When you accelerate the engine makes a sound, but when you use the engine-brake, is another sound. What determines this sound?
Engine brake? Are you using the decompressor to 'slow down' the engine? Not a good idea, since you're opening a valve when it's normally not open - if the engine is revving, you can bend a valve. The noise you hear when you activate the decompressor while the enigne is running is actually the sound of combustion. The exhaust valve is usually closed when the spark ignites the gas, but with the decomp pulled in, the exhaust valve is open, so that POP POP POP is the sound of the fuel combusting.
When you release the throttle, the air intake valve slams closed, the piston is still going up and down trying to suck air in, but now it can't get enough because the valve is shut so it starts to create a vacuum. This takes up work instead of producing it so the engine is basically working against the direction of rotation and it slows you down. If I had to describe the sound, I guess I would say it sounded kind of like the engine is straining but muted and slowing down. I've also had people tell me they use the decompress to slow down, but I've never followed that procedure. I tried it a few times (it should be safe for the engine), but the lack of compression can cause the engine to die. The purpose of the decompress is to prevent the compression (duh, lol). Since this prevents the vacuum from building up, you would get a lot more braking ability by just releasing the throttle.
Way too complicated a matter to discuss in accurate complete detail here. Accel and Deccel are 2 entirely different modes,,practically alternate universes In Accel,,,the Intake Velocity is very High and Voluminous. Combustion Chamber Pressures are High ,,and Rising by the RPM. Exhaust is Pressurized to the point it constitutes a Flow Restriction,,,,Likewise on Intake Side'sVaccum being restricted by mass volume demand. Intake Gulp Frequency is communicated all the way thru to altmosphere On DECCEL,,Air Intake is almost NIL,,,, Carb Downstream Intake tract is under Extreme High Vacuum while Carb Intake is under a near NO Flow/Atmospheric Pressure condition. It's virtually Isolated from atmosphere Combustion is almost NIL,,hence Pressure is limited to engine's physical displacement. And even THAT is being starved by Closed Throttle. Engine is pumping NO Air to compress,,,None to support combustion. PSI entering Exhaust falls thru the floor. What WAS The Restrictive Exhaust under ACCELERATION,,, instantly becomes Relatively CAVERNOUS compared to the Mass of Air it must pass under Deccel. Or I should say LACK of Air The Reduced AirFlow Mass,,,Drops the Velocity at exhaust outlet. With practically NO Back Pressure on exhaust,,,the Pop of the engine's pumping is communicated thru the exhaust with additional resonance due to the rapid Rise/Fall of system pressures. Not reffering to lean misfire pop,,,but the popgun/bottle cork rarefaction type of pop Lots more complicated and complex than that,,, but it all boils down to the effects of Air Mass,and Pressure,and Velocities at different point of the engine /Air Intake/Exhaust. All of which behave entirely differently during accel/deccel
