Will start only with starting fluid and then runs fine BS 18HP twin 1979

[mitchstein443]

The newer carbs have a solenoid valve hat jambs rod up the main jets rectum to stop the carb passing fuel through the venturi while it is spinning with the magneto turned off.
The unburned fuel sits in the red hot muffler till it dilutes to a combustible ratio at air pressure then ignites - bang.
The new carb is probably jetted leaner but just does not have the solenoid.
So to avoid the after fire you need to run the engine for long enough at low speeds to cool the muffler down far enough ( and risk overheating the engine ) or live with the bang.
The other way to avoid it is to fit a tap in the fuel line and starve the engine off

The new carbs have the solenoid.. I was thinking I could put a switch to turn the solenoid off before shutting down the ignition... no fuel in should result in no bang out... my hythesis anyway (I'd say theory but I don't have enough experience for that yet lol)

 

[Rivets]

Here is a manual that should help you out. As to my opinion on afterburner in the muffler, I say idle down and live with it. It’s not going to hurt anything, except some ears.

https://drive.google.com/file/d/0B6NaqjIxWV1ybHphbUNTQUNNR2s/view

 

[bertsmobile1]

The new carbs have the solenoid.. I was thinking I could put a switch to turn the solenoid off before shutting down the ignition... no fuel in should result in no bang out... my hythesis anyway (I'd say theory but I don't have enough experience for that yet lol)

Sounds reasonable, they are just a strait drain of 12V off the battery.
only down side of an external switch is leaving it on because if the solenoid drains the battery completely, most times the battery will not be able to be recharged.