Mariner 8hp 4-stroke outboard stumbles (bogs down) when revved from idle.

[fredrussell]

My 2009 4-stroke Mariner 8hp has a flat spot that developed last season. Its at low revs, when going from idle/neutral and into gear as throttle opened. It happens less when engine is on choke but is still there. If you throttle engine rapidly from idle it'll stall completely, but if you 'nurse' it up the rev range you can get past the flat spot. Mid to high rev running is spot on.

I'm going to strip the carb tomorrow as my hunch is its a carb thing. The fuel filter is spotlessly clean. Plugs are very clean too, in fact the whole engine is fairly pristine be honest. For my sins I used to refurbish old VW engines so I know the principles of a carb fed 4 stroke engine. On the engines I used to work on a fault such as this would most commonly be caused by the accelerator fuel pump failing to give a squirt of fuel as the throttle was opened. This will be my first line of enquiry upon removing carb but if anyone has any wisdom to share on such an issue, I'd love to hear it.

 

[RichardS]

I'm also familiar with carb fuel pumps but I don't think that small 4T outboard engines have pumps although the symptoms are similar.

However, I think you're on the right track and a thorough carb clean and check will probably solve your problem.

Richard

 

[thinwater]

Please look in the bowl for evidence of corrosion/pitting. Ethanol in the petrol tends to increase this, particularly if just one drop of seawater finds it's way in, and the corrosion products are great at clogging jets. Since they are inorganic, they are hard to dissolve with most cleaners.

How common is ethanol in petrol in your area?

 

[VicS]

My 2009 4-stroke Mariner 8hp has a flat spot that developed last season. Its at low revs, when going from idle/neutral and into gear as throttle opened. It happens less when engine is on choke but is still there. If you throttle engine rapidly from idle it'll stall completely, but if you 'nurse' it up the rev range you can get past the flat spot. Mid to high rev running is spot on.

I'm going to strip the carb tomorrow as my hunch is its a carb thing. The fuel filter is spotlessly clean. Plugs are very clean too, in fact the whole engine is fairly pristine be honest. For my sins I used to refurbish old VW engines so I know the principles of a carb fed 4 stroke engine. On the engines I used to work on a fault such as this would most commonly be caused by the accelerator fuel pump failing to give a squirt of fuel as the throttle was opened. This will be my first line of enquiry upon removing carb but if anyone has any wisdom to share on such an issue, I'd love to hear it.

This carb looks quite a complicated device for a small outboard carb.

You might be glad of the workshop manual http://boatinfo.no/lib/mercury/manuals/8-99.html#/96 assuming this describes your carb

 

[simonfraser]

Sound like a blocked jet

 

[fredrussell]

Reporting back. The carb was stripped and there was a very minor amount of sediment in the float bowl. The accelerator pump was working fine but some of my jets were a bit furred up. The main jet, marked as a 71 would not allow my .70 micro drill through. The air corrector jet was marked as 37, and again, a 35 drill would not go through by hand. Once these were drilled out, she ran like a goodun. No flatspot.

Thinwater, not sure of prevalence of ethanol over here to be honest, but as you suggest whatever was blocking jets took more than carb-cleaner to clear.

VicS - cheers for the link, saved for later.

 

[thinwater]

Reporting back. The carb was stripped and there was a very minor amount of sediment in the float bowl. The accelerator pump was working fine but some of my jets were a bit furred up. The main jet, marked as a 71 would not allow my .70 micro drill through. The air corrector jet was marked as 37, and again, a 35 drill would not go through by hand. Once these were drilled out, she ran like a goodun. No flatspot.

Thinwater, not sure of prevalence of ethanol over here to be honest, but as you suggest whatever was blocking jets took more than carb-cleaner to clear.

VicS - cheers for the link, saved for later.

My understanding is the UK petrol is ~ 4.7% ethanol by law, and there is some question as to whether UK is going to follow their EU commitment to move to 10% this year.

What color were the deposits? I think one of the reasons they are becoming more troubling is that they are a mix of gum and corrosion.

What are the popular petrol additives? Lucas makes several, and there is Soltron. Others?

 

[gordmac]

I believe super unleaded isn't so contaminated with alcohol, might be worth using it for applications where fuel lies about in engines?

 

[VicS]

I believe super unleaded isn't so contaminated with alcohol, might be worth using it for applications where fuel lies about in engines?

Some brands may be ethanol free....... the difficulty is finding out which !

 

[PotatoNavigator]

New ignition leads.

 

[thinwater]

I believe super unleaded isn't so contaminated with alcohol, might be worth using it for applications where fuel lies about in engines?

I would NOT leap to that conclusion; ethanol boosts octane. This is NOT the case in the US--super contains just as much ethanol, where it is used. Always has. There are ethanol free areas, but it has nothing to do with product grade.