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I have retuned the carburetors to within spec after changing the number 1 carb with the latest version (superseded 7 times) from Yamaha.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 the superseded parts 7+ versions show that there is a problem here.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.

Thoughts?

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Dec 28, 2020
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Update
by: Mark

Since the post submission from this past summer, I was able to get the carbs tuned properly by using the service manual and a couple of tools that are required. A RPM gauge, a vacuum meter and the extension tubes to attach behind the carbs. With much patience, try and a few errors I managed to get the engine to run acceptably. However the key really was using Recreation fuel ONLY and getting the carbs tuned properly.

I think the ethanol has damaged the middle and lower carbs to the point that they will never run perfectly. But I changed the top carb and that helped quite a bit along with a good vacuum balance setting between all 3 carbs.

Ethanol is the enemy with this motor. It causes permanent damage to the carbs in very short order. I used a very High concentrate of Seafoam and ran the motor on it at a high idle for an hour to clean it out. It had to be in gear on the boat lift to get this to work as I needed the motor under load. This works specific areas of the carbs that needed to be cleaned out for it to run right. This was part of the prework to getting the carbs balanced.

I hope this update helps someone else out there with this motor. We can now use the boat without fear of it dieing on us out in the lake.

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