Our club has a 2004 Yamaha F40 -- 40 hp, 4 stroke, 3 cylinders, 3 carbs. The engine starts and idles beautifully. With the fast idle lever, we can rev it in neutral. We put it in gear and try to advance the throttle and it simply bogs down. We used the boat weekly, even over the winter, and this behavior has come on gradually.
In our desperation to figure what is going on, we attached a vacuum gauge to the intake manifold. In neutral we see good manifold vacuum. When we put it into gear and try to advance the throttle the vacuum drops to very low levels and does not increase. This does not seem right. What can cause that? Do exhausts get clogged?
We have checked everything we can think of short of taking apart the carbs. We can't believe that fuel is the problem. It is perfectly clean. No water at all in the separator. We can see the accelerator pumps squirting gas.
In our desperation to figure what is going on, we attached a vacuum gauge to the intake manifold. In neutral we see good manifold vacuum. When we put it into gear and try to advance the throttle the vacuum drops to very low levels and does not increase. This does not seem right. What can cause that? Do exhausts get clogged?
We have checked everything we can think of short of taking apart the carbs. We can't believe that fuel is the problem. It is perfectly clean. No water at all in the separator. We can see the accelerator pumps squirting gas.