F90TLRB Fouling Plugs 1 &2

[Nowisthetime]

Have a Yamaha F90TLRB which is a 4-stroke 2003 carburetor version outboard motor. Cylinders 1 & 2 spark plugs show fouling (black) whereas cylinders 3 & 4 are fine. Engine runs very rough and misfires constantly. I know the power packs are okay as the top one fires cylinder 1 & 3 and the bottom one fires cylinder 2 & 4. Fuel is non-ethanol and has been since the engine was new. Fuel is filtered with after market fuel/water separator system. I disconnected the electronic choke thinking it was malfunctioning and continuing to pump excess fuel but that did not solve the problem. Have checked all of the fuel line connects and everything seems in order. Over the winter, I removed the 4 carbs, inspected them and reinstalled them. All floats open and close the same. This problem began last fall which was the reason for the carb removal. However this spring the problem is still there.

[panasonic]

Did your inspection include disassembly and cleaning the carbs?

[rodbolt17]

if you clean the carbs they MUST be set with a 4 channel manomer or it never runs correctly.
\not a tough job typically takes me about 4 hours.

[boscoe99]

How do you inspect ****y tiny internal passageways within a carburetor that are hidden from sight and touch?

[99yam40]

I do not know but it looks like it will take at least 4 hours

[cleddau1]

Unlikely for both carbs to become a problem at the same time and in the same way. Fouling plugs on adjacent cylinders suggest loss of compression due to leaking head gasket between cylinders. Compression/leak down testing needed........

[panasonic]

Unlikely for both carbs to become a problem at the same time and in the same way. Fouling plugs on adjacent cylinders suggest loss of compression due to leaking head gasket between cylinders. Compression/leak down testing needed........

Good point Mr Cleddau.

[99yam40]

that motor shows to have a prime start enrichment system not a electric choke.
I believe disconnecting the electrical to it should make it keep sending extra fuel to all cylinders not stop it, if it is similar to the system on my C40

maybe Rod or Boscoe and tell us what #19 does on this parts break down and if it could cause 2 cylnders to burn badly
https://www.boats.net/catalog/yamaha...b/carburetor-2

[walleye1]

It has to be hooked up and engine running to close the prime start. [extra fuel]

[walleye1]

I charge an hour per carb to clean and sink, not counting parts.

[cleddau1]

Counterintuitively when no power is applied the Primestart is in the fuel enrichment, cold start position. Once power is received from the charging coil/alternator the needle retracts over several minutes to the warm running position and stays there as long as power is applied. If there is a cracked lead the Primestart will stay in the fuel enrichment position. If the Primestart itself fails it will most likely do so in the fuel enrichment position. Both Primestarts on 3&4 would have to fail to explain the problem, unlikely to happen simultaneously but quite possible sequentially, although I understand they are usually reliable. Very easy to test them.

[boscoe99]

Given enough time and heat from the engine the Primestart device could very well close without any electricity being provided from the lighting coil. Too many things that we need tested but don't have test motors available to us.

[boscoe99]

maybe Rod or Boscoe and tell us what #19 does on this parts break down and if it could cause 2 cylnders to burn badly
https://www.boats.net/catalog/yamaha...b/carburetor-2

The gizmo you are referring to is part of the accelerator pump mechanism. Provides additional fuel when the throttle is quickly opened to prevent a lean fuel mixture. Also acts as a dashpot when the throttle is quickly retarded. Here is what the training manual says:

This carburetor assembly uses an accelerator pump to ensure that the proper amount of fuel reaches all of the carburetors during rapid throttle openings, preventing temporary lean conditions.

As the throttle is opened, the throttle lever rotates and pushes the throttle lever link rod. The throttle lever link rod then pushes the diaphragm in the dashpot, forcing out the air. The air flows into the
carburetors and mixes with the fuel from the main jets. This air/fuel mixture is then injected into the venturi, ensuring that enough fuel reaches the engine.

As the throttle is closed, the throttle lever rotates back and the throttle-lever link rod returns causing the diaphragm in the dashpot to move back. As the diaphragm moves back, suction causes
the air from the venturi of carburetor #4 to flow through a one-way valve in the carburetor and back to the dashpot. The in-line, one-way valve prevents the air in the hoses attached to the carburetors
from flowing back to the dashpot. This also prevents fuel from flowing back into the air hoses attached to the carburetors.

Also, the diaphragm functions as a dashpot during quick deceleration preventing the engine from stalling. A restriction in the one-way valve at the carburetor prevents the air from entering the
dashpot too rapidly. This in turn slows the closing action of the throttle preventing engine stall when decelerating.

[99yam40]

well it sounds like it is dealing with air in those hoses, so that should not be a problem.

maybe the op can post up a picture of some of the plugs so everyone can see what kind of fouling they are getting.

what all can foul plugs?
too rich on fuel to air
oil
water
pics will help

[rodbolt17]

very likely both carbs are nasty. do a leakdown test to insure it can mechanicaly seal the do the carbs.
if the enrichment device faikls in the open position it runs two cylinders rich. the can be tested as per the SM for needle extension.