It is not as simple as just "lean tuning" the carb. The fact is that the loading of an engine is NOT CONSTANT and neither is the amount of heat generated by an engine under different loading conditions. The amount of horsepower your "20 HP" engine might produce can vary from under 1 HP at idle with no PTO load applied (the load of water pump, compression and alternator cannot be removed) to the theoretical maximum of 20 HP at wide open throttle, cutting heavy grass in 3rd gear. That is far too great a range of heat output to simply "lean tune" a carb for. Not even with electronic fuel injection could you ever hope to compensate the air fuel ratio to a working mix that would allow for elimination of the thermostat. It was proven a long time ago that that the most efficient temperature range for an ICE (whether burning gasoline, propane, diesel fuel oil, or alcohol) was in the 180 to 220 degrees Farenheit range. In this temp range, you are able to coax the most power out of the fuel and still be able to manage the task of shedding surplus heat of combustion (and compression). If you try to operate at a lower temperature, you will not get the maximum efficiency out of the fuel (essentially dumping more unburned fuel out the tail pipe). If you try to operate a liquid cooled engine at a higher temperature, you run the risk of raising the temperature of the coolant above the cavitation point (where the coolant boils in contact with the water jacket). Once the coolant begins to boil, it no longer cools and the engine is at risk of failure. The argument about "cooler is better" has never been true. It is just an often repeated misconception. For as long as ICEs have had thermostats.....there has been a small group of people who believe thermostats can be thrown away with no ill effects. Now, if I remember something from past arguments, this is about where somebody should jump in and say that if you run an engine without a thermostat, it will cause the engine to overheat (because the water flows too quickly through the water jacket and therefore does not "have time" to absorb any heat). That too is fallacy.

OK rusty jones, you mentioned that hand crank Model T Ford.....Made me think back to a story one of the "older guys" in the shop told in the lunch room back in 1971. He said he and his year older brother shared a Model T to get them to their jobs when they were both young (so it had to be about 1930 when it took place). Both of them still lived at home (with their parents) at the time. He said his older brother work a night shift job and got home about 6 AM. My friend had a day job where he had to be at work by 9 AM. He said it was a frosty morning in November when he went out to hand crank the old Model T. He described (step by step) how he retarded the manual spark advance a bit, set the throttle up some, trickled a bit of gasoline into the carb (all these "hand controls" were in the cab), and then went to the front of the car with crank in hand. He said he rolled it over to get it between compression strokes and then gave it a mighty twist. The engine caught instantly, but it seemed as if something was loose under the hood from the noises and thumps he heard. Just as he reached to open one side of the hood......his Mama's big ole tomcat streaked out from under the car....running for the nearby woods like hell was chasing it......every hair on it puffed up like a bird on a cold day. He looked at the engine (which now sounded normal) and saw lots of cat hair all over the top of the cylinder head. He said he figured that cat had gotten into the habit of stretching out there on cool mornings after his brother came home and parked it. He said the cat just lingered a little too long that morning....stretch out there amongst all those spark plugs on that flat head 4 cylinder. He reckoned that ole cat learned a lesson about magneto ignition the hard way that day. He said from that day forth, you couldn't get that cat to go near that car.

A "cylinder leak down" test (or reasonable facsimile) using compressed air can be done to identify the route by which the compression is escaping. To do a "bona fide" leak down test requires a "percent of leakage gauge". A close approximation of the test can still be done if you have (1)Compressed air available, and (2) a threaded adapter fitting that will screw into the spark plug hole and also receive an air hose, (3) a means of securely locking the crankshaft/flywheel in the "TDC of compression stroke" for the cylinder to be tested. Locking or holding the engine from turning when compressed air is applied to the cylinder is very important, both for personal safety, and, for accurate test results!! Actual testing is simple and amounts to listening to determine where the air is escaping when compressed air is applied to the cylinder through the spark plug hole adapter fitting. As follows: Turn the crankshaft/flywheel to position the piston of the testable cylinder to TDC of the compression stroke. Here's a hint, the valve train for both valves of that cylinder will place the valves in the closed position. you determine this by watching the tappets through the open valve compartment (if applicable) or by watching the valves with the valve cover removed. When both TAPPETS or lifters, are retracted toward the camshaft, and you can detect clearance between the end of valve and the tappet.....lock the flywheel in place so the crankshaft can't turn when air pressure is applied to the cylinder. Failure to securely lock the flywheel against moving could result in personal injury if the crankshaft rotates in reaction to the compressed air applied to the cylinder. In addition to the locking of flywheel, keep your hands away from the flywheel and crankshaft until air pressure is disconnected from cylinder. With the flywheel locked securely, thread the adapter fitting into the spark plug hole (threads are 14mm dia. X 1.25 pitch when you ask for the adapter fitting) and tighten it so it can't leak air. Connect the air hose from a source of compressed air (60psi to 90 psi recommended). IF, there is a loss of compression, you will hear air leaking from somewhere. Carefully determine where the air is escaping by using a 2 foot length of hose or tubing. Place one end of the hose or tube close to an ear (but not inside your ear) and then move the other end of hose to the exit portal of the exhaust system. If air is heard loudly escaping here, the defect is in the exhaust valve or seat. Check the intake valve condition by moving the end of the hose to the throat of the carb (throttle must be held wide open, choke butterfly open, and any crankcase breather hose must be disconnected from the carb, if carb is mounted to the intake manifold). If air is heard loudly here, the defect is in the intake valve or seat. If you do not detect air escaping from either of those places, check to see if you can hear air escaping from the crankcase breather hose (or pulse hose if impulse fuel pump is present). If you hear air escaping from the crankcase breather hose, or pulse hose, the defect is in the piston or cylinder wall. Always remove the compressed air supply from the cylinder before unlocking the flywheel. A leaking head gasket serious enough to cause loss of compression would be obvious without any test equipment, and you know it right away when the engine is running.

This problem may require the services of an expert. Erratic brakes and a stumbling engine are ingredients that can lead to an accident. I'm no expert in this arena, but here are my thoughts. On startup, you are missing the vacuum boost to the brake system, and somehow, later on, some or all of the boost is regained. The engine stunbles consistent with a vaccum leak. You've checked for vacuum leaks, but may have missed the "big" one, the vacuum boost unit itself, or the vacuum accumulator and associated vacuum lines. At this juncture, I suspect that the vacuum boost unit of the power brake assembly has failed, and admits an overload of air to the intake manifold (when the brake pedal is depressed) causing the fuel mixture to lean out. Often associated with the vacuum booster is a small vacuum tank called the 'accumulator'. It will be attached to the vaccum lines that supply the vacuum booster. It's purpose is to have at ready a goodly supply of vacuum volume to handle a suddent demand, and to keep the heater controls happy during this step demand. This tank is very important if the the vacuum fitting on the manifold is 'ported', that is, has a orifice within it to restrict the amount of air flow in case of total vacuum system failure. This tank may have a one way valve. This is the vaccum source that aids one more application of the brake after the engine dies. This tank may be made of two plastic halves and ultrasonically welded together. A leak along this seam is possible. Do you have a hand pumped vacuum tester with vacuum gauge? If you do, remove the vacuum line that feeds the vacuum booster and apply vacuum directly to the booster. Pump down to a vacuum level, say, 10 psi, and hold. Can the booster hold vacuum or does it leak down fast. If it can not hold vacuum, it is leaking. You may have to block the brake pedal down past the valving point for this test. There is a valve in the booster unit that applies vacuum when the brake pedal is depressed, and it relieves vacuum when the brake pedal is let up. This valve can be a problem source.