4-stroke slow running problem

[Inselaffe]

Murphys!
After running my 1995 4-stroke Yamaha 9.9 HP for half an hour in a bucket at the farm last week, and pleasing myself with it's sweet-as-a-nut performance, it wouldn't run at low revs at launch (with a big que of boats behind us for the hired road crane!) on the weekend, and when it died became very difficult to (hand) start! Made my first experience of handling a boat under power somewhat interesting but we got there in the end, if not with an '11' for style!

-I can smell it's too rich, and the plugs were black,
-If we could get it to start it had to be with full throttle,
-The 'automatic choke' seems to be pulling down and off as the motor runs as it should do,
-When it runs at higher revs it runs very smoothly,
-We had initial problems with starting last week on dry land, but a strip down and cleaning of the carb sorted that, the float was not full of fuel,
-When we had real troubles restarting we got it to start again by draining the float chamber.
-The motor is in a well but this is totally open and has plenty of air. However, it did seem to run a little bit better without the cowling on, although I guess this is just because it is running too rich, not because of air starvation.
-There appears to be only a wire grill air filter


I guess it's the float needle valve and will clean and check the carb again ,but I'd be most grateful for any comments or suggestions that might give me other ideas that I have missed.

Also, I couldn't find a mixture adjustment screw like I'm used to on cars and the explosive diagram in my Clymer outboard manual doesn't show one. Is this right or have I missed the glaringly obvious (once again)?

Thanks

Leigh

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[andyball]

Mybe your choke is different from the one we had on a later engine, but it was still a manually operated choke, but with a spring loaded bypass (like lots of cars) so it didn't run so rich at higher rev's.

Your description "The 'automatic choke' seems to be pulling down and off as the motor runs as it should do," makes me wonder if the actual choke is still on.

Just a thought?


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[boatless]

If it ran well last season, it's unlikely that any settings should need adjusting. The fact that it starts when you emptied the float bowl could point to water in the fuel. Drain the bowl again, but into a jar, and see if you can see any water in the bottom.

And before anyone says "boatless always says that". it's only the second time.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[Inselaffe]

The choke is a diaphragm above an extra channel that gives rich fuel to the cylinders. when the motor gets going the diaphragm pulls down due to the flow in the fuel rich channel and cuts off the rich fuel supply (I guess it doesn't cut it off completely as if it did then the flow is stopped completely and there is no downwards force).

You can fix the operation of this choke by a clip that hold a bar attached to the diaphargm (you can hold the fuel rich channel open or closed). We tried this but it didn't seem to make any difference)

But as the motor starts it does pull down, so I would say the choke is off.
However, all of this was done under slightly pressing circumstances so I'll check it all out again..

Thanks!

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[Inselaffe]

Don't know if it ran well last season, boat new to me!

It did run well on the trailer though, I had a thought (ouch!) - since I ran it I changed the trim angle slightly to keep the leg away from the front of the well, also the boat presumably sits at a different trim on the water to on the trailer, if the float level is near a critical point could this change push it over the edge so to speak? How sensitive is the float level on a boat motor to trim etc changes?

If water in the fuel then why are the plugs black and I can smell it's rich and then it runs ok with higher revs? Also the fuel is new in the can.
But a good possibility to eliminate, I will check for water in the float chamber thanks!



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[boatless]

Wouldn't have thought trim should make any difference. Main jet is usually (roughly) in the the middle of the bowl.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[andyball]

They're not that sensitive to trim....It hasn't been tilted right up with the fuel turned on? (if float valve leaks, then might be petrol in the oil)....I guess not, if it's in a well.

I can see how vacuum will pull the diaphragm down, but is there no temperature dependent part, or it'd come on full when you close the throttle at speed,surely?.





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[steve28]

How is the engine fitted to the boat, If its in a well it may be choking itself or it could be that there is to much back pressure on the leg as it was too far in the water.

steve

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