Have a mild build 350 in my boat and sometimes (most times) it has a hard time idling down. Sometimes I have to shut it off and restart a few times to get it down to a reasonably safe shift rpm. Engine has LT4 hot cam, 650 demon with electric choke, v6-14 ignition module. Base timing is set around 22 degrees. Could the choke linkage be hanging or maybe to much timing? I've killed 2 drives in the last year and I'd like to get the idle down to about 650. Also, the idle screw adjustment on the throttle linkage is backed all the way off

possible throttle cable problem so unhook the cable and work the throttle by hand blipping it and see if idle returns how it should ?

Also check for resistance in the throttle plate shaft possible bent or stuck or corrosion issues

also adjustment on carb out of spec for your app

also a vacuum leak somewhere at base of carb or intake

Also check timing

make sure your distributor can not rotate - check hold down clamp

87 Magnum - is this right after running WOT ?

Most BG vacum secondary carbs, especially with lighter vac secondary springs, will do this.

The trick; shift to neutral, shut off engine and restart....idle should go back to normal.

You will kill an Alfalfa almost instantly with too high rpm shifting into gear.

BTW: choke has nothing to do with it as I remove the chokes from most all carbs and these will still do it.

22* base timing?

I was thinking at start up - and after engine was warmed up and choke fully opens - does your choke fully open - and he is waiting to put it fwd gear or rev off the trailer ?

so when does the high idle always happen

Backing off the idle speed screw won't lower the idle rpm without adjusting the cable too. Take a look, you'll see why.

Hey guys I got this one...Lol. Sounds funny, but Yes, I'm serious.

The secondary spring doesn't have enough tension to close off the secondaries with air running thru it. Shutting off the engine, stops the airflow. The low tension spring can now close the secondary throttle shaft.

I run into same issues with some throttle bodies with weak throttle shaft springs. And this make carburetor.

^^^^^ exactly and the reason I said to unhook the throttle cable - that will tell you right away if your cable is out of adjustment or bad / needing replacement - the boat is an 1987 is this original throttle cable ? 19897 very possible throttle cable problem

post 7 - I am referring to.

Also when is your high idle ?

29 year old throttle cable or NO - new one was installed ????

And throttle cable correctly adjusted for aftermarket carb whether the cable is old or new - Yes or No ?