Carburetor question on restoration I’m working on 1992 40 hp MSHQ Yamaha

[Douglasdzaster]

Pulled and cleaned the carburetor bodies as I replaced everything thing else. All new jets, everything. Boat ran great until the third time out started idling high and hard to start. I did a link and sync by the manual the other day and it idles smooth and starts first pull. I also adjusted the polit screws for smoother running. The top and middle carburetors made the difference. The bottom one didn’t seem to change anything when I started adjusting it. Even though it’s running a lot better should I pull the carburetors and clean the bottom one again or might I have another problem? Any advice is greatly appreciated. One more question. I have a new epa portable fuel tank that doesn’t vent until 5 psi. So I installed a shut off valve for when I’m not running the motor because the pressure will flood the engine. Could this thing be causing problems when I’m running the engine? If so I’ll have to break down and by the fuel demand valve they’re pushing with the new tanks. Any reply’s are greatly appreciated. And thank you Sea Rider for guiding through this under taking.

 

[99yam40]

Tank pressure should not flood a motor.
I had an old metal tank before on a 35 hp Evinrude that the primer bulb blow up on when I stopped at my parents house and let it sit in the sun for a few hours, but it never flooded the motor

 

[Douglasdzaster]

Tank pressure should not flood a motor.
I had an old metal tank before on a 35 hp Evinrude that the primer bulb blow up on when I stopped at my parents house and let it sit in the sun for a few hours, but it never flooded the motor

These things swell up and looks like a basketball. They’re supposed to be built with heavy material to take it. But the pressure will push fuel through the motor. There’s several articles about it. You’re supposed to put this fuel demand valve between it and the bulb and the valve only lets fuel through when the motor demands it.

 

[99yam40]

no worse than metal tanks back in the day

 

[Douglasdzaster]

no worse than metal tanks back in the day

People are wishing they had those old tanks back because they had a vent.

 

[Douglasdzaster]

People are wishing they had those old tanks back because they had a vent.

EPA Requirements for Portable Fuel Components | West Marine

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I also tried the sprayless connectors mentioned here and they still leaked.

 

[FishMan001]

Fuel pumps can leak into cylinder 3 if the diaphragm is damaged.
Take it off (2 bolts), but leave the fuel lines attached.
Pump the fuel bulb and see if ANY fuel comes out of the hole on the back.
If it does, then a new pump, or repair kit is needed.

 

[Sea Rider]

Those Yams have only a main fuel pump, if flooding one carb will flood the others as well. Thought that have removed and bypassed the valve found in between the tank and motor connector. Luckily don't run those EPA tanks and hoses, old tanks with air vents and standard fuel hoses are preferred and fully available.

What you can do to rule out if the lower carb's needle is not closing firm against the new seat is with bulb's arrow facing upwards prime it till firm and once there prime it real hard as much as your hand allows for an extra fuel pressure and check if with fuel spills, if any blame the EPA tank for it...

Assume that when rebuilding the carbs replaced the 3 bowl's o'ring/gasket and float's needles as well ? with correct float height adjustment, new seats, needle and bowl's o'ring gasket carbs should not be leaking a single drop..

With my electric air pump used to inflate my Rib which delivers 3.0 PSI can blow all what lays around my shop, now imagine what a 5.0 PSI of pressure accumulated inside those EPA fuel tanks and hoses will do...

Happy Boating​

 

[FishMan001]

Those Yams have only a main fuel pump, if flooding one carb will flood the others as well. ​

I will bow to your greater knowledge, but I was under the impression that the fuel pump it has, works by taking the vacuum from the No. 3 cylinder crankcase, just like most of the other 3-cylinder 2T yams. If so, then a leaking diaphragm empties fuel into the No. 3 cylinder only. It's happened on my 50G and 70B.
If I'm wrong about the 40MSHQ, I apologise for wasting time, and I'll shut up now.

 

[99yam40]

Those Yams have only a main fuel pump, if flooding one carb will flood the others as well.
​

I am not sure what you are trying to say.
are you saying that one fuel pump will flood all carbs?
the other poster was talking about if the pump diaphragm leaks, it will flood #3 crankcase and not affect all 3.
which is correct.

 

[Sea Rider]

I am not sure what you are trying to say.
are you saying that one fuel pump will flood all carbs?
the other poster was talking about if the pump diaphragm leaks, it will flood #3 crankcase and not affect all 3.
which is correct.

That motor has a built in fuel pump located at the lower carb, if the diaphragm were shot the motor will stop working soonafter what's been initially fuel primed has been totally burned, will need to constantly prime the bulb hard to keep it running, can't see any crankcase being directly flooded in such case scenario as there's a fuel bowl in which the diaphragm empties fuel into.

Seems there's to be a wrong misinterpretation with posters about the fuel leak/flooding whichever is happening. Doug, is the fuel leaking out third carb around upper or middle portion onto the pan, if so, the excessive tank's 5 PSI built in pressure surely accounts for that..

Happy Boating​

 

[Douglasdzaster]

That motor has a built in fuel pump located at the lower carb, if the diaphragm were shot the motor will stop working soonafter what's been initially fuel primed has been totally burned, will need to constantly prime the bulb hard to keep it running, can't see any crankcase being directly flooded in such case scenario as there's a fuel bowl in which the diaphragm empties fuel into.

Seems there's to be a wrong misinterpretation with posters about the fuel leak/flooding whichever is happening. Doug, is the fuel leaking out third carb around upper or middle portion onto the pan, if so, the excessive tank's 5 PSI built in pressure surely accounts for that..

Happy Boating​

Okay. The only time it flooded the engine was when I forgot to shut the valve off between the new tank and the motor when I stopped to fish. I’m trying to figure out if the pressure could be affecting idle as well . If so I’ll install the fuel demand valve.(see link in post#6)The problem with the bottom carburetor is the polit screw doesn’t seem to affect the engine while adjusting it. I adjusted the other 2 the other day after the link and sync and it’s running smoother than ever. Just concerned about the third not adjusting. Thanks for all the help. I appreciate y’all.

 

[Sea Rider]

Okay. The only time it flooded the engine was when I forgot to shut the valve off between the new tank and the motor when I stopped to fish. I’m trying to figure out if the pressure could be affecting idle as well . If so I’ll install the fuel demand valve.(see link in post#6)The problem with the bottom carburetor is the polit screw doesn’t seem to affect the engine while adjusting it. I adjusted the other 2 the other day after the link and sync and it’s running smoother than ever. Just concerned about the third not adjusting. Thanks for all the help. I appreciate y’all.

Upload a pic of the lower carb showing what you mean by polit screw to have a better idea what looks like. If you adjust the upper or middle carbs idle settings will the motor be affected or will remain unaffected as the lower carb ? Have you removed all 3 spark plugs to check their current tip condition from new installed, will indicate which cylinders are burning fuel efficiently each single carb is delivering

Have you tried bypassing the fuel demand valve and operating the motor with the tank's fuel cap bit unscrewed from full screwed and check what happens. If all good to go, blame the EPA tank and hose for it...

Happy Boating​

 

[99yam40]

That motor has a built in fuel pump located at the lower carb, if the diaphragm were shot the motor will stop working soonafter what's been initially fuel primed has been totally burned, will need to constantly prime the bulb hard to keep it running, can't see any crankcase being directly flooded in such case scenario as there's a fuel bowl in which the diaphragm empties fuel into.

Seems there's to be a wrong misinterpretation with posters about the fuel leak/flooding whichever is happening. Doug, is the fuel leaking out third carb around upper or middle portion onto the pan, if so, the excessive tank's 5 PSI built in pressure surely accounts for that..

Happy Boating​

Yamaha mechanical fuel pumps use the pulse from one of the cylinder crankcases to operate the fuel pump.
most are mounted on the side of crankcase, but this one is mounted on the bottom carb so I assume it is still ported to either the crankcase or the the reed chamber to get the pulses.
if the diaphragm gets a small hole in it, there will be fuel sucked into the place the pulse is coming from causing an over rich condition to that cylinder due to the extra fuel that is not metered thru the carb and the pump can still pump fuel from the tank to the carbs as long as the hole or rip is not too large.

If the lower cylinder has good compression, good spark at the correct time, then he is left with too much or too little fuel getting into that cylinder if it is not firing properly.
finding which it is, and what is causing that condition is the main problem.
there are also seals at the top and bottom of crankshaft that can leak air and there are seals between the cylinder crankcases that can go bad.
also crankcase recirculation check valves that might affect things

He complains of the pilot screw on the bottom carb not changing the motors running at idle like the top 2 did.
either the pilot jet/passages are not allowing a change in the too lean condition due to blockage or it is running too rich and closing that screw off is not taking enough fuel out of the equation to help.

with all of this said, the need to know what is flooding while sitting needs to be looked at also
where is the pressure build up in the fuel tank pushing fuel into?
is it affecting one or more carbs / cylinders.
is there visible fuel coming out of the bowl vents or maybe out of one or more carb throats?

 

[Douglasdzaster]

Upload a pic of the lower carb showing what you mean by polit screw to have a better idea what looks like. If you adjust the upper or middle carbs idle settings will the motor be affected or will remain unaffected as the lower carb ? Have you removed all 3 spark plugs to check their current tip condition from new installed, will indicate which cylinders are burning fuel efficiently each single carb is delivering

Have you tried bypassing the fuel demand valve and operating the motor with the tank's fuel cap bit unscrewed from full screwed and check what happens. If all good to go, blame the EPA tank and hose for it...

Happy Boating​

 

[Douglasdzaster]

The other two pilot screws are good for adjusting. I do need to pull the plugs and see how it’s burning fuel. I found an old picture you can see it on top of the carburetor. I’ll also post an older picture from when I did the thermostat to show y’all where the fuel pump is mounted to the cylinder. I don’t have one of the new fuel demand valve. This what I’m trying to decide on as it’s supposed to only let the amount of fuel in that the pump is demanding. All I have now is a Manuel shut off valve.

 

[Faztbullet]

Never seen a 3 cylinder Yammy of any HP with fuel pump on carb. That motors pump is on block. Find a older Merc or Johnson tank and ditch the EPA junk. These new tanks build up so much pressure if you dont unplug them and use demand valve, it will flood motor as pushes needle off the seat. Also run the risk of diaphragm rupture and float level being tweaked. My duck hunter customers hate em

 

[FishMan001]

I found an old picture you can see it on top of the carburetor. I’ll also post an older picture from when I did the thermostat to show y’all where the fuel pump is mounted to the cylinder.

Your second picture shows the fuel pump on the block, not on the carb.
If the diaphragm is damaged, it will dump excess fuel into cylinder 3 only.
Please see my post #7 on how to check if it's damaged. It's quick and easy and will rule out pump problems.

 

[Sea Rider]

Yep the consensus states to get rid of the EPA tank and fuel line or at least test as indicated in post 13. After removing the spark plugs to check their condition, perform a cylinder compression test if countig with a compression tester which don't remember if was previously done ?

Happy Boating​

 

[boscoe99]

That motor has a built in fuel pump located at the lower carb, if the diaphragm were shot the motor will stop working soon after what's been initially fuel primed has been totally burned, will need to constantly prime the bulb hard to keep it running, can't see any crankcase being directly flooded in such case scenario as there's a fuel bowl in which the diaphragm empties fuel into.

Seems there's to be a wrong misinterpretation with posters about the fuel leak/flooding whichever is happening. Doug, is the fuel leaking out third carb around upper or middle portion onto the pan, if so, the excessive tank's 5 PSI built in pressure surely accounts for that..

Happy Boating​

That is not what the parts catalog or the service manual shows. What be we missing?