Introduction: Tuning Two-Stroke Engines

As promised last week, here is the instructable inspired by the violent failure of this instructable

Step 1: Why It Failed

The faliure of the 50 mph trike was most likely not caused by wear and tear on the engine, or even damaged parts, but by a simple mistake in the tuning process. Two stroke motors are VERY touchy in the tuning process, if they run too rich, they bog down and quit. if they are too lean, they will over-power themselves, and 90% of the time they explode violently. I'm not a certified mechanic in the least. but I do know what I am doing when it comes to re-building and tuning small engines, mainly two strokes. The reason the trike's motor blew up, was because it was running too lean, and it was a racing motor (potentially deadly combination).

Step 2: Two Stroke Theory

here is an overview of how two stroke motors work. Two stroke motors are more efficient that four strokes for a few reasons. 1 is the displacement, they generally have smaller combustion chambers, which means less fuel burned. 2 is the fact that they have fewer moving parts, which decreases the constant load on the engine, allowing it to burn the air-fuel mixture more efficiently, and reach much higher revolutions per minute.

Here is the process of the engine.

Intake, The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft. During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer/Exhaust. Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well. I'm going to interrupt here for a second to explain how that fresh mixture is prevented from being expelled.

so, you think an exhaust pipe is just meant to get the exhaust away from the engine, right? well in the 4 stroke application that's completely true, but in the two stroke, you must use a specially formed pipe, that uses something called backpressure, to force the expelled mixture back into the combustion chamber, this can be seen in the animation quite clearly.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture, insuring that the mixture explodes properly. (At the same time, another intake stroke is happening beneath the piston).

Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.

Step 3: The Nitty Gritty

so folks, let's get down to the nitty gritty. the engine I am using is from my chainsaw, which is a Husqvarna. safety first, you will need to remove the chain, and bar from the motor. I removed the bar, because it was in the way. next, you will need to remove a plastic shroud that sits in front of the carburetor. on the body of the carb, you should see two screws, labeled L, and H. L stand for low speed (idle) and the H stands for high speed. you will want to gently turn both screws clockwise, closing off the fuel to the engine. Then , turn the L screw counter clockwise two and a half turns, opening the passage for the idle circuit. attempt to start the motor after priming it, by pressing the rubber button 3-4 times. it should at least sputter, if not, then tighten the screw 1/4 of a turn, and try again. contunue to do this until it starts. if it does not start, you will need to close it again, and turn it out two and a half again, prime it, and loosen the screw 1/4 turn at a time. once it starts, it should promptly die when you try to give it throttle. this is good. now, with the motor running, you will want to tighten the screw very slowly, and you should notice a difference in the speed of the engine. keep tightening it slowly, until it bogs down, or quits, now loosen it 1/4 of a turn, no more, no less. now, do the same with the high speed circuit, except when you do this, have it at wide open throttle. when it starts to get really powerful, turn the screw out until you notice a decrease in power.

Step 4: You Did It!!

ta-da! you've just tuned a 2 stroke motor properly, so you'll never have to worry about it exploding on you. now don't you feel better about a motor running at five-thousand revolutions per minute next to your delicate organs? I know I do, infact, I'm overjoyed!