Yamaha f40tlra 2002
by Mark Romanowski
(Houghton Lake Michigan, USA)
Engine will not accelerate under load. I have to pump the drive control arm to prevent it from stalling. If I very slowly increase the throttle arm, like move it slightly every 30 secs or so, I can eventually get it to full throttle.
This is a carbureted 3 cylinder. These carbs are notorious from what I can read online. We have had two different Marina's rebuild them and the problem does not go away.
I have retuned the carburetors to within spec after changing the number 1 carb with the latest version (superseded 7 times) from Yamaha. Although this seemed to help with making the motor start more reliably and easily, it does not solve the power under load issue.
I have cleaned every electrical connection to ensure there wasn't any corrosion causing issues. (nothing was obvious)
I have rebuilt the fuel pump. Changed the inline filter, and the entire hose and connectors from the tank to the motor.
I changed the plugs as well. None of these things has changed the behavior. I have had it work properly 1 out of 20 tries.
We are on a fresh water lake. and the motor is winterized each year. This problem has been an issue for at least the last 4 years.
I have done more testing today to try and find my next steps. With a timing light I was able to see that when I rev the engine, at high rpm (3500-4000?) the light shows misfires on all cylinders. I don't know though if this is because the brain is trying to limit the rpms so you don't hurt the engine? The engine does seem to miss fire on one of the cylinders just above idle, while under load. Increasing the rpms slightly will get you past this point but it seems suspect to me that this would occur at all. (coils are on order...)
I have read about the Yamaha issues with these carbs.. class action lawsuit etc. the superseded parts 7+ versions show that there is a problem here. But I can't deny that the spark issue could also be a factor here. Question is what to test / change and in what order to minimize the cost of repairs.