4hp two stroke outboard

[corinthmistral]

Grateful for thoughts on the following. I have a 4hp 2 stroke mercury short shaft outboard, 2003 model. This season and last, I cleaned the plug (new plug) drained the old fuel from the tank and the carb and refuelled with new fuel, mixed about 80:1. Appreciate it should technically be 100:1 but I prefer to run it a bit rich rather than risk not giving it enough oil. (A few years ago the Yamaha dealer at the boat show actually said their tests demonstrated 60:1 was the optimum for good running vs engine wear.) Anyway ... It starts and runs perfectly every time. I used it on our tender today (and a few times last year) and after it has run flat out for about 5 minutes the engine cuts out. If I leave it for a couple of minutes it restarts and runs ok again. It always runs fine for prolonged periods up to about 3/4 throttle. I'm just trying to work out what is causing this. All fuel pipes are ok and no obvious air leaks etc. engine runs smoothly and properly (other than when it cuts out per above). One thought I have is that the spark plug is an incorrect temperature rating (I think they have different ratings?) and at prolonged high revs it gets too hot and cuts out and then restarts ok after a couple of minutes when the plug has cooled down again. Grateful for any ideas.

 

[rbmatthews]

Any possibility that the engine itself could be overheating (as opposed to the sparkplug)? Is water coming out of the tell-tale?

R.

 

[Stemar]

Favourite for me would be a partial fuel blockage somewhere. I had a problem with my OB because there was a bit of crud in the (well hidden) filter above fuel tap. Like yours, it would run for a while quite happily then cut out until the float chamber refilled. Another possibility is the air vent in the fuel cap. Run the engine, vacuum builds up in the tank, fuel stops flowing. Engine stops, vacuum fades, engine restarts.

There's almost certainly one recommended plug for the beast and that will be the right heat for normal use, but running with extra oil means the mixture's a bit weak, so it may run a little hotter, but I've vague memories from a youth mis-spent trying to run aging 2T bikes, that plugs cooked to the point where they give up tend not to recover, but it was a long time ago. A hot-running plug will have a singed look about it, with white deposits. Too cold and it'll oil up on a 2T, just right should have a light tan deposit, but again, all this is dredged up from the fading depths of my memory.

Final thought. Does it turn over easily when it's just died? If not, it would suggest it's on the verge of seizing up, 'cos it's running way too hot. If that's the case, your relationship with the motor is, I fear, liable to involve big bills.

 

[Seajet]

Are you sure the air inlet screw in the tank is opening and letting air in ?

 

[corinthmistral]

Thanks all. Definitely pumping water ok - a good flow through tell tale. Fuel filter is clean/not blocked. Will have a good look at the air vent as a couple of replies suggest this as a potential culprit. I open it a few turns, but will check it's not gummed up on the underside. Many thanks .

 

[Spyro]

If it's not blocked air breather to the tank Could it be the carb bowl is emptying quicker than it can be filled at full throttle suggesting a blockage somewhere.

 

[ProDave]

Thanks all. Definitely pumping water ok - a good flow through tell tale. Fuel filter is clean/not blocked. Will have a good look at the air vent as a couple of replies suggest this as a potential culprit. I open it a few turns, but will check it's not gummed up on the underside. Many thanks .

Including the little bowl on the bottom of the fuel tap? Mine was clogged once, but you have to remove the tap from the casing in order to unscrew the bowl.

 

[VicS]

Grateful for thoughts .

Follow up on suggestions that there might be a fuel flow restriction. Check/ clean/ replace (in-line ??) the fuel filter.
I take it you are just using the internal tank, not a remote tank.
If using a remote tank look for air inleakage esp at the connector ... and check if pumping the bulb will keep it going

Check that it has the correct plug, NGK BPR7HS-10 or Champion RL82YC I think you will find, gapped at 0.040" (1.0mm) But check with the owners manual which you can download from http://download.brunswick-marine.com/download/preparesearch?mod=4&lang=EN if your have mislaid your copy.

Well worth fitting a new plug. We so often find that symptoms that point to fuel problems are often solved with a new plug!

If problem persists it worth testing the spark with an inline spark tester

Personally I'd use a 50:1 fuel mix, as recommended for commercial use, esp if often running it at WOT but fuel mix is not your problem. I take it you are using a TCW3 outboard engine oil.

 

[macd]

The heat rating of the plug is most unlikely to be the OP's problem. 'Hot' plugs are used in low-tuned engines (like the outboard in question) where they need to run hotter to reduce the risk of plug fouling. 'Cold' plugs, conversely, are used in highly-tuned engines where a hot plug might contribute to detonation. The difference is typically in the length of central electrode covered/uncovered by ceramic insulation. Most reputable plugs made in recent decades have had a far wider effective heat range than hitherto. This, along with better oils and leaner oil ratios has made plug fouling much rarer than it was.

 

[Rigger Mortice]

A few years ago I had a Mariner 8hp which developed exactly the same problem. It took me ages to diagnose but it turned out to be the diaphragm in the fuel pump. Over time it had become "baggy" so the pump could not always cope. I don't know if the 4hp has a similar pump but, if it does, check it out.

 

[corinthmistral]

sorted! kept happening so I went to change the plug and the little cap thing that screws on the top of theplug was loose. tightened withpliers, hosed the thing in wd40. run like a dream ever since

 

[Hadenough]

Until the next manifestation of lack of maintenance appears.