Hitchcocks Forum

I have fitted a Amal concentric carb and I have been having a lot of problems getting the bike to run properly.
When starting from cold the engine sometimes spits back through the carb which would indicate an air leak. When I manage to get the engine hot it runs fine.
I took care when fitting the carb but may have some distortion of the flange/adaptor.
Is there anyone else who has had similar problems and are there any cures?

What bike? and carb size would help, is you have gone to a bigger than standard carb? this could be your problem.

The fact that it runs fine when it's hot is telling. Are you using the choke when cold? If you are perhaps it doesn't need it?

If it is spitting back the mixture is too lean. Are you using the choke correctly if at all?

Why does a weak mixture spit back through the carb?

To explain it fully would take a lot of typing and this board doesn't allow much time. The simple explanation is that a weak mixture burns hotter than desirable and even after the exhaust stroke the air left in the cylinder can be hot enough to ignite the new mixture coming in.

When using an Amal concentric, I have always used the choke just for experimentally adjusting the mixture.For cold starting, with the carb set up right, tickling on its own seems to do the trick.

Don't know if this will be of any help, but this is from a few days ago. Regards, Paul. ..... https://www.youtube.com/watch?v=IBOQEbonTK0