Outboard fuel starvation or is it?

wombat88

wombat88

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I have a 6hp Tohatsu with external tank. Serviced this year but without tank and line.

The engine starts quite easily and runs well at first, idling nicely. Given any amount of throttle it will run without missing a beat for hours and hours. Drop back to idle afterwards and it will start stalling and needs choke to start when hot. Great when going through locks into marina berths.

Based on the above I assume fuel starvation. Do outboard fuel pumps become less efficient when hot?

Before I rush off and spend money buying another fuel pump is there anything else I should try?

The fuel tank is approx a foot below the motor, the fuel line is quite long...perhaps I should try experimenting with the tank higher up but apart from cluttering the cockpit if it did work what is this telling me?

Any suggestions most welcome.
 
Instead of the choke, try squeezing the primer bulb?
If the pump is the problem, filling the carb via the primer bulb should start the motor with no choke.
If the pump is working properly, then the bulb should go firm with very little squeezing.

Yes it's possible the pump is letting the fuel run back at low rpm.
Could be a valve not sealing, an air leak etc.
 
I agree with the above 2 suggestions

Squeezing the bulb or rising the tank abov the motor should overcome a defective fuel pump and fuel line air leaks ( the connector is the most likely place for these)

Squeezing the bulb also gives you an immediate short term solution.

The bulb should go firm as soon as the carb bowl is filled if its valves are good and there are no leaks. It will not remain firm while running but if it is sucked partly squashed then check the tank pickupt for blockages and that the vent is open properly.
I take it you have checked the fuel filter(S). Inline between pump and carb, and/or in tank pick up .
 
Indeed, filters checked etc. I will try the suggestions and see, many thanks.

The fact that fuel starvation/stalling at idle happens when the engine is hot suggest to me that the pump starts to fail or become inefficient when hot and when at idle hardly pumps at all.

Am I making sense?
 
wombat88 said:
I have a 6hp Tohatsu with external tank. Serviced this year but without tank and line.

The engine starts quite easily and runs well at first, idling nicely. Given any amount of throttle it will run without missing a beat for hours and hours. Drop back to idle afterwards and it will start stalling and needs choke to start when hot. Great when going through locks into marina berths.

Based on the above I assume fuel starvation. Do outboard fuel pumps become less efficient when hot?

Before I rush off and spend money buying another fuel pump is there anything else I should try?

The fuel tank is approx a foot below the motor, the fuel line is quite long...perhaps I should try experimenting with the tank higher up but apart from cluttering the cockpit if it did work what is this telling me?

Any suggestions most welcome.
Click to expand...

Quick answer, air vent in fuel tank cap, bit of rubber under screw is blocking hole, engine uses fuel to produce a vacuum and fuel stops running to engine.

Brian
 
wombat88 said:
Indeed, filters checked etc. I will try the suggestions and see, many thanks.

The fact that fuel starvation/stalling at idle happens when the engine is hot suggest to me that the pump starts to fail or become inefficient when hot and when at idle hardly pumps at all.

Am I making sense?
Click to expand...

Yes but the checks suggested will confirm a bad fuel pump or eliminate it from the equation if it is OK

Taking the cap off the tank is the simple way to check the vent.

I assume the engine is a 4 stroke but you have not said, nor have you told us the year model ITYWF that fuel pumps on 4 strokes are mechanical operated from the cam shaft not from crankcase pressure-vacuum pulses like two strokes so dont suffer from quite the same problems.
 
wombat88 said:
Thanks Brian, will double check that one!
Click to expand...

Had it years ago outboard just serviced, half way to boat engine starts to stutter, then dies slowly, row the other half of the way. Take cap of check for petrol no problem, half way to shore engine dies, row to shore, check fuel pull cord engine starts, it the twigged.

Brian
 
Before you start spending any money - make sure you have decent quality fuel.

- always buy 98 octane for outboards, whether 2 or 4 -stroke. Lower octane fuels have too much water in them to work reliably in marine environment. The higher octane fuels also reduce build up of waxy hydrocarbons in carburettor.

- never store your fuel in plastic containers! These allow higher octane fractions to evaporate over time, and add to waxy hydrocarbons. Store fuel in good quality metal jerry cans.

- Fill your outboard tank via a filter-funnel which removes debris and water.

- Stop your outboard by turning off the fuel supply so that the carburettor float bowl fully drains.

If after doing that you still have outboard problems then maybe you have a pump problem.
 
wombat88 said:
Indeed, filters checked etc. I will try the suggestions and see, many thanks.

The fact that fuel starvation/stalling at idle happens when the engine is hot suggest to me that the pump starts to fail or become inefficient when hot and when at idle hardly pumps at all.

Am I making sense?
Click to expand...

It's a possibility, but I would not rush to pull the pump apart before eliminating other things.
Outboard fuel pumps seem to always have fragile diaphragms in them that tend to get damaged on dismantling.

When it won't start, whip a plug out.
Is it damp with petrol?
Is it the exact correct plug?
If it's a 2 stroke, IMO, you want NGK plugs.

I'd also take the float bowl off and empty it into a jam jar or something.
A drop of water in a carb jet has a dozen different ways of messing you about.
 
Just been down and done some checking.

Behaviour impeccable when cold and slightly warm.

When Hot:

With the tank in the original position fuel can be seen in the inline fuel filter when under load. When idling it dwindles away.

With the tank raised the filter is still fuller under load than when idling.

Fuel pump I'd say???
 
Filter is before or after pump?
The air space in the filter should not be getting any new air from anywhere.
If it's after the pump, the pump should pressurise it a little, shrinking the air space, but only by a few psi, so the air gets maybe 20% smaller if that?

I would be thinking about air leaking into the suction side.
When the float bowl is full, can you disconnect the feed to the carb and direct it into a jar or something. Start the engine, you will see if the pump works. Get rid of the air, see if more appears.
Check all hose clips and joints on the tank side of the pump, back to the tank.
Check the o ring on the fuel connector at the engine.
Sometimes these things leak under vibration.

If you have a suitable tube, try running straight from the tank to the pump inlet.
 
I have a Tohatsu. I find that the original hose has a finite life. In the tropics this is about 2 years then the hose starts to crack. I suspect that you may have a hose with cracks. Inspect carefully and work the hose at the joints. The most likely culprit is the joint closest to the engine.
 
Some hairline cracks in the cheapie plastic fuel filter look suspicious so I've binned it and all replaced the hose so we'll see how it goes.
 
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