clear the circuit. Use a #78 drill bit, which is .016". It won't be long enough to reach across the passage alone. A spray tube from a can of WD-40 makes a good handle.

Later Amals have the pilot jet pressed in across from the screw and a plug pressed in behind it. Some drill out the plug and install a grub screw so the jet can be cleaned. The hole is already threaded.

Regards,
Rob