2-stroke intermittent running - why?

[Babylon]

My 2.5hp Mercury 2-stroke has started to run only intermittently. Was okay on half- to three-quarter power but would cut out at full chat, but now cutting out at only half-revs. It is a 2006 unit which had a full service a year ago, including clearing the blocked cooling channels in the head - water now flowing normally, so not a cooling issue. Fuel is all fresh this season, 1:50 oil as usual. When about to falter I can sometimes keep it running by working the throttle up and down repeatedly, but not reliable for use when anchored in windy conditions!

What is the likely cause of the intermittent cutting out? Water in fuel? Carburettor gummed up? Blocked filter somewhere?

 

[Stemar]

Fuel is the most likely culprit - crud in a jet, or a blocked vent in the filler cap, etc. However, I had an intermittent fault on my Tohatsu 3.5 2T that gave all the symptoms of fuel starvation, but wasn't cured by stripping the carb twice. I turned out to be corrosion on the earth terminal of the coil. When I remade the joint, all was well again.

 

[VicS]

Certainly sounds like a fuel related issue

Check fuel tank vent
Check filter ( on fuel tap ) sticking up inside the tank
Rinse out tank to get rid of ant debris or water

Check fuel flow to carb

Maybe carb needs cleaning .. if so applying a bit of choke may help and confirm.
Sticking float or needle valve maybe

Always worth trying a new plug when little 2 strokes play up even when apparently fuel related.


( BTW water flowing from the telltale only really confirms that the pump is pumping water up to the power head. It does not confirm full cooling flow through the engine. If any problem with cooling suspected check for over heating. It should be possible to touch and maintain brief finger contact almost anywhere on the powerhead . If not it's overheating. Steam from anywhere indicates overheating. )

 

[Babylon]

Thanks for the feedback. I don't think its an overheating issue, as it also happens when the engine is still cool; when warmed up with the choke off, the water flowing out from the little hose is warm rather than cold or hot.

Will work through the checks, starting with the plug (new last year and still very clean when checked earlier this summer), then proceed through the fuel system from vent through to carb needle.

Will also check coil negative - where would I find this on the engine?

 

[VicS]

Thanks for the feedback. I don't think its an overheating issue, as it also happens when the engine is still cool; when warmed up with the choke off, the water flowing out from the little hose is warm rather than cold or hot.
Will work through the checks, starting with the plug (new last year and still very clean when checked earlier this summer), then proceed through the fuel system from vent through to carb needle.
Will also check coil negative - where would I find this on the engine?

I assume the ignition system looks something like to diagram here Mercury Mercury & Mariner Outboard Parts by HP & Liter 2.5HP OEM Parts Diagram for CD Ignition | Boats.net
If so there are three earth points to check.
The exciter coil (#2) earths through its mounting
The CD unit ( #8) via an earth lead and its mounting screw
The HT ignition coil (#6) via an earth lead and its mounting screw.

If there is any suspicion that it is not firing all the time also check the stop/kill switch

 

[KompetentKrew]

When my two-stroke had this it was crud in the carb (in the jet?).

 

[SteveA]

I've had similar recently and it was the spark plug breaking down - try changing it

 

[TernVI]

Take the plug out after it's stopped.
If it's wet with fuel, look at the ignition.
If it's dry, look at the carb etc.

 

[Babylon]

Thx for the extra detail - feel better equipped to troubleshoot it systematically.

 

[Babylon]

I think the problem has been (easily) solved!

Opened the vent in the filler cap as fully as it would go, then motored around on all throttle positions without any faltering at all. BUT, from the fully open position, the vent screw could be clearly seen rotating down, i.e. closing itself due to vibration. As I'd only previously normally opened it a couple of turns or so, this was the clear culprit!

?

 

[Stemar]

 

[NealB]

I think the problem has been (easily) solved!

Opened the vent in the filler cap as fully as it would go, then motored around on all throttle positions without any faltering at all. BUT, from the fully open position, the vent screw could be clearly seen rotating down, i.e. closing itself due to vibration. As I'd only previously normally opened it a couple of turns or so, this was the clear culprit!

?

Excellent work, Mr Holmes!