FT9.9 1998 AEX European Running Rich

[cleddau1]

Starts fine but sooty exhaust, misfiring, plugs sooting up, automatic "choke" ok, carb checked and cleaned, mixture adjustment no effect, compression 110psi both cylinders, leakdown test valves ok but air going into sump and up oil channels in head.
Obviously have worn cylinders/piston rings but compression does not seem low enough to cause problem. Any advice before I start stripping down the motor.

[99yam40]

Starts fine but sooty exhaust, misfiring, plugs sooting up, automatic "choke" ok, carb checked and cleaned, mixture adjustment no effect, compression 110psi both cylinders, leakdown test valves ok but air going into sump and up oil channels in head.
Obviously have worn cylinders/piston rings but compression does not seem low enough to cause problem. Any advice before I start stripping down the motor.

what do you mean by automatic Choke OK?
prime start system ?

too much fuel could be from that system
carboned up rings are a possibility from what you are saying.
I think 4 stroke compression is a lot higher than 2 stroke motor
110 is OK for a 2 stroke

[cleddau1]

The F in FT means 4-stroke. Yes Primestart but have checked that and it is ok.

[robert graham]

Is primestart in the “normal” position?…..try turning primestart in the “off”position and running the motor….if rich mixture stops the primestart could be the issue. Check fuel pump for bad diaphragm, which can cause a very rich condition. Is your fuel fresh? ….old gas can cause carbon…Check the air intake passage in your upper cowling….bugs/birds nest can plug it up and cause rich mixture….

[cleddau1]

The cold start mechanism is a thermomechanical device, might not be called Primestart, have checked with a 12v supply and needle moves correctly. The power supply to this device from the engine is ok also now.

[boscoe99]

The cold start mechanism is a thermomechanical device, might not be called Primestart, have checked with a 12v supply and needle moves correctly. The power supply to this device from the engine is ok also now.

That is exactly what Yamaha calls it.

Any Yamaha outboard motor that has automatic fuel enrichment for a cold start condition (instead of choke plates) is called PrimeStart. Two stroke carburetted, two stroke EFI, two stroke HPDI, four stroke carburetted, four stroke EFI, it does not matter.

[99yam40]

OK so lets see what can cause an overly rich condition.

Float or needle and seat allowing too much fuel into the carb bowl maybe,
but I would think that would cause fuel to flow out of the bowl vent,

bad fuel pump leaking extra fuel into crankcase if this motor has a pulse driven pump.

prime start system allowing too much fuel for too long.
is there a way to turn off the prime start system on this motor?
( My C40 has a switch to turn to 3 different positions)
is there a diaphragm involved that could go bad?
could someone change the jet /orfice that would cause to much fuel to flow?
does the op know if 12 volt power gets to the system while motor is running?

could someone modify the carb jets causing too much fuel to flow?
(someone drilled the main jets in my C40 before I got my hands on it)

maybe something restricting the air flow acting like an old choke plate.

anything else come to mind of others reading this thread?

[cleddau1]

Have checked float and replaced needle; 12v going to Primestart and needle retracts ok. If fuel enters oil via a bust fuel pump diaphragm then how does it get to the cylinders? Engine has been out of my possession on only one occasion since bought new and that was 2 years ago when I sent the carb only to the dealer for an ultrasound clean and service because of poor idling. Any comments on the compression and leakdown? Odd that the mixture control screw makes no difference except when screwed almost right down or almost right up.

[99yam40]

Sorry my thoughts on leaking diaphragm into crankcase were on a 2 stroke motor, forgot yours was a 4 stroke.
you did not say if there is a way to shut off the enrichment system on this motor

[cleddau1]

Not possible to run engine with Primestart device removed, if wiring disconnected it will enrich the mixture even more.

[cleddau1]

This the type of carb

[99yam40]

my C40 has a red lever with 3 positions auto, open ,& off
so I can turn the system completely off so it does not add and extra fuel.
I was just trying to see if yours has something similar

[cleddau1]

Rodbolt Panasonic where are you?

[99yam40]

Rodbolt is long gone, but I have hopes for Pan showing up

[cleddau1]

Rechecked compression as forgot to do it with WOT last time, values 150psi in both, ignition timing ok. Checked and reset tappets. Primestart Starter Set moves 9mm or 0.354inches with 12v applied. Have stripped and cleaned carb. Engine running better but still seems rich. Still suspect carb problem.