Join Date: Jul 2016
Location: North Louisiana
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Is there a choke on that center carb ?
Sorry !'m not familiar with Tri-power carbs.
I do know that some Q-jets have a high idle screw, connected to the choke. It's supposed to hold butterflies open enuff to idle, when cold, then release the butterflies, when engine warms & choke opens completely. The screw adjusts this mechanism. So, there can be some sort of malfunction of the choke mechanism.
Again, this may not apply to your carb.
Since adjusting the throttle screw increases your idle speed, that pretty much rules out a fuel supply to the carb problem, IMO, since if the problem was fuel supply, just turning the screw would not increase fuel supply to the carb.
I just read a post that said when you are chasing an idle problem, it's best to remove the end carbs & seal off the holes with block plates. Then solve your idle problem BEFORE putting the end carbs back on. Sounds reasonable to me.