I saw this post on a Seadoo Forum from one of the best 2 stroke mechanincs in North America, Bill O'Neal. Thought I would share this with you.
You need to study up about fuel curves for carbureated engines. Without a drawing, I will try to describe an example of a typical fuel curve of a carbureated engine.
Assuming that the jetting is correct for the application,
1. The two places that the curve is the richest, is at idle and at full throttle.
2. With a pencil and paper, draw a large U on the paper.
3. Write RICH at the top of one leg of the U, and write RICH at the other end of the U, at the top.
4. As the rpm moves from richest ( idle ) towards the bottom of the U, it is leaning out. So at the bottom of the U, write LEAN. The bottom of the U represents half throttle.
5. As the rpm rises from the bottom of the U to the top at full throttle, the curve goes from leanest to richest.
6. The reason that the motors do not sieze up at half throttle is at half throttle, the heat generated in the combustion chambers is not so hot that the cooling system cannot handle it. At half throttle, there is not enough air passing the diffusers at a high enough velocity to pull fuel from the carbureators fuel circuits, and the throttle plates are far enough open to lose the manifold pressure that was in the carbs when the throttle plates were closed or nearly closed. Manifold pressure in laymen's terms is vacume.
7. As the rpm moves from half throttle to full throttle, if you hold the rpm for too long at or near 3/4 throttle, enough heat is generated in the chambers to allow the heat to sink into the pistons and sleeves. As the pistons overheat, they out grow the sleeve bore and eventually grab the clyinder wall and stop the crankshaft from rotatiing.
8. At full throttle, the fuel curve has returned to rich as with maximum air velocity passing the fuel diffusers in the throat of the carbs will pull alot more fuel from the fuel circuits inside the carbureator body and sprayed out the diffusers
9. You should now understand that the fuel delivery in a carbureator depends on vacume at closed throttle plate travel up to about 15% throttle, then lessens until there is very little vacume to pull fuel from the fuel circuits in the carbureator. From 15 % to half throttle, the air velocity passing the main fuel diffusers is very low and it stays too low until you reach full throttle, where the fuel flows freely and at a much richer mixture ratio with the incoming air.
10. Heat in the motor is controlled by two things, water coolant and fuel mixture. Higher rpm without the correct air/fuel mixture will cause major problems in the clyinders as they get too hot to control the tempetures. This is when an engine pre-ignites, detonates and ruins motors.
You need to study up about fuel curves for carbureated engines. Without a drawing, I will try to describe an example of a typical fuel curve of a carbureated engine.
Assuming that the jetting is correct for the application,
1. The two places that the curve is the richest, is at idle and at full throttle.
2. With a pencil and paper, draw a large U on the paper.
3. Write RICH at the top of one leg of the U, and write RICH at the other end of the U, at the top.
4. As the rpm moves from richest ( idle ) towards the bottom of the U, it is leaning out. So at the bottom of the U, write LEAN. The bottom of the U represents half throttle.
5. As the rpm rises from the bottom of the U to the top at full throttle, the curve goes from leanest to richest.
6. The reason that the motors do not sieze up at half throttle is at half throttle, the heat generated in the combustion chambers is not so hot that the cooling system cannot handle it. At half throttle, there is not enough air passing the diffusers at a high enough velocity to pull fuel from the carbureators fuel circuits, and the throttle plates are far enough open to lose the manifold pressure that was in the carbs when the throttle plates were closed or nearly closed. Manifold pressure in laymen's terms is vacume.
7. As the rpm moves from half throttle to full throttle, if you hold the rpm for too long at or near 3/4 throttle, enough heat is generated in the chambers to allow the heat to sink into the pistons and sleeves. As the pistons overheat, they out grow the sleeve bore and eventually grab the clyinder wall and stop the crankshaft from rotatiing.
8. At full throttle, the fuel curve has returned to rich as with maximum air velocity passing the fuel diffusers in the throat of the carbs will pull alot more fuel from the fuel circuits inside the carbureator body and sprayed out the diffusers
9. You should now understand that the fuel delivery in a carbureator depends on vacume at closed throttle plate travel up to about 15% throttle, then lessens until there is very little vacume to pull fuel from the fuel circuits in the carbureator. From 15 % to half throttle, the air velocity passing the main fuel diffusers is very low and it stays too low until you reach full throttle, where the fuel flows freely and at a much richer mixture ratio with the incoming air.
10. Heat in the motor is controlled by two things, water coolant and fuel mixture. Higher rpm without the correct air/fuel mixture will cause major problems in the clyinders as they get too hot to control the tempetures. This is when an engine pre-ignites, detonates and ruins motors.
