which engine? lots of engines on 86, lots swapped too.
to work on any car engine wise this is first, ID the engine.
then the Fuel system. carb or EFI.?
lean fuel mix can be caused by.
well gee what engine, and is it EFI, nah , not in 86 its a carb.
if a carb you really need to say so as many a mech never seen one.
today. gone now for 27 years. 87 to present.
i cut teeth on carbs. in 60s
a love/ hate thing,,, today i have fancy tools, that mean them easy.
my comments are USA only , as export jeeps are vastly diffr.
I had a 88 (2 of them same time) then a 99, then a 2002 then 2008
1/2 million miles on them or more.
vacuum lines are easy, replace em or inspect test them.
if they look cracked they are roll them between fingers.
or use a hand vacuum tool attached and do a leak down test on all .
or pinch them running to see if RPM changes radically.
ill answer for CARBS (in order of likely)
- dirty carb. (inside)
- jets clogged or partially.
- flow level set too low.
- float valve sticking closed. gummy. see #1
- fuel pressure low. to CARB. the FSM has spec, here .
- lean under heavy load is power jet stuck closed , see #1
- lean only for an instant, fast right foot, means the accelerator pump is dead, fix it . clean carb and kit it. and it need to be adjusted per the manual. FSM.
- sure any air leaks below the carb cause lean... lots of ways.