[mikesands]

I could have pranged my boat today. Manoevreing slowly the engine coughed out , but fortunately I was pointed exactly in the right direction and was able to grab the pontoon rail to stop the bow hitting a stanchion. I've got to do something about this poor low revs runnning on the Mariner 60hp Bigfoot. I have now tweaked the minimum revs up to 900 instead of 600-700. This might help but it still often coughs out for no apparent reason. And I'm slightly nervous that there may be a maximum advised number of revs before putting it in gear.
Someone suggested a couple of weeks ago that I should use hotter running plugs. Anyone care to tell me why this may be a bad idea .... I guess it might put a hole in the piston running for long period at peak power ?
Mike

 

[alana]

its the plugs that run hotter not the motor. to many revs will make the gearbox suffer when you put it in gear . have you checked the amount of oil in fuel does it smoke a lot on tickover

 

[Afishionado]

Sounds like your slow running jets are blocked. Many carbs (if not all these days) have two sets of jets. One slow running and another as the revs pick up and the butterfly choke opens, that add extra mixture for fast running. Before fiddling about with hard and soft plugs (which could lead to a piston burn out) get your carb checked out.

Afishionado

 

[David N]

If it is not your slow running jets that are blocked. It may well be. But using a single grade hotter plug may prevent oiling up at low revs. I certainly won't burn a hole in your piston. The combustion temperatures do not change only your plug central electrode temperature.
2 stroke petroil mix is a much "colder" fuel to ignite than straight petrol. The oil mixed in is more reluctant to burn than petrol. Inefficient burning of the fuel occurs particularly at low revs and lower combustion chamber temperatures caused by less efficient cylinder scavenging of the unburnt fuel. Thus a fouled or "drowned" plug. My own old Evinrude uses a grade hotter than the manufactures recommended grade. The slow running performance is noticeably better. There is no difference at high revs except possibly a slightly better fuel consumption.

It would certainly do no harm to check your carb out and if that does not sort your problem and trying out the hotter plug option does not work, the engine might possibly need a good decoke?

 

[mikesands]

its the plugs that run hotter not the motor. to many revs will make the gearbox suffer when you put it in gear . have you checked the amount of oil in fuel does it smoke a lot on tickover

The oil is injected - it's a 60hp Mariner Bigfoot about 6 years old. It does smoke a bit - particularly when it coughs.
Mike

 

[David N]

That's because of the unburnt fuel not being ignited from the previous cycle.
Your injection pump may be the cause of your problem. Too much oil in the mix?

 

[ebgb]

just had almost the exact same problem on a mariner 50 4 stroke. runs get all day but when manoevering at low revs, cough cough splutter splutter. It was he choke butterfly on the bottom carb sticking, had the carbs stripped and cleaned and its perfect again now.

One thing the engineer asked me but I hadn't realised ws, had my fuel consumption gone up recently, don't know if your has but, if so it might be the same fault

 

[mikesands]

If it is not your slow running jets that are blocked. It may well be. But using a single grade hotter plug may prevent oiling up at low revs. I certainly won't burn a hole in your piston. The combustion temperatures do not change only your plug central electrode temperature.
2 stroke petroil mix is a much "colder" fuel to ignite than straight petrol. The oil mixed in is more reluctant to burn than petrol. Inefficient burning of the fuel occurs particularly at low revs and lower combustion chamber temperatures caused by less efficient cylinder scavenging of the unburnt fuel. Thus a fouled or "drowned" plug. My own old Evinrude uses a grade hotter than the manufactures recommended grade. The slow running performance is noticeably better. There is no difference at high revs except possibly a slightly better fuel consumption.

It would certainly do no harm to check your carb out and if that does not sort your problem and trying out the hotter plug option does not work, the engine might possibly need a good decoke?

It's just been serviced but I doubt they'll have stripped the carbs. Is it an easy job for me to do? I think I'll try the one grade hotter plugs first, as that would seem easier. I doubt it needs a decoke as It's always had this problem even when I got it and it had then done only 150 hours. The manual says NGK BP8H-N-10, I can find BP7H-S-10 plugs on line The N plug seems to have a 'strong ground electrode' whereas the S plug seems to mean 'standard copper'
Mike

 

[David N]

I would check your carbs first.
Afraid I don't know your competence in working with mechanical things. So difficult to advise if it is easy. I love tinkering and enjoy pulling things apart and putting them back together again. You may not?

ebge has posted he had a similar problem and sorted it out with a strip and rebuild of the carbs . So I would go down that route first.

I stripped and cleaned my carbs first to be sure it was running as it should before I changed the plug grade.

Just remember reassembly is the exact reverse of disassembly! Take notes of what goes where if you DIY.

 

[Nos4r2]

Before you take them off and disassemble them make sure you know how to balance the carbs properly when you reassemble.
You'll be a long way down on power if they go out of balance and it's not good for your engine.

 

[David N]

It's just been serviced but I doubt they'll have stripped the carbs.
Mike

Just noticed this part of one of your posts. I had not realised you had had the motor serviced.
Part of a service would have involved setting up your fuel system properly I assume?
I have never had an engine professionaly serviced so can't advise.
Someone will know what a service involves I'm sure?

If this problem was happening before and after your service, I would think you have good reason to complain to whoever serviced your motor and assuming it was tested that things are not right.:g:

 

[mikesands]

Yeah - I didn't complain because I'd come to the conclusion that all 2 strokes do this and if I wanted good low speed running I'd have to buy one of the new design engines ..... but when I thought about it, that can't be right or too many people will have bent their boats in low speed shunts.
Mike

 

[David N]

I can't speak for other owners 2 strokes but I have three that all run reliably at slower speeds. All of my ones don't run efficiently at slow speeds and prefer higher revs, but they don't stall or stop like your one. (Which is one of the reasons why I use the aux for trolling as we previously discussed)
When you open the throttle back up for higher revs they do smoke because of the unburnt fuel in the cylinder/s from the slow running but that clears after a bit and they run clean again.
I have no idea how long a warranty you would get on the service job. Has it been some time from the service? A few weeks? You probably can take it up with the people who did it if it has only been a short while.

 

[mikesands]

Service engineer visited the mooring today and reset all 3 of the idle mixture screws. That seems to have sorted it, but it's something he did on the service about 15 hours ago, so I'm not sure why they should have gone astray meantime. Anyway he told me how to do it for the mercury 60hp 2 stroke. Locate the 3 carbs -one above the other when viewed from the right hand side of the engine stood in the boat looking backwards. It's fairly easy to spot the idle screws on the sides of the carbs facing you.
Wind the screws fully home then back one and a quarter turns. Start her up, and now adjust each carb in turn by turning the screw slowly in (clockwise) until the engine just begins to bump and roughen. Now back off the screw a quarter of a turn. Do the same for each of the other two carbs and it should run fine on idle. Set the idle speed to 750 to 800 rpm. It's blatantly obvious how to do this if you blip the throttle and watch which cam moves.
Hope I got the instructions right. I'll know how to do it myself next time and he only charged me £15 which must have hardly covered his petrol.
Mike