Carb adjustment question

On my Farmall 400 the book says that when used lightly the Main Adjustment screw can be used to restrict the flow of fuel. I take it that is some sort of fuel saving thing? It says for max engine power to screw it out 5 turns. Why not just always have it 5 turns out? Is there a possibility of blowing up the engine by leaving it at 5 turns and then having the motor at full throttle? I don't understand why it isn't always at 100% of capability.

No , that is how i have been setting them for years because it is only going to use the fuel that it needs when it needs and tryen to lean one out can cause more problems then it is worth. I like to set a carb up for the best slowest idle and then for max performance at the top end.

If you're gonna work the tractor hard all the time your approach is the way to go.

However if the tractor is will be running long periods under light load (raking, pulling a hayride, on a tractor ride, etc) leaning the power mixture noticably reduces fuel consumption. It also helps prevents plug fouling and causes less fuel dilution/soot winding up in the crankcase oil. Also it makes the exhaust smell a bit less objectionable.

Running the fuel mixture too rich will not cause the engine to blow up. It only wastes fuel and actually REDUCES the maximum power developed.

FWIW I run the power mixture on my Super M (same 264 engine as your 400...) about 4 turns open only for plowing and at tractor pulls. Otherwise I run it about 1-1/2 turns out.

sometimes when you have the carb set rich(5 turns in your case)and you are not using full power of engine it will make them hard starting as they're is to much fuel in the manifold when you had shut it off last time.

The plugs in it are D16 Champion's. They were gapped at less than .025 and I put them up to .030 before getting it started. Any idea what the gap should be? The plugs where caked and fouled. I haven't checked them now. I used the tractor to mow for about 2 hours and it ran fine. Except when I got it in the barn it acted like it was running out of gas but it has over half a tank in it.

Idea's?

Max power is when every molocule of oxygen gas a gasoline molocule attached to it. To make this happen there needs to be surplus fuel (rich). Some of the surplus fuel goes up the stack or into the crankcase unburned or partly burned. Engine makes max power. Exhaust contains high % of carbon monoxide. HP per gallon per hour drops. Black soot can be seen and coats engine components.
Engine combustion chamber temps drop some as the rich mixture burns cooler and the raw unused fuel absorbs some heat. Less prone to detonation.

Max economy is when every molocule of fuel has an oxygen molocule attached. To make this happen there has to be a surplus of oxygen molocules(lean). Less carbon monoxide up the stack. Less crankcase oil dillution with raw gasoline. Engine bore/ring life is longer. Exhaust valve life can be shorter due to higher operating temps. Engine can be more prone to detonation/knock/ping. Engine will have best HP per gallon per hour rating. Peak HP will be a few % less than at max rich.
Extra cost of wasted fuel making the rich mixture and max power doesn't pay. Unless tractor pulling and that extra 1-5HP is needed.

Great answer Buick... That helps make things make sense to me... So I should be going with a lean mix since I don't do anything really hard.. That mix is controlled with the main fuel metering valve. I think it is at about 1 or 1.5 turns out right now.

Sounds like the plugs are fouling up again.

Recommend switching to a hotter plug (Champion D21, or other mfr's equivalent). Gap the plugs to .025.

Now with the tractor fully warmed up turn the power mixture screw in until the engine stumbles a bit when you snap the throttle open from idle. Then open the mixture back up 1/2 - 3/4 turn and go to work!