Marina 6HP Fourstroke Cutting Out

[gandy]

Hi,

This outboard is around four or five years old, owned from new. It's a 6HP four stroke, with external fuel tank. Fuel tank was emptied this winter (old fuel's in the lawnmower) and 10L of new petrol stuck in.

Just this season it's started cutting out after a few minutes of running, sometimes it falters very briefly before cutting out. It will restart in a few pulls almost straightaway, but dies again in somewhere between a few seconds or a minute or so. I'm thinking fuel, mainly because if it was ignition then I'd expect some misfiring and maybe difficulty starting, or misfiring at higher revs. However the symptoms are a little odd, for example once restarted it doesn't seem to last longer at idle than if it's given more power, and pumping the primer bulb doesn't seem to help.

I've got the outboard home and stupidly left the fuel tank at the yard so I can't test the whole set up. Inside the cowling there's the connector for the fuel line, then hose to an inline filter then hose to the pump, then to the carb. If I connect a length of pipe to the filter and stick that in a petrol can, the motor starts readily and runs, even without being able to prime, meaning it needs to suck the fuel up all on its own. I've run it for maybe 10 minutes or so at fast idle, but need to find a bigger tank or bucket before I can put it in gear and run on load. However so far the fuel system within the motor itself seems to be fine.

Really I'm looking for suggestions for known trouble areas. For example could a dodgy coupling on the fuel line (that I've left at the yard) cause this sort of problem, by drawing in air for example.

Or any other checks I should make?

Thanks,
Tony S

 

[steve yates]

I had very similar to this after the ob was serviced. Turned out that the central jet in the carb had not been screwed in properly and had just dropped out. Engine would run, then suddenly stop. Always started again no problems, but lacked power.

 

[gandy]

Cheers, I don't think ours is down on power but I'll add that to the check list.

 

[VicS]

Hi,

However so far the fuel system within the motor itself seems to be fine.

Really I'm looking for suggestions for known trouble areas. For example could a dodgy coupling on the fuel line (that I've left at the yard) cause this sort of problem, by drawing in air for example.

Or any other checks I should make?

Thanks,
Tony S

Yes it could easily be drawing air in.... the connector(s) would be the prime suspect(s) due to a bad O ring. The O ring(s) may be replaceable . If you have to buy a complete new connector get a genuine Mercury/ Mariner one as some of the aftermarket replacements are very poor.

If air inleakage ie the problem you should be able to keep the motor running by pumping the priming bulb constantly.

Check the tank vent is opening properly ( take the cap off) Also check the fuel line over generally

 

[Fmurchu]

I'd give the carb a good cleaning first.

 

[VicS]

I'd give the carb a good cleaning first.

Simple things first. Tear into carb later if necessary

Other simple checks could include checking for a good spark that will jump best part of 1 cm.
and
Compression check ( 42psi ±14 psi )

 

[GrahamM376]

Many 4 strokes have sump oil level sensor and will cut out if oil low. Engine may run OK when upright but cut out when tilted a bit.

 

[geem]

On that engine there is a tiny filter in the fuel cock. It will be blocked if you have never cleaned it in six years. The symptoms you describe are identical to the ones we experienced

 

[gandy]

Cheers all. I don't want to rule out anything out of hand, but I'd be surprised if there's much crap in the carburettor as the filter is very clean, and nothing except clean fuel came out when I drained the float bowl.

 

[gandy]

On that engine there is a tiny filter in the fuel cock. It will be blocked if you have never cleaned it in six years. The symptoms you describe are identical to the ones we experienced

There's no fuel tap, ours just has the connector to the external fuel tank, and an inline filter in the pipework before the fuel pump. Looking at the manual it doesn't appear there's a filter in the pump.

 

[Topcat47]

Don't forget your Starter for 10, the breathe hole in the filler cap.

 

[VicS]

Don't forget your Starter for 10, the breathe hole in the filler cap.

See #4

 

[gandy]

Check the tank vent is opening properly ( take the cap off)

Don't forget your Starter for 10, the breathe hole in the filler cap.

Oops, that is one thing I didn't check on the water. I don't normally close the vent on the tank, but while carting it backwards and forwards and getting fresh fuel then I may well have done so. I don't know if I should be pleased or annoyed if it turns out to be so simple.

 

[gandy]

So, no wiser I don't think. Heading out to the boat I was 100% sure I was going to find the tank vent screwed shut. It seemed that would explain exactly what I found, running for a few minutes before cutting out, then restarting OK but only running 15 or 20 seconds before cutting out again. However the vent was fully open. The only test I could think of doing onshore was to connect up the fuel tank, pop the fuel line off the carburettor and try the priming bulb - that gave a plentiful squirt of fuel suggesting fuel line and filter and pump are all clear of blockage. I know the pump works because it ran at home without priming, so the pump had to do all the sucking up. So other than an internal problem with the carb, I can't see where an issue could lie in the fuel system.

On the water today, no problems at all. It's only a short run to and from the boat but bear in mind that last weekend I had to restart at least 5 times on the way over.

So the only thing I've positively done is fit a new spark plug. Could that have been the cause? I'm tempted to stick the old one back in and see.

 

[VicS]

So, no wiser I don't think. Heading out to the boat I was 100% sure I was going to find the tank vent screwed shut. It seemed that would explain exactly what I found, running for a few minutes before cutting out, then restarting OK but only running 15 or 20 seconds before cutting out again. However the vent was fully open. The only test I could think of doing onshore was to connect up the fuel tank, pop the fuel line off the carburettor and try the priming bulb - that gave a plentiful squirt of fuel suggesting fuel line and filter and pump are all clear of blockage. I know the pump works because it ran at home without priming, so the pump had to do all the sucking up. So other than an internal problem with the carb, I can't see where an issue could lie in the fuel system.

On the water today, no problems at all. It's only a short run to and from the boat but bear in mind that last weekend I had to restart at least 5 times on the way over.

So the only thing I've positively done is fit a new spark plug. Could that have been the cause? I'm tempted to stick the old one back in and see.

I dont think what you have tried has eliminated the possibility of air being sucked into the fuel line at , for example, a bad connector. If this is the problem repeated squeezing of the priming bulb will keep it going.

Maybe just a dodgy plug but symptoms suggest a fuel issue.

 

[gandy]

Maybe just a dodgy plug but symptoms suggest a fuel issue.

I agree - it doesn't "feel" like an ignition issue. The connector is still a concern, it's not particularly positive. I might replace it just as a precaution. Last Sunday I did try the priming bulb to see if that would keep the engine running, without complete success but I could persuade myself it did help. I guess another possibility is that the vent was blocked, and checking to make sure it was open cleared the blockage in some way.

 

[gandy]

Back again. The outboard behaved perfectly after replacing the spark plug, until this Saturday when it started misbehaving again in a similar but not quite identical fashion. This time it cut out when running at full power and took quite a lot of attempts to restart. Once running it would continue for a while, then cut again and could be restarted relatively easily. When running I could hear and feel it falter now and then. As before pumping the primer bulb didn't make any difference, and in fact is was very firm suggesting that the engine had all the fuel it needs.

So next weekend it's getting a new spark plug and we'll see if it does the trick again. If not I'll need to bring the engine and fuel tank home for more investigation.

Anything else worth checking at this stage? I know the possibility of air leak was discussed but I'm not sure the best way to check that. There is always an air space in the filter, I don't know if you'd expect that to be completely full. I must say that other think like our mower always show air (or vapour) in their filter as well.

Thanks, Tony S

 

[Martin_J]

What did the used spark plug look like when removed.... was it oily looking or clean/normal?

I'm just wondering if you have topped up and put too much 4-stroke oil in recently..

 

[gandy]

Good question. Back in 2017 the plug looked pretty normal. This time I haven't yet checked. And yes the oil has just been changed, although I don't think it was over filled. Both will be checked this weekend.

Do you know if the engine oil level is supposed to be checked with the dipstick fully screwed in, or just resting on the threads like Honda motorcycles? The manual does not say. If it turns out to have been over filled, I can suck a bit out of the sump to resolve the underlying issue, and I assume that running the engine for a few minutes will clear out anything left in the combustion chamber. Or is there some other step I should take?

 

[Martin_J]

Screwed in or not.. Good question. I know my Yamaha motorbike is a sight glass (for level), the Volvo Saildrive is with the dipstick resting in place, but the Yamaha 4 stroke outboard is with the dip stick fully screwed in - madness!

A previous ybw thread says it needs to be screwed in...

dipstick-marks-mariner

As to sucking it out of the sump. I changed the oil on a second hand 6hp 4 stroke yesterday. The day before I tried reducing the oil level bit by bit with a short tube and finger over the end to create a vacuum. Took ages at a teaspoon a go.... I'll keep a syringe on board next time.

When I actually drained the oil via the sump drain plug, 800ml came out. 200ml more than the capacity of 600ml. No wonder it ran badly when I tried it previously! Oily plug might have been a giveaway.