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  • 1 Post By davidpm
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Old 06-25-2012
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Mercury 3.5 4 stroke

I've been fighting with a Mercury 3.5 4 stroke about 2 years old and we are pretty much at a barely acceptable stand-off.
When I picked up the motor the previous care-taker said he was never able to get it to run right. I looked at his shop and it was obvious he was a very experience gear-head so I dropped it off at the outboard repair facility.

They said the carburetors were flukey at best and they usually just replaced them which they did on this one.
It would start and idle well but stall on giving it gas.
I took it back and they adjusted it and I picked it up again.
Exact same behavior.
I took it back again and he adjusted it while I watched and "helped".
He said he did something he was not supposed to do breaking epa rules to make a screw work that would let the carburetor run a little richer. The screw is supposed to be sealed.
He adjusted the rich/lean screw and the idle screw and gave it back to me.

I actually built a testing tank so I could test it in comfort.
This model has a built in fuel tank.
The best anyone can get it now is to start well and idle OK. You have to play with the choke and throttle just right to get it to not stall for the first 30 seconds or so. I've finally got the touch, most of the time.
There is always a flat spot where if you just punch the throttle you can get it stall. But going too slowly will make it stall also. It is sort of a punch release feather type of motion.
The second issue is since it is a for sailboat tender I have unship it off the dinghy to the stern-rail then back to the dinghy. It is impossible to keep it straight up and down during this process. Once back on the boat it it really hard to start, I'm assuming it is flooded even though I turned off the fuel lever and shut off the tank.
How about if I shut off the fuel and tank while the motor is running and let it burn out? Is that a good idea.

Also any ideas about the general touchiness would be appreciated. Would higher test fuel be a good idea?
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Old 06-25-2012
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Re: Mercury 3.5 4 stroke

If it runs OK under full throttle conditions it may be some debris in the low speed circuit. But before you tear it apart you might try richening up the idle mixture screw about 1/4 of a turn at a time until it starts to roughen up a little. Then lean it down just to the point it smooths out. Generally, counter clockwise richens, clockwise leans. I try to get them to run as rich as I can without running rough. Make sure it is fully warmed up before making the adjustment and watch out for the EPA police looking over your back fence. Running the fuel out of the carb shouldn't hurt anything as would draining the carb if it isn't going to be used for several weeks or so. I doubt if a different grade of fuel will effect it. Until we have to have an annual smog check for outboards making your own adjustments probably won't land you in jail, maybe. Let us know how it comes out.

Paul T
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Re: Mercury 3.5 4 stroke

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Originally Posted by dabnis View Post
If it runs OK under full throttle conditions it may be some debris in the low speed circuit.

Paul T
It runs Ok idling and OK full throttle.
It is process in getting from one to the other where it likes to stall out.

As far as tearing it about, exactly what would I be looking for in a brand new carb.
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Old 06-26-2012
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Re: Mercury 3.5 4 stroke

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Originally Posted by davidpm View Post
It runs Ok idling and OK full throttle.
It is process in getting from one to the other where it likes to stall out.

As far as tearing it about, exactly what would I be looking for in a brand new carb.
I have had the same engine and it never missed a beat. Now I have a Suzuki 2.5 and it had the same problem you have from new, now fixed with a new carb.

I think the best explanation is that the jet is blocking up at least partially. Maybe there is debris in the fuel line that blocks up the jet, and even if you fit a new carb it just blocks up straight away. I have had precisely this issue on a car. In that case the debris was getting in via the crankcase ventilation system - it was full of grit.

Now assuming you get it running right, here are my tips to keep it that way :

1) Use ethanol stabiliser in the fuel.

2) Never use fuel more than 2 months old. Put in in your car when it hits that age.

3) Always run the carb dry.
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Re: Mercury 3.5 4 stroke

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Originally Posted by davidpm View Post
It runs Ok idling and OK full throttle.
It is process in getting from one to the other where it likes to stall out.

As far as tearing it about, exactly what would I be looking for in a brand new carb.
Mark's suggestions are good. About the carb, The jets in that size engine are very small. With one motor I had, a Honda 2HP 4 stroke, I had a similar problem and although I cleaned out everything in the carb I could never get it to run spot on. A new carb for about $100 solved the problem. You have nothing to lose by taking it apart and do a thorough cleaning and blowing it out. But before you do that you might try running a 50% mix of fuel and carb cleaner through it, let it set for a day or so, drain the carb, start with fresh fuel and see what happens. I have used Berryman's fuel additive for many years with good success. My problem was from letting old gas stay in the engine too long, my fault. All this assumes you adjusted the low speed needle as rich as possible without it running rough as described in my earlier post.

Paul T
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Old 06-26-2012
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Re: Mercury 3.5 4 stroke

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Originally Posted by MarkSF View Post
Now assuming you get it running right, here are my tips to keep it that way :

1) Use ethanol stabiliser in the fuel.

2) Never use fuel more than 2 months old. Put in in your car when it hits that age.

3) Always run the carb dry.
I have the same motor with similar performance problems. These 3 things helped.

1) I use Sta-bil

2) It does not like old fuel.

3) I try to close the fuel valve and the tank vent when we're done for the day. Letting the fuel burn out of the carb.

These motors don't like being turned upside down or even horizontal. Avoid it if you can.
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Re: Mercury 3.5 4 stroke

After leaving my Suzuki on it's side (the correct side, btw) for just one hour, the whole cylinder was full of oil and the engine wouldn't turn over at all - hydrolocked by the oil.

Removed the spark plug, cleaned it, let the oil drain out via the spark plug hole, and it started right up. Ran a bit smoky for a while, needless to say.

Yes they really don't like being on their sides.
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Re: Mercury 3.5 4 stroke

There is a special tool that is made to clean the jets of those small carbs. It is a necessity if you are going to clean them. I've soaked them and blown them out only to have the same issues. 90% of the time you can't see the spec that is clogging the jet.
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