Appreciate suggestions re my 1981 RX7.
Always has idled very smoothly at 750 rpm.
Recently will, after several seconds, drop suddenly to 500 rpm and concerns me that it could stall.
Perhaps adjustment of carb needed? Float?.
Thanks for suggestions. Car has only 51,000 original miles. I am original owner and bought new in 1981.
-Thanks!

 

My guess would be vacuum leak. Since your average mileage is about 1100 miles per year, theres probably a bunch of rubber hoses that need to be freshened. Rubber after 44 years is going to be pretty crispy. This includes vacuumm lines, fuel lines, brake lines, coolant hoses, anything rubber actually. Have you had any of these refreshed in recent years?

 

This sounds familiar. When stopping at a light, toes on break and heel on gas.

Checking all the vacuum lines, rats nest solinoids, and diaphragms, I found the trailing advance was leaking but not the main cause. I replaced it anyway.
Turned out it was the #2 choke diaphragm located on the front-right top of the air horn. I capped off the line to confirm that was the cause and then purchased a used one from the Forum Marketplace.

 

Hey, Barry - welcome aboard! First off, you're in the right place with many of us owning these cars since new - stick around.

As to the idle condition, I would advise against doing ANYTHING with the car just yet. If it has run and idled great at 750rpm for the past 44 years - it's not the carb! As the others have suggested, it's more likely to be a small vacuum leak that has developed from a broken connection to a solenoid or a hose thats become dislodged or cracked. Have you been working anywhere in the engine bay recently? Usually what happens is that a crumbly rubber vacuum hose will get knocked when a wrench slips or when reaching down to unclog the Distributor Cap when doing a tune-up and goes unnoticed. At that point, the mechanic attributes the wonky change to what he did and not an incidental change.

Vacuum leaks are common sources of idle inconsistency and are somewhat easily found by spraying carb cleaner around the possible hoses until the idle speed is affected. This gets you in the right general area to do a,closer inspection of all lines and connections. Good luck, post back,