2003 Yamaha 4-Stroke Intermittent Power Loss - HELP!!

[Square Grouper]

Purchased a 1720 Key West with this engine about 6 mos ago. Guy said he never had any issues with it. 30 gallon tank was about half full when purchased. I don't believe he used Non-E fuel. I've always used Non-E so I topped off with it.

Engine seemed to run awesome first couple trips. I continued putting Non-E fuel in whenever It needed and have so far ran probably 50 gallons through it.
After the first couple trips we were out one day and stopped to fish. Used the *****ing motor for a couple hours then fired up the engine. Got on plane fine and a few minutes late experienced a power loss and drop of RPM's. It essentially will drop off until it stalls if you don't back off to neutral. It will only do it a couple times in a day period and seems to occur after being turned off for an hour or more. But never does it first thing in the am after sitting all week. So far I've done the following without any change:

Fuel/Water Separator
Engine Fuel Filter
Plugs

Last Sunday it ran fine from 7am until about noon after we fished on the *****ing motor for an hour and a half. Then while on plane it did it again. If you bring it down to neutral then try and power up it runs rough like it's starving for fuel. But if you try to power through it, it will stall. If you run at about 2000 rpms for 5 minutes or so it eventually smooths out and is good again. Also, when it did it a second time my wife pumped the primer bulb and that seemed to correct it.

It's so intermittent, I can't see it being filters, plugs or even carbs. Any thoughts? I'm stumped!

[fairdeal]

my reading comprehension is sloppy -

is there a model # or
HP mentioned here?

[Square Grouper]

Sorry my typing is no better!

90HP 4-Stroke Carbureted

I believe F90TLRB

[rodbolt17]

if by pumping the primer(you bypassed the fuel pump) and it ran I would suspect a faulty fuel pump or an air leak on the suction side.

[Square Grouper]

Replaced the primer bulb last night, checked all lines and tightened fittings and removed the anti-siphon valve on the tank. Will run it this weekend and see.

[Square Grouper]

Well, I forgot to remember the K.I.S.S. method. Sometimes I get ahead of myself and think the worst. The primer bulb was the culprit!

[99yam40]

I hope you are correct about the bulb being the fix and you are good to go now,
but I do not understand how it could have been the problem if pumping it helped before as you said

[Hollowman]

I hope you are correct about the bulb being the fix and you are good to go now,
but I do not understand how it could have been the problem if pumping it helped before as you said

It's the check valve that is what usually fails in the primer bulb,causing the line to LOSE prime and let the fuel drain back.

[bajakeith]

Tell Ya a lil Secret

Not only does the P/B have a arrow for fuel direction,, But, They also work much,much better facing up.. as in skyward...FACT.. I keep one in my moble rig and show guys all the time.. and demo for many wanna be back yarders and shops .. see installs from many,many factorys rigged wrong !!

[99yam40]

It's the check valve that is what usually fails in the primer bulb,causing the line to LOSE prime and let the fuel drain back.

this would not affect a motor that was already running as the OP said

[Hollowman]

this would not affect a motor that was already running as the OP said

Actually it can.

It happens after his motor is off for awhile. If the check valve allows drainback, the carbs and even the fuel pump and internal fuel lines can still have enough fuel to run, but shortly ,they will become starved unless re primed, or the pump itself recovers it depending on how much was lost. Which also explains why when he let it run at much lower RPM, it eventually did, on its own.

Been there, done that.

[Hollowman]

Not only does the P/B have a arrow for fuel direction,, But, They also work much,much better facing up.. as in skyward...FACT.. I keep one in my moble rig and show guys all the time.. and demo for many wanna be back yarders and shops .. see installs from many,many factorys rigged wrong !!

I had seen this about these, not so long ago.It makes a huge difference on how easy it is to prime when facing up.

Question...does this also apply to normal operation? Mine normally just lays in my splashwell horizontally. Should I mount it vertically somehow?

[boscoe99]

Actually it can.

It happens after his motor is off for awhile. If the check valve allows drainback, the carbs and even the fuel pump and internal fuel lines can still have enough fuel to run, but shortly ,they will become starved unless re primed, or the pump itself recovers it depending on how much was lost. Which also explains why when he let it run at much lower RPM, it eventually did, on its own.

Been there, done that.

"this would not affect a motor that was already running as the OP said"

You missed a key point. The point about the motor already is running. In which case the orientation of the primer ball is not a factor.

[99yam40]

Got on plane fine and a few minutes late experienced a power loss and drop of RPM's.

when it did it a second time my wife pumped the primer bulb and that seemed to correct it.

I do not see a primer bulb causing his problem if both of these statements are true.

the motor would not get up on plane and run fine for a few minutes before stumbling there is not enough fuel held in the system to do that and the pumps should pick up the fuel within those few minutes.

and pumping a primer bulb that was defective would not do much good like it did on the outing like he said it did

[fairdeal]

while the two valves of the priming bulb could theoretically work as "check valves" to prevent the fuel in the lines draining back into the tank -

it seems to me, that if the hoses, connections, pump(s) etc are in "proper condition" - i.e. - not allowing air leakage -

there should be no need for such a check valve function?