Help Setting up carbs after clean

[Disschord]

Hi I have a Yamaha f40bet 2008 serial: 67C L 1041598 H

I have stripped and cleaned the carbs with an ultra sonic cleaner and replace most the gaskets and O rings as well as the bungs that cover the jets.

When removing the pilot screw I counted 1 1/2 turns from closed (I was expecting about 2 1/2)

I have put the carbs back together and I am ready to set her up.

I have a vacum gauge to balance the carbs but I am unsure how to set the pilot screw.

Many thanks

[boscoe99]

2.0 turns outward from being lightly seated is the specification.

Plus or minus 1/2 turn.

Why bother replacing the plugs that are covering the idle mixture screw if that is what is meant by the "bung"?

Good luck with those carburetors. They can be a bit fiddly.

[Disschord]

2.0 turns outward from being lightly seated is the specification.

Plus or minus 1/2 turn.

Why bother replacing the plugs that are covering the idle mixture screw if that is what is meant by the "bung"?

Good luck with those carburetors. They can be a bit fiddly.

I had an over fuel problem on the bottom carb. The float appeared to be operating normally the bungs that cover the jet inside the carb seemed a little perished so I figured just change them and be done with it.

I haven't found anything conclusive causing the over fuel issue but I am hopping the clean and new parts will sort it out.

My question though is how to set up the carbs do I set them all at 2 turns out then balance the butterflies with the vacum of do I balance with the mixture?

[boscoe99]

I had an over fuel problem on the bottom carb. The float appeared to be operating normally the bungs that cover the jet inside the carb seemed a little perished so I figured just change them and be done with it.

I haven't found anything conclusive causing the over fuel issue but I am hopping the clean and new parts will sort it out.

My question though is how to set up the carbs do I set them all at 2 turns out then balance the butterflies with the vacum of do I balance with the mixture?

Yes, the idle mixture screws are set two turns out. The sync and link procedure to balance the carburetors does not involve a change to the idle mixture screws. Only the throttle valves are adjusted.

The F40 seems to run lean. Very lean. If for instance you have a stumble upon acceleration you might just find that turning the screws outward a bit might help solve that issue. Or if the motor is hard to start sometimes a richer idle mixture can help. I have been known to install larger jets to make a POS non-running motor turn into a purring kitten.

I find that it is important to follow the procedure in the service manual to sync the carburetors.

[exeter27]

My hunch is that the 'bung' that he references is really a welch plug. But, advice provided is right on. I'd suggest getting the mixture set properly on all carbs before doing the sync/link. My experience shows a bit rich is better than a bit lean. Lean is tough on operator (operationally) and also hard on engine - particularly lean sneezes and lubrication.

[Disschord]

Thanks for all your help

[99yam40]

My thought about the bung ,
is it is a rubber plug that sits over the idle jet forcing the fuel to be drawn from the main jet thru a hole between them.
It keeps the pilot/idle jet from drawing fuel directly from the carb bowl

[Disschord]

My thought about the bung ,
is it is a rubber plug that sits over the idle jet forcing the fuel to be drawn from the main jet thru a hole between them.
It keeps the pilot/idle jet from drawing fuel directly from the carb bowl

Yep that's the one. I have replaced it because I hope that was what's causing my over fuel problem

[99yam40]

I installed new ones in my C40 when redoing the carbs. I did find some that came back out after pulling carbs back apart.
I think it will only cause too rich at higher speeds as the fuel is drawn from both places not just thru the high speed jet

[Disschord]

I installed new ones in my C40 when redoing the carbs. I did find some that came back out after pulling carbs back apart.
I think it will only cause too rich at higher speeds as the fuel is drawn from both places not just thru the high speed jet

That sounds promising because my over fuelling issue only happens when trying to get up to speed. At idle and low speed the engine was fine

[boscoe99]

How did you determine that the bottom carburettor was over fueling? What exactly did you see? Feel? Smell?

[Disschord]

How did you determine that the bottom carburettor was over fueling? What exactly did you see? Feel? Smell?

First I noticed fuel was leaking out into the sea and in the cowling around the bottom of the engine.

When back on land I found the bottom spark plug was very wet with fuel and lastly when turning the engine over with out the spark plugs fitted the bottom cylinder was chucking out lots and lots of fuel out.

[99yam40]

That would not happen from the rubber plug being dislodged from the pilot jet opening where it belongs.

sounds like trash getting into the needle and seat not letting it close off

[Disschord]

That would not happen from the rubber plug being dislodged from the pilot jet opening where it belongs.

sounds like trash getting into the needle and seat not letting it close off

Thanks I have stripped the carbs again now and cleaned with an ultrasonic cleaner so hopfully I am good to go.

I will hopfully get them back on by the weekend and take her for a blast.

Fingers crossed.


Thanks everyone for your help