wallfishing
Recruit
- Joined
- May 7, 2008
- Messages
- 4
I have been working on my Yamaha 30 ELG (1988) engine since it will run but has no power under a load. It is a 3 cylinder, 3 carburetor unit. The electrical all demonstrates a nice spark at each cylinder and I have replaced all three plugs. The compression on the cylinders is 115 (top), 125 (middle) and 135 (bottom). I started to work on the fuel since electrical and compression appeared to be fine. I replaced the fuel filter, drained the tank, put in fresh fuel, and completely rebuilt the carburetors making sure all ports, especially the idle and high speed ports, were clean. The fuel pump is working fine as all bowls have fuel and if I crank the engine with a fuel line disconnected, fuel comes out. After all of this work the condition did not improve.
My next step was to remove spark plug wires while the engine was running with no load.
Top cylinder - killed the engine.
Middle cylinder - no effect on engine performance
Bottom cylinder - no effect on engine performance
My next step was to block the air intake on the carburetors
Top cylinder - again, this killed the engine
Middle cylinder - no effect on engine performance
Bottom cylinder - no effect on engine performance
Next I disconnected the throttle tying all three carbs together and then while the engine was idling, I manually opened the throttle for the middle cylinder. After a brief delay the engine RPMs increased as expected. I repeated the same process with the bottom carb and had the same results. Thus the spark, compression and carb were working for the middle and bottom cylinders when isolated. I assumed the slight delay in increased RPMs was due to the need for the cylinder to create enough vacuum to pull in the air/fuel mixture.
My conclusion is that I have an air leak somewhere in the engine case that is severe enough that the motor is not pulling air/fuel from the middle and bottom carbs into their respective cylinders. I have sprayed a little carb cleaner at the carb-intake manifold junction of the suspect cylinders and had no effect on engine performance. I have reached the conclusion that I must have an air leak in the crankcase cylinder. Presumably it is in the upper casing gasket since that is closest to the cylinders that are not firing.
My questions are:
Is my conclusion logical based on the diagnostics I have performed?
Do I need to perform additional diagnostics and what would they be?
Do I just need to replace the upper casing gasket or are there gaskets on the flywheel side of the engine that need replacement?
My next step was to remove spark plug wires while the engine was running with no load.
Top cylinder - killed the engine.
Middle cylinder - no effect on engine performance
Bottom cylinder - no effect on engine performance
My next step was to block the air intake on the carburetors
Top cylinder - again, this killed the engine
Middle cylinder - no effect on engine performance
Bottom cylinder - no effect on engine performance
Next I disconnected the throttle tying all three carbs together and then while the engine was idling, I manually opened the throttle for the middle cylinder. After a brief delay the engine RPMs increased as expected. I repeated the same process with the bottom carb and had the same results. Thus the spark, compression and carb were working for the middle and bottom cylinders when isolated. I assumed the slight delay in increased RPMs was due to the need for the cylinder to create enough vacuum to pull in the air/fuel mixture.
My conclusion is that I have an air leak somewhere in the engine case that is severe enough that the motor is not pulling air/fuel from the middle and bottom carbs into their respective cylinders. I have sprayed a little carb cleaner at the carb-intake manifold junction of the suspect cylinders and had no effect on engine performance. I have reached the conclusion that I must have an air leak in the crankcase cylinder. Presumably it is in the upper casing gasket since that is closest to the cylinders that are not firing.
My questions are:
Is my conclusion logical based on the diagnostics I have performed?
Do I need to perform additional diagnostics and what would they be?
Do I just need to replace the upper casing gasket or are there gaskets on the flywheel side of the engine that need replacement?