F40tlrx runs rough between idle and 3000 rpm

[Djs1969ss]

My 1999 F40TLRX has never run real good. I inherited this engine as part of a 20' pontoon package from my father. I have been able to get it to run reasonably well for awhile by draining the fuel bowls and squirting carb cleaner into the bowl drain holes watching for debris. I've also completely disassembled the carbs, including the plugged orfices, being careful to return the screws to their previous settings. I use rec 90 fuel and stabilizer after taking it to an authorized Yamaha repair facility and being told that problem is bad gas. Nothing I do seems to remedy the problem long term. Start up this spring was extremely difficult. I believe it was due to excess fogging oil in the plugs. Spraying the plugs with cleaner resolved the no start. It now starts fine, but runs extremely rough between idle and 3,000 rpm. It runs great anywhere above 3,000rpm. I've seen some you tube video's about installing larger jets from a F25LA on the F40TLR's. Could I benefit from the same rejetting? The articles, and videos I've seen recommend changing the #38 and #95 jets to #45 & #108.
Is rec 90 a good fuel to use? Is Sea Foam ok to use? I read in another post about using Yamaha's "ring free". Would this additive be helpful in this situation? Also read about the ECM being a culprit.
Please, someone, HELP!
Thanks

[99yam40]

how did you drain and clean the fuel tank ?
Just removing some of the fuel does not work well

did you open up the fuel pump or replace it?
stuff tends to hide in pump and lines and find its way into the carbs again after cleaning

swapping jets may be a good idea
having a service manual and following procedures to link and sync after installing cabs is needed every time

[rodbolt17]

if you do not have and use the multi channel manometer you simply CANNOT get it right and it runs rough at low speeds.
best I recall the old Fmerc motor used 87 octane.
any fuel additive wont help if you have contaminated fuel,fuel lines,pump cavities or carbs.

[Djs1969ss]

Thanks for the replies!
Just emptied the tank. Didn’t do anything to flush it or the fuel pump. The fuel pump is original and hasn’t opened up or anything done to it. Carbs have not been synchronized after disassembly.
Should I do something with the pump?
Thanks for the help!

[99yam40]

Rod normally replaces the pump when he does a carb job from what he says.
keeps people from bringing them back for a do over.

but if you are inclined you can take it apart to clean and rebuild and flush /clean lines when you clean the carbs again.
people seem to dislike paying someone high dollars to play with that, so he just replaces with new.
labor is more than a new pump

how big is the tank?
how did you drain it?
if you did not drain completely and clean it may still have old fuel and trash in it when you put new in.

[Djs1969ss]

Tank is s 6 or 7 gallon. Replaced the original and have since emptied the current one just by pouring out all the fuel, after problems persisted. Sounds like a fuel pump rebuild or replacement is in order , as well as carb rebuilds again? Would a rejetting as I described above harm anything?
Thanks

[panasonic]

As Rod and Yam40 said everything has to be clean from fuel tank to carbs and everything in between. (Fuel Pump)

I would rebuilt the fuel pump or replace depending on cost.

As for the Jets, the engine was designed to run on the Jets installed and I am sure there are many owners of your exact engine model whose engines run just fine.

I believe what Rod said is true. You have a four cylinder with 4 separate carbs.If they are not in sync then it is not going to run properly. I have had my share of 4 cyl 4 carb motorcycles and know what he is saying...

Here is a inexpensive 4 channel manometer if you chose to do it yourself.

Do you have Yamaha service manual?


https://www.motosport.com/product/?a...badger#reviews

[99yam40]

just do not tell the EPA

[Djs1969ss]

I don’t have a service manual. Anybody have a link where I can get an online version?
thanks to everyone for the help!

[rodbolt17]

without a manometer your pissing into the wind.

[zenoahphobic]

without a manometer your pissing into the wind.

I think I read somewhere that it is more politically correctly known as a personometer, and that we should all be urinating sitting down.

[rodbolt17]

the USN taught me the difference from pissng winward and pissing leeward.

[panasonic]

I don’t have a service manual. Anybody have a link where I can get an online version?
thanks to everyone for the help!

https://www.vansoutboardparts.com/se...0&limitstart=0