[Tom Vaught]

Originally Posted by bobby326

I have an afr gauge in my 67 Lemans and at idle it reads 16.8-17.5. At cruise at 2K it reads 13.8 ish. Im having a running hot issue, around 220 on hot days and 200 at night when its cooler out. Did all the normal checks, timming is 15 initial. Divider plate is as close as possible without rubbing, flowcooler pump and griffen alluminum rad with mark viii fan. So i was thinking that the afr might be a little lean. Motor is a 67 400 with E heads and 2802 cam. Carb is a holley 780 with #70 front jets. Should i change the jets to get the idle afr more towards the 14.5. Would that move the cruising afr to 11.5-12. Would that be a better aft for idle. 14.5vs 16.5 and cruise 11.5vs13.5. Maybe that would help a little with the temp problem. Did just biy a new rad with 1.25 tubes vs the old 1” tubes. Any help with what afr would be best to aim for would be appreciated. Thank you

I personally think 16.8-17.5 is way too lean for idle. That air/fuel ratio WILL cause Idle Temp issues. Especially on a Holley Carb. Carbs work based on air and fuel MASS going thru the engine.

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every ONE GRAM of FUEL MASS, to 14.7 grams of AIR MASS are required.

You gain nothing by being leaner than 14.7 ratio because there is no FUEL MASS left to burn.
There is a ideal burn rate vs push on the piston for given modes of engine operation.
Think about it. At idle, even with the PCV Valve supplying 3 cfm of Inert Gas to the combustion process you are only using and burning 16 cfm of air and fuel mixture and 3 cfm of Inert gas.

So, again, math wise, if you have a fresh mixture of Air and Fuel equal to 16 cfm and 3 cfm of burned Inert gases and oil mist you get 19 cfm at idle.

Off idle, the burned Inert gases and oil mist continue to be at 3 cfm and the Mass of air and fuel mixture goes up to the maximum fuel capability of the fuel and air control orifices.

So at your 16.8-17.5 mixture ratio you are using less than one volume of fuel vs 16 volumes of air in the engine at idle. Perfect combustion is again 14.7 quantity of air mass to one quantity of fuel mass. Anything above 14.7 is just bragging rights on an air/fuel gage and does nothing for you. Can't burn a mixture and make push on the piston without a combustion event. Anything above 14.7 and you cheated the fuel out of a chance to have a proper push on the piston for that air mass going thru the engine.

So OEM Manufacturers today use modern computers to run at 14.7 Air/Fuel under all modes of operation today. They use other devices to help the engine run in those modes vs resorting to max power air/fuel ratio modes. They use transmission gearing and EFI to make it work. You can make a EFI unit run just as well as a carb but in both cases you have to put the right parts in the carb and the right calibration in the EFI system

Just my comments.

Tom V.