Running problem

I am having running problems with my fuel injected 1976 westie!
When first started and the fuel mixture is rich she runs perfectly. After 20 - 30 mins she judders, shudders, kangaroos at low revs. in second, third and fourth gears. Is this carburettor related and could be down to jets, or regulator or f.i. combi relay or is it likely to be electrical

 

why do you have the f.i relay if you've gone over to carbs?

 

No, I haven't gone over to carbs, I still have the fuel injection.

 

you said is it carb related in post one, no jets on f.i just injectors. check for air leaks and throw some injector cleaner in.

 

will do, thank you