TOPIC:

Do you have another set of carbs (any kind) that you could use for a test? I'm thinking if you do you could put another set on, without hooking up the cables, start the engine and blip the throttle at the carb. If the idle quickly returns to normal you could for certain rule out any ignition. Then if you put the BS34's back on without the cables and blip the throttle and the idle hangs high the problem must be with the carbs. On the other hand, if the problem persists with the other carbs you may need to look a bit closer at the advancer to ensure it isn't slow to react due to weak springs, stickiness, etc. Of course if neither carb set hangs with the cables unhooked it may just be a cable or throttle tube issue (although this seems highly unlikely). Ed

Another swag --

Possible over-zealous cleaning of pilot air jet whereby it's been reamed out too large.

Good Fortune!

it may be posible that there is too much ware on the slides ,and u are getting air bypassing the slides??

Just tossing random thoughts --

Perhaps one or more of the throttle slide springs has weakened, and slow to restore the slide to its idle position.

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Got a different set of BS34. Cleaned them up and put them on, guess what same thing so it's not the carbs. I again re-checked the manifolds, no leaks. I am at a total loss here.

Checked all vacuum lines? Also the rubber sync plugs will dry rot letting in air.

There are no vacumn leaks.

Possibly weak springs on the timing advancer. Or pivots needing lube.

Good Fortune!

Just checked the timing advancer again, even replaced with a know n good one. No change.

If not already done, would test run with alternative fuel supply and cap on the vac nipple that usually supplies vac to the petcock.

This is to rule out any possible vac leak attributable to the petcock or vac hose to petcock.

Good Fortune!

Have you tried raising the float level a little? Could be starving the pilot circuit at idle.