Tolitson hr idle screw

I have a 1977 elan 250 twin deluxe with a Tolitson HR carb. Engine will not rev and runs ok at idle. It runs better when the idle mixture needle in fully seated and will marginally rev better. The carb has been rebuilt on many occasions and only performs when idle screw is fully seated. This model of carb only has one adjusting needle for low speed only....

Any suggestions for that aggravating issue?

Thanks...

Tolitson hr idle screw