Welcome to our Big Bronco site where we try to DIY!
So a previous owner or shop converted the 91 to carb!
Here is a summary of smoke symptoms and causes:
Less $ first (less worries as well!)
PCV System: A stuck closed PCV valve will cause excessive crankcase pressure resulting in blue/gray smoke.
Oil burning in the combustion chamber. Pull a few spark plugs & check iaw Spark Plug Diagnostic Chart
Oil Deposits Symptoms: Oily coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the combustion chamber. Causes hard starting and misfiring.
Valve Seals: Leaking valve seals will cause blue/gray smoke at startup because oil leaks past the seals into the cylinder after the engine shut down.
Valve Guides: Excessive clearance between the valve stem and the valve guide allows oil to leak past the gap into the cylinder.
Piston Rings: Worn or damaged piston rings will cause blow-by resulting in blue/gray smoke.
Worn Cylinder Walls: Worn cylinder walls cause blow-by resulting in blue/gray smoke.
Here is the 1990 F Series/Bronco Pre-Delivery Shop & Electrical & Vacuum Troubleshooting Manual (EVTM), Partial by member Kingfish999 in Google Drive (similar to your 91)
This has much info applicable to your 91.
Slow scrolling for my slow Comcast Highest Cost Blast Internet..
Find page # of item desired in contents, then flip screen flip down like spinning a bearing then stopping periodically to see what page you are on.
Forum faqs @ https://www.fullsizebronco.com/forum...lies-faqs.html
... includes for example;
How to use search (Advanced) @ https://www.fullsizebronco.com/forum...-advanced.html
Baba Looey's Favorite FSB Links (lots and lots of tech links) @ https://www.fullsizebronco.com/forum...ech-links.html
How to Post Pics @ https://www.fullsizebronco.com/forum...-pictures.html
...The crew here likes pics!!!
Free registration for some wiring diagrams (86 through 96) and Technical Service Bulletins, (80 through 96), same as by Ford @ BBB Industries- Premium Alternators, Starters, Power Steering Products and Turbochargers | TSB's & Wiring Diagrams
➡️ Please find time to participate and VOTE
in our Full-Size of the Month (F.O.T.M.) & Full-Size of the Year (F.O.T.Y.) Contests @ https://www.fullsizebronco.com/forum/27-f-o-t-m-voting/
Contests will be held in JANUARY, MARCH, MAY, JULY
& SEPTEMBER. Each month in-between will be used for nominating contestants to run in the following Month.
You will get ideas by those competing
. Also see the prizes⚠️ They are awesome as compared to other sites' "contests"!
Then get ready for the Full-Size of the Year (F.O.T.Y.).
GL with the inspection!