My 1997 bighorn's idling is eratic. observed that the rpm goes up and down [surge motion] what steps do i have to make to eliminate the problerm

there is an idle up solenoid on the the carburetor. adjust the idle speed at 800-900rpm. RPM up and down is due to a/c compressor engaging. if the rpm drops too low, a/c compressor disengages. observe your idle speed with engine warmed up, turn on airconditioner and adjust the idle compensating vacuum diaphragm adjustng screw underneath the carburetor to the same rpm when a/c was off. that should correct your rpm problem

well gee, there are 50 reasons.
did you scan the PCM yet, if not ,why not?
when the idle surges, that means.
NO RPM ranges stated, if super low rpm check EGR.

1: engine lean and ISC dead, or cant handle the huge engine power drop..The PCM can even make this worse!, buy hunting the ISC to find spec, idle and failing... to...
2: or any of 50 other things causing huge engine power drop and PCM can't maintain idle controls.
super lean burn.(bogging)
super rich burn, misfire for 50 reasons.

i cant hear your engine,nor feel it, or see it shake from gross misfiring.
no can i drive it and see/feel power loss,
can I?
asking net bunny's this (blind as bats)
is "OVER THE TOP" no?
so here , ill toss the dice for you
and guess gross misfiring.
ever get a tuneup this decade, if not bingo , no?

and the others...

http://jeepdied.com/Misfire101.html

link even has the 50 cause list, to show the futility of guessing!
we do 1 to 10 tests. and find it.
for sure.

as you can see EFI systems are complex.
gue$$ is not only expensive but can net you 2 problems easy.

From my experience with my wife's 97 Wrangler look at the intake manifold for 2 vacuum lines. They will be just under the throttlebody One of them is broken or has come loose. We had this problem several times, idle high, running eratic, until I put a smaller rubber hose on the fitting. Has run good since.

try to set the idle screw on the carburettor

I don't really know where they should be in idle on your car, but if you don't apply your gas pedal, the RPM's should be low. My 18 wheelers RPM's are down to 500 in idle position and go up to 2300 when I step on my "diesel" pedal. If in neutral or park and you step on the gas...do your RPM's go up? If yes, all should be okay.

It may be one of two things. A intermittent problem with the idle air control sensor or the throttle position sensor. Have the cars computer scanned at a local auto parts store to see if there is a pending code that has yet to turn on the check engine light.

This really sounds like a vaccum leak at the intake. Just spray a bit of starting fluid at the intake manifold, with the engine at idle. You will hear a change in the engine. Replace necessary gaskets.

IAC motor

Sounds like a possible vaccume hose leak?

Sounds like the idle air control valve...buy a repair manuel and it will walk you through the steps to change it...it's not hard.