1981 Civic won’t start when hot
 

1981 Civic 2nd Gen EN1, Hondamatic, 110,000km (used only 3-4 times per year, ~200-300 km/year).
Fuel used is 91 RON unleaded (with “Flashlube Valve Saver Fluid” added as a precaution).
Ambient temp. ~25 Celsius (SE Queensland, Australia).
History: (1) Air Con was removed a few decades ago after a radiator leak and I decided it wasn’t needed.
(2) Mechanic installed an additional fuel filter between the fuel pump and the carburetor a few decades ago after a contamination issue.

I love this little car ... BEST vehicle I have ever owned and it has been super reliable and has always started effortlessly at all times in summer (35 deg C) and winter (15 deg C) until a few months ago.
The problem now is, the engine starts easily from cold, and idles and runs faultlessly on the road, and at full throttle up a hill, but when the engine is hot it won’t restart after a short break (10-20 mins).
The starter turns the engine easily, I hear the engine fire 2-3 times, but then it just dies.
After 6-8 attempts, and much fiddling with the accelerator, the engine will run for a few seconds, but only at high revs, then it dies again when the accelerator is released.
After a few more attempts, the engine will eventually start and keep running and then it runs faultlessly again.
Only after a break of about an hour will it start normally.

My local mechanic suggested it might be a stale fuel problem with a loss of volatile components. However, whenever I open the fuel tank, there’s always a release of pressure so the tank is pressurized and I don’t know how I could be losing volatiles.
So I checked my receipts and discovered that I last filled the tank about 18 months ago, so I thought, ok maybe that’s the problem.

I estimated there was about 15L of “stale” fuel in the tank, so I drained 5L (no sign of contamination and it ran a 4-stroke lawnmower without problems).
I added 10L of 98 unleaded to make 50-50% mix of old 91 and fresh 98 (41L tank was now half full) and added 50ml of “Fuel Doctor” Fuel System Cleaner-Conditioner-Stabiliser (shock treatment dose), but the problem persists.

I ran a test in my garage:
1. Started the engine and let it idle until it was hot and both radiator fans were cycling on and off.
2. Switched off and let it rest for 10 mins.
3. Attempted to restart ... Fail.
4. Waited an extra 15 mins (25 mins total) and attempted to restart ... Fail.
5. Waited an extra 35 mins (60 mins total) and attempted to restart ... Success!

I don’t think it can be a vapor lock problem because fuel vapor should condense again after several minutes shouldn’t it? And as I said before, I’ve never had any problems previously.
The car is 38 yrs old so I’m suspecting it might be age related and some carburetor component has failed, but I have no idea where to start looking. Any suggestions would be much appreciated.

Re: 1981 Civic won’t start when hot
 

After I told my mechanic about my test, he suggested perhaps the idle cut-off solenoid might be the problem.
Perhaps gum or varnish was causing it to stick closed when the engine was hot, which would explain why the engine ran with a bit of accelerator, but not at idle.
He suggested I try some carby cleaner spray.

That made sense so I bought a can and gave the carby a good spray and ... BINGO!!! ... problem solved and my beautiful little Civic is now running as sweetly as ever again. :)

Re: 1981 Civic won’t start when hot
 

glad you got it figured out :tup: