just filled up with mid grade gas. no more run after sign off

I have the run on issue as well, my 292 is quite stock but no blowy,good comp.i kept blaming the ign. switch. after reading these posts I followed the advice ted had given. retarded the timing a bit,got idel to 550 no run on,canadian gas which is terrible with all the additives. seems betters also had a flat spot   when upshifting and engine cold. (manual 4spd with od in 4). I also hooked up the exhaust flapper to get a bit more heat to the intake. still have to go for a good  run but appears to be much better intake with  Holley 600cfm 4ve.

Excellent point Ted, thanks.

1955 312 T-Bird Warwick, NY

This brings us full circle and now back again to the quality of the fuel.  Have you tried fuel from another location?

I bring this up as I had a 454 BBC on the dyno that had a serious ‘run on’ issue after making the first dyno pull.  Once it was verified that the ignition timing and idle speed were not the problem, a quick check for the ethanol content in the gasoline being used showed it had 35%.  The fuel was purchased by the customer from his local station and was advertised as 93 octane Premium on the pump.  This was from pumps with stickers staying ‘May contain up to 10% ethanol’.  I had the customer go to the nearby local grocery store and purchase five gallons of 93 Premium and a quick check showed it had only 5% ethanol.  That fuel in the same engine without any other changes solved the ‘run on’ problem.

Lorena, Texas (South of Waco)

I have a relatively new conventional conductive timing light.

1955 312 T-Bird Warwick, NY

Are you using a conventional or ‘Dial Back’ timing light?  If a ‘Dial Back’ light, then I’ll suggest checking the timing with a conventional light just to ensure that that the ignition timing is the same with both timing lights.  ‘Dial Back’ lights are notorious for giving erroneous readings as they get older.

Lorena, Texas (South of Waco)

I add an oz of Sea Foam to every gallon, I recently replaced both exhaust manifolds and put my finger into several exhaust ports finding only a tiny trace of carbon. The cylinders used to be choking on it.

1955 312 T-Bird Warwick, NY

Hmm, I’m at 12ish BTDC, no pinging or hesitation, great acceleration. The idle is comfortable at 500 on the factory tech that sits at 100 with the engine off but I’ll try shaving a few RPM and see what happens. Thanks much Ted for the feedback.

1955 312 T-Bird Warwick, NY

Gut feel says you’ll not see a difference between 91 and 93 octane fuels.  That’s assuming those fuels are as advertised.  Idle speed being too high is still at the top of my list.  Have you looked at your ignition timing?  Could simply be too much ignition advance.

Lorena, Texas (South of Waco)

I’m running Sunoco 91 octane, in addition to reducing idle speed is there any harm in trying the 93?

1955 312 T-Bird Warwick, NY