Only runs on starting fluid.

old old old car.38 years later.

when did it last run good, last week , last yea,r last decade/?
fast idle is vacuum leak, or bad IAC. 1500rpm cold is normal not 3000
overheating has many causes. (huge) need a list or just wan to buy tools to test it and find it.> ???
gue$$ing is hard, work and a real slog. hear goes 1 slog1.
due to age here is the list (a test each)

running 38 year old antifreeze? Lol ?
38 year old thermostat? 7 years is design life EOL ,oops.

engine compression test passes? 180-PSI? wot done?
rad and engine block flushed out of red rust mud.
38 year old water pump is no good nor is the belt. now.

RaD LEAK DOWN TEST, A TOOL; PASS OR FAIL.?
RAD , gas test (exh) a tool that see exhaust fumes in the RAD coolant, green/blue tester all shops have all this here. I typed.

water leaks anywhere under car or passenger toes. , not the side tank blow out you told and is clear
see milk-chocolate for oil in pan, oops leaks. coolant.
see milk-chocolate in tranny pan (did sticked , oops)
Rad CAP must be replaced this old. the rubber gasket in it is dead.
next is see oil or carbon dust in the RAD neck cap off dead cold engine. and full to the top of said neck. (free test eyeballs)

ok all is good above and does it overheat only parked or driven or both or only AC on. or all the above, I bet yes all.
if overheats parked, the fans must all be good are they.?

If still overheats we use IR gun tool and run the beam ever every inch of RAD TUBES. and must show 50f degree variance or more
per tube end to end if not that TUBE is clogged up, RAD BAD./
that is all for now I can do more. if need be..

on simple cause is not checking RAD water level dead cold engine
and making sure that side spill tanks has good tubes inside and work (as engine cools it sucks back coolant.) if tubes bad fix them
or check coolant like 1929 days,RAD cap off. look cold.
Coolant expands naturally hot. that side tank captures it for next cool down cycle and bad rad caps to wreck this cycle.
full stop fingers tired.

pretend caddy here,,, funny then and Diane Keaton hot .(x100)

the bug only needs a fan belt to stay cool.... (owned 2, 1970 to now)

https://youtu.be/Xo2kSu6O8cU?t=2

keep cool.
the side tank blow outs are also warped heads and or bad headgasets
if you see STEAM at tail pipe end hot engine the head gaskets are bad. 1 or both. or far worse crack block.or cracked heads.
id have looked at the tail first. (steam in front yes, look in rear)

this is HP computer forum , not BOATS

but can answer ,
bad fuel!! all boat fuel goes bad, parked, so long, ALL DO.
too much oil in fuel (is it 2 stroke engine? no year given,so.,....)
carb, filthy, im cleaning my 40 hp now, in the garage, and wheel grease.. (fishing next,cat fish... hummm)
can be bad spark plugs. ever tune it up. with new ones?
water in fuel. rust in tank if tank is steel.
wrong fuel someone put in diesel
low compression (3 cylinders like mine?)

I had a similar issue. Tiller starts and runs for 20 secs. or so then dies. Found the problem! In turning the tiller on its side to check model numbers etc. the fuel line had shifted inside the tank and was stuck just about at the TOP of the fuel level. When I gently pushed the fuel filter and line back to the BOTTOM of the fuel tank, the tiller now runs beautifully! So, check that the fuel line placement inside the fuel tank is correct.

Running only on the prime is caused by the main jet in the carburetor being blocked. This can happen if you put dirty or old fuel in it or let it sit for a long time with fuel in it. Today's ethanolated gas goes bad quickly and absorbs tons of moisture which causes water to settle to bottom of the fuel tank, which in turn prevents the main jet from lifting fuel, it picks up water instead, thus causing it to die. So, check the tank for water, drain. Check the gas can, and get rid of the old stuff. If debris in either of them, you may have to take carb apart to clean the main jet.

Hi there is a small stone fuel filter in the tank. this filter stops fuel from running back into the tank from the carb check to insure it still on and make sure there are no cracks or hole in fuel line that should fix your problem

-How about spark plug...? Did you change as well...?
-Make sure turn on the gas line.
-Try again.

Good luck. It's closed for garden time.

Mai

I have the same FG 100 tiller with the same problem. I did replace the carb with a brand new one and still can not get full RPM. It does try to go to full RPM, but drops back. I even checked the transmission and clutches very thing works very well.

take the gas cap off and look into the tank and see that the fuel filter is on the bottom of the tank and still secure to the fuel line.
Your tank will be slightly pressurized be sure your gas cap gasket is there and pliable.
same for your gas lines be sure they are relatively soft and dont crack easy if you move them?

try these couple items and see if any will help the cause

Most likely the carburetor jet is still clogged after sitting for years with fuel in it. Putting injector/carb cleaner in the fuel tank may help, but if not, the jet needs to be soaked some more -- or replaced.

Follow mixing proportions on the label for injector cleaner.

I had the same problem a few years back. Check the rubber elbow coming from the gas tank to the carb, it's probably cracked and sucking air instead of fuel from the tank.