2 year old F40TLRB carb problems...

[LakeLanierTom]

2003 motor, 2yrs and the same problem. Any help or suggestions on my "no midrange" problem???

Have a 2003 F40TLRB and I am INTIMATELY familiar with the carbs now... They are super clean, never had a water-in-gas issue, no corrosion, and again; fresh out of the ultrasonic cleaner, meticulously assembled and look like a new set. For the past two years I have nursed this motor along with practically no midrange capability. Cold start is a little bad, but works and warms up fine. Idles fine for hours on end in or out of gear, runs W.O.T. just fine, but if I go from idle to WOT slowly it dies, and if I come down from WOT to idle slowly the motor dies. Throttle settings between 1800rpm and 3000rpm lead to a stalled out engine. I have nothing from 1800 to 3000 rpm unless I push the throttle quickly to WOT or if I chop the throttle from WOT relatively quickly to idle speed.

I own a VacuumMate tool and have synchronized the carbs several times and get a great idle and smooth accelleration to WOT if I do it quick. I have been setting the carbs as such: #3 27 cm/hg, #2 28cm/hg, and #1 29cm/hg at 1000 rpm in the water, in gear, on the water (not tied to the dock).

My bowls are super clean, no corrosion at all, and I hear the check ball rattle when I shake them. Someone mentioned a ball (ball bearing) in the accellerator plunger but I do not have one and haven't seen one on the parts listing.

My WOT rpm is 5100 and the motor is installed on an 18ft pontoon (kind of a heavy one in pontoon terms). I'm running a 4-blade Solas 10" pitch propeller and I'm thinking about changing back to a 3-blade to get up to 6K rpm WOT.

[boats.net]

May have to let a dealer check the accelerator pumps are in fact working and still plugged up.

[rodbolt17]

the mercury designed F40 and F25 have the most complex carbs in all the line ups.
careful investigation and a FIRM understanding is the key.

[LakeLanierTom]

I'm ready to learn! I can perform the tests & do the cleaning/inspection etc...

Was hoping to get some of the gurus to make some suggestions here. I'm a little frustrated that I can figure this out on my own and have given in to these Keihin carbs.

If I "goose" the throttle, this motor runs right up to WOT easily. It got better today and I have from 1200 to 2500 as my "dead zone". I can run all day on either side of those rpms, but the motor will slowly proceed to shut down if I set the throttle inside that range.

Can anyone provide fuel flow diagrams for these carbs? I assume the accellerator pump is working since the plungers are free and operating. Also, when I goose the throttle I get good accelleration to WOT.

I would like to see the factory service bulletin regarding where to drill out the plug and clean the fuel passages in the carb bowls. Anyone have a scan of it or a link where I can find it?

Thanks,

Tom

[99yam40]

My bowls are super clean, no corrosion at all,

My WOT rpm is 5100 I'm running a 4-blade Solas 10" pitch propeller and I'm thinking about changing back to a 3-blade to get up to 6K rpm WOT.

The bowls are not the things that plug up and cause the problems, it is all the different passages.

Does it run better with the 3 blade?

Make sure you have good compression, good spark, and the timing is advancing properly. If all that is up to specs then it is a fuel to air ratio problem. You may need to take it to a professional if you can not fix it.

Check with you Yamaha dealer to see if they will share the factory service bulletin

[LakeLanierTom]

Yes, actually; The bowls do have passage ways...

<<The bowls are not the things that plug up and cause the problems, it is all the different passages. >>

Yeah, actually the F40 carb bowls do have some pretty intricate passageways, and some sort of check ball. I have cleaned them out best I can without drilling out any plugs or lead shots... Does ANYONE have the service bulletin available from Yamaha regarding these carbs showing which lead shot to remove to gain access to a troublesome passageway that needs additional cleaning??? OR could you reference the service bulletin number so that I can approach the local dealer to ask for it.

Please? At this point, I have purchased a 1997 T50TLRV outboard and replaced it on my pontoon today. It runs GREAT! I still would like to sort out this stinkin' F40TLRB, or else I'm going to just part it out on ebay.

Thanks,
Tom

[rodbolt17]

there are no bullitens.
there is a circuit drawing,I think, in one of my fuel systems training manuals.
however I have no way to scan and post it.
like I said, they are among the toughest carbs to deal with and can only be synchronized with a 3 channel manometer.