2008 Yamaha F40 running rough

[outfishing13]

I recently purchases a fishing boat with a 2008 Yamaha 40 4-stroke model F40BMLH. This is the first 4-stroke that I have owned. I have had it out three times trolling in the last three weeks. The motor starts up on the first pull and I let it warm up, as recommended. It seems to have plenty of power to get my 16 foot Alumicraft 1650 up and going. The prop on it is a 4 blade 11.8x9 and top speed is about 22 mph with 3-4 adults.

I have talked to other owners and they all say how nice it is to troll with these because you can run them for hours and they are quiet and smooth. Anyhow, mine shakes quite a bit at idle while trolling. I can troll right around 1.7-1.9 mph at the slowest, but I have heard you can troll them as slow as 1.4 mph. Some searching indicates that this rough running at idle seems to be a common problem with the carburetors on the F40s getting gummed up. When I bought it, the engine had only run 2-3 times in the last 2 years, but before I bought it it started right up on the second pull. The previous owner said he put Sta-bil in the fuel and I put 3-4 new gallons in the 6-gallon tank when I topped it off the first time this season.

I pulled the plugs today and all three plugs are just a tan to light tan on the insulator, so that seems like it is okay. The 2nd cylinder plug is a little darker than the other two, but not by much.

My plan is to change the plugs, dump the fuel(use it in my mower), change the fuel filters and add some Sea Foam to the tank and just keep running it to see if the rough idle at trolling speed will clear up.

I'd like to hear what other F40 owners have done to cure this little problem, which seems to be pretty common ailment of these yamahas. I sure would like to steer clear of having to take it to a shop or take the carbs apart.

Thanks,
Matthew

 

[Capt Ken]

Re: 2008 Yamaha F40 running rough

Most of the times the roughness is caused by the carbs being out of balanced. You need a vacu-mate to perform this task. Also if the blocks on the low speed mixture screws haven't been removed, you need to remove them and try to enrich the mixture just a bit.

 

[outfishing13]

Re: 2008 Yamaha F40 running rough

Capt Ken,

Thanks for the tip. After closer inspection, I noticed that the yellow paint on the middle carb adjustment screw spring and head were not lined up like the other two carbs. I started wondering if the previous owners had adjusted it thinking that might be the idle adjustment. (seriously there are a lot of screws in that area!) Anyhow, I decided I would adjust it back. So I got the motor in a tub of water and warmed up to operating temperature and screwed the second middle carb adjustment screw clock-wise half a turn lining up the paint and it ran a little better. So I screwed it another turn and it ran better yet. I adjusted the idle down and turned it a full turn again and it ran smoother yet. I really think someone had adjusted it thinking that was the idle adjustment. Now it runs much smoother at an idle. Now I just need to get it out on the water and see how it performs.

I can't seem to find any information about adjusting those carbs. I have a service manual, but it has no information.

I wish I had a vacuum mate to check everything, but I think I might just see what it costs to get the carbs adjusted at the local Yamaha Dealership. Heck, it seems to run really well now, so I might just leave it and not worry about it.

It seems like if the middle carb was running to rich it would cause that cylinder to flood, making the engine run rough. That would also explain why the middle spark plug was a little darker than the others. What do you think?

 

[boscoe99]

Re: 2008 Yamaha F40 running rough

I have found that a vaccumate is essential to getting that model motor to run smoothly at idle.

The carburetor synchronization procedure is shown on pages 4-15 and 4-16 of the Yamaha service manual.

Another possibility is to go back to the original idle jet size for your motor. It is now a #38. It was a #40. It will also work well with a #45.

 

[Capt Ken]

Re: 2008 Yamaha F40 running rough

If you look on the dude of each carb, you should see a slotted screw head down in a hole. If not, then you will see a brass plug. Drill a hole in the plug, screw a screw in just a bit and pop the plug out. Under them is the idle mixture screw.