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setting up an F head in a M38A1 Cdn2 .. carb recently rebuilt ... but the vent line on top of the carb connected to the crossover tube to crossover tube was stalling the carb - unhooked the one end that went onto the crossover and idles fine now- [checked the crossover tube vent line hookup and is clear of any obstructions] ... put the line back onto the crossover ,it sputters and stalls... .. could timing on the engine be off requiring this vent to be 100% open at the top of the YS carb ?? Idles and works mint now but why cant I put the line back onto the crossover again ?

Is the elbow tube that is welded into the crossover, inside the crossover clear?

There are two 90 degree tubes inside the crossover tube. One is for the air in to the distributor and the other is for the carb bow vent and the open ends face the incoming air. If they are plugged or installed facing the wrong way they can cause your symptoms. IE if the tube is facing downstrean a negative suction will be applied to the bowl causing a low flow through the main metering and the idle metering systems.

They are clear - one to the dist and the other to the carb vent ... the elbows are pointing towards to air cleaner away from the carb

The only way that tube can have any effect on your idle is if it is blocked. Otherwise it makes no difference weather you vent at the carb or at the crossover pipe. Have you adjusted the idle mixture screw yet? It may be set to lean and is sensitive to bowl pressure.