I have a 180E 1991 model that is hard to start and when it does start it has a bit of a miss sometimes when starting it backfires/pops through air cleaner, others days just won't start

Sounds like a lean misfire condition. Have you run a fuel pressure test? Should be 55-60 lbs. If pressure is ok during idle and reving, Ttry disconnecting MAF and see if any difference on accel.

check for loose, cracked or disconnected vacuumn lines or connectors that are missing a vacunmn line.

nope, distributor plugs and wires, change those and the pcv valve. Run you total no more than $100 for parts if you shop around a bit, since parts prices vary greatly. the spark is not entering the cylinders correctly to burn the fuel properly, replace those items and you will not have this issue unless you have some rare deadly engine disease- :-) Hope it helps and have a nice day!

Try a crank sensor.

Give it a good tune up. Change the cap and rotor, plugs. Make sure that the plug wires are in good shape and not broken from heat.

Things that cause this are usually carburetor problems....sometimes the fuel/air mix is off and the timing too.

What have you done so far to the engine that coould be related to either of these?

Try this, remove (MAF) Mas Air Flow sensor,(be sure key is OFF) so you don't set a trouble code when disconecting wire conector, with some throttle body/air intake cleaner from the outlet side of MAF, spray the sensor clean, let dry, reassemble.Remove snorkle from throttle body, and with a old tooth brush, clean throttle body& plate with cleaner, may take a couple of cans if throttle is badly 'coked' up. Hope this helps.

probly jumped timeing

hi if the engine light comes on you need to get the diagnostic tester to find out what the fault code is otherwise finding the fault can be a bit hit or miss and end up costing you more money then it might have you had the engined tuned [seviced] as this is the first thing to get done its the basics then look deeper if need be yates210456

You just go to the link I attached I'm sure you will get the solution of your problem 1991 LS400