What is causing the problem with our 3.5hp Mercury 4 stroke?

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Oscarpop

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Our outboard has a problem. When you turn the throttle from idle upwards the engine dies, unless you do it really really slowly.

Have stripped the carb, changed the fuel and changed the spark plug. Still happens.

Any ideas?
Poor quality fuel?
 
Jets cleaned. Totally patent . Use a piece of thread .
No i didn't change the gaskets. Little chance of finding them out here without having to sell a kidney
 
It certainly sounds like a carb problem, either a partially blocked jet, an incorrect fuel level in the float chamber, a fuel flow problem or a mal-adjusted jet, if they are adjustable.

The best way to test it is to get someone to squirt some fuel into the air inlet, whilst you whip open the throttle. If the engine responds properly for a few seconds then you can rule out an ignition issue and concentrate on the fuel system.

Richard
 
Oscarpop said:
Our outboard has a problem. When you turn the throttle from idle upwards the engine dies, unless you do it really really slowly.

Have stripped the carb, changed the fuel and changed the spark plug. Still happens.

Any ideas?
Poor quality fuel?
Click to expand...

I would think its a carb or fuel flow problem. May have to strip and clean the carb more thoroughly

Check the float level and that the little clip between the float and the valve is in place.

If you have any means of checking the spark while running eg a timing light it might be worthwhile monitoring that
 
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Normal starting and dying on larger throttle openigs are classic symptoms of not enough fuel getting through the main jet. The usual cause is the jet itself being blocked, but if that's ruled out, something else is obstructing the fuel flow. Work through the system methodically as Vic advises.
 
I have had similar problems on the high jet of a Honda. I tried carb cleaner , compressed air, ultrasonics etc to no avail.

Close inspection of the high jet orifice with a magnifying glass showed a layer of hard calcium type deposit lining the jet and sufficient to constrict it slightly.

I used a strand of copper wire to pull through the jet to clear it and have not had a repeat of this problem for the last 12 months.

I suspect this was corrosion of the carburetor metal bowl that takes place even if you routinely run the engine dry .
 
catmandoo said:
Id you check the fuel tank , has it an integral or external filter . Is the fuel in it last years or fresh from the pump ?
Click to expand...

IThe parts diagram does not show a filter at all but maybe one inside the tank
 
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Not adequate fuel flow which it means that your main jet is partially blocked or the fuel filter which is in-line and located behind the fuel shut-off valve. Carb jet blockage is the most common problem with these small outboards.
 
Almost certainly a partially blocked jet. I bought some carb cleaner from my local car spares place, stripped the carb and soaked for 3 days agitating the liquid every time I passed. Cured my problem - but it did take 3 attempts!
 
TSB240 said:
I have had similar problems on the high jet of a Honda. I tried carb cleaner , compressed air, ultrasonics etc to no avail.

Close inspection of the high jet orifice with a magnifying glass showed a layer of hard calcium type deposit lining the jet and sufficient to constrict it slightly.

I used a strand of copper wire to pull through the jet to clear it and have not had a repeat of this problem for the last 12 months.

I suspect this was corrosion of the carburetor metal bowl that takes place even if you routinely run the engine dry .
Click to expand...
Youve got it Steve, classic signs of main jet blockage. Ive used a wire pulled out of a wire brush to clean main jets since I was an apprentice. Never buggered main jets as the so called experts say!
Stu
 
-- The filter is a strainer in the tank. it is a VERY good idea to add a little lawn mower filter in-line on this engine. Really reduces problems. Do it TODAY.

--No fuel pump. Gravity.

--I don't think these come with a remote option.

Having lived with this motor, it is almost certainly the main jet or the channel leading to it. Tank strainer is possible. Also bad fuel; they will get like that if the gasoline/ethanol have separated. So dump out the tank (only holds ~ 1 liter), remove the strainer and clean, install the lawn mower filter, and try again. You will have ruled out much.

The key to avoiding bad fuel is to close the vent and fuel valves every time it is not running. Really helps keep the moisture out.
 
I have the same outboard as you. Mercury 3.5hp. It wasn't used for a good while and must have got some rain water or moisture in and the needle float valve corroded. Replaced that and it was fine again. Cost about £15.

Symptoms beforehand were the same as yours.
 
Friend bought one of these and had persistant problems with fuel. The second time we pulled it to bits, I found some fibrous material around the inline filter which was not properly 'snapped on' to the upstream side of the fuel tap. Some of this had got passed and caused problems with the float valve and jets. Seems it and the miss-assembly had got passed quality control at the factory.
 
Sound like its running weak, If you have'nt tried fresh fuel do that first. Pump fresh and not been unopened for 6 months fresh. If that doesnt cure it check the inlet / carb gaskets. If that doesnt solve it its either low fuel level in the bowl, or a narrowed main jet caused by fuel residues. It will be one of those problems
 
VicS said:
IThe parts diagram does not show a filter at all but maybe one inside the tank
Click to expand...

I need help with the following: - I went ahead and cleaned the carburator, main jet, set screw, etc - all of it - and it starts right away at low setting, but the second I try to rev the engine, it just dies. I am able to run the engine in forward also only in the lowest setting.

I read somewhere the set screw is 1.5 turns - I have tried several things without success. I have not soake the main jet and main nozzle, but I have use plenty of carburator cleaner on it. I have not tried yet to clean the low air jet - i do not know how to paste in a message but is the one in front of the choke valve
 
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