Dieseling after 5 - 10 minutes of driving (‘75 Eldorado)

[Benji808]

Carb is rebuilt with Cliff’s stuff, tuned, starts and runs great. On the first start of the day (two pumps and she starts right up), if I only drive it for 5-10 minutes, it will sometimes diesel on shutdown. If I start it and drive longer 15+ minutes), then no issue at  shutdown. I’ve thought that maybe the choke is running rich, but it runs so smoothly I’m hesitant to touch anything without more information.

Any ideas on what it could be or things to check?

[Cliff Ruggles]

Start with basics, check initial timing.  It may benefit from a few more degrees at idle to lower the throttle angle slightly.

Custom tune the idle mixture screws  with the engine fully warmed up and well heat soaked.  Set for the most vacuum at the leanest settings.  This also helps reduce throttle angle which helps avoid running-on or "dieseling" after shut-down.

Also make sure the timing is rock solid at idle speed and not moving around or at a different setting each time the engine returns to idle speed......

[Jeff K]

Until you fix it, shut it off with the transmission in drive to put load against the motor and keep the rpm down.
I wonder if the vacuum motor on the exhaust heat is sticking.. If you still have one its best to remove it all together.. I'm thinking this has a big 500 engine.

Those converters in those cars were trouble also. Make sure it has a newer converter or maybe you can remove it. But check with your state laws. The converters they have today are pretty efficient..

[quadrajam]

Sounds like your idle speed is too high at shut down. Is the choke staying on too
long? Is the fast idle cam sticky?