SC Idling Low
 

My 1979 SC has begun to idle low once it is warmed up.
Normally it will idle high at 2K for the first 30-90 seconds, depending on the outside temperature. Then it will drop to 800-900.
Starting about a week ago (incidentally, when the weather started getting hotter), it now idles around 400-500, low enough that it is rather uneven. After it has been running for a while (1+ hours) it gets back close to normal idling range.

Should I adjust the idle speed, or might this indicative of an actual problem?

likely the mixture is off, but why? I would start with a fuel pressure check to see if the WUR is spec for control pressure, then start looking for vacuum leaks and such.

I did put pure 100% gasoline in it recently. Normally I can only get the ethanol blend (stupid Milwaukee). It had about 13 gallons in addition to the other 7 of ethanol stuff.

Possible vacuum leak? If you take off the oil reservoir cap does the idle stay the same? That could be an indicator for a vacuum leak. Idle should go down.

check fuel pressures and vac leaks. DO NOT make any adjustments until you do those 2 things.

I would and it might unless your problem is solved by so doing.

I'll check for vac leak via the oil cap test when I get off work. How do I best check the fuel pressure?

Silly me. When I put a quart in last weekend I must not have put the oil cap in correctly.

A minute ago I took it off and the idle didn't change, so I carefully put it back on and everything is back to normal. It didn't seem poorly seated, but it must have been.