Mercury 3.3 woe's.

ianc1200

ianc1200

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Bought another 3.3 Mercury 2 stroke last week. Cheap at £70 from 2nd hand section of the marina chandlery, as having a fault it would only run with choke 1/4 on. So replaced the carburetor with one I already had and trust, replaced spark plug & emptied fuel replaced with new. Ran it a lot in my wheelie bin, would run fine for forty minutes or so in neutral, and engaged gear fine etc. Put it on the dinghy to take dog to beach, and twice it faded away and stopped within a hundred yards. It will start again immediately. Tying the dinghy up and replacating it happened a lot more times. Any advice what the issue might be? Should say the engine seems to kick out a lot of smoke (perhaps too much oil) but if the plug was oiled up it surely wouldn't start again so easily? Plus in taking the OB back to the workshop noticed a spring clip missing where fuelpipe joins fuel valve, so will try and relpicate in the tank and see if when it stops any fuel in the bowl.
 
Yes, sounds similar because that (ie the solution of split hose) is where the clip is missing. Will do & report back later this week. All other issues on Seasick Steve's checklist been covered/already checked.

First things first:
  • Did you open the tank vent and, if applicable, the fuel cock?
  • If you’re using an external tank, check that the fuel hose isn’t pinched anywhere.
  • It really sounds like fuel starvation, so confirm the fuel is flowing freely.

Next steps:
  • Inspect the fuel filter and replace it if clogged.
  • If the engine still stalls, open up the carburetor and clean the jets and passages thoroughly—varnish or debris can easily cause this issue.
  • Check the fuel pump diaphragm for wear or cracks; a weak pump can’t keep up under load.
  • Verify the spark plugs are clean and properly gapped.
  • Look at the air intake for obstructions and ensure the choke isn’t sticking.
  • Finally, consider the age of the fuel—old or stale fuel can cause poor combustion. If it’s more than a couple of months old, drain and refill with fresh fuel.
 
Put the clip on the hose onto the fuel valve, replaced the fuel cap in case the vent wasn't opening or blocked, ran in the wheelie bin for over 30 mins at 1/3 - 1/3 throttle and in gear (anymore and was losing too much water over top of bin) and no issues. Need to put on the dinghy and try the trip again.
 
Consider the angle the engine is operating at. If the problem is due to fuel shortage, that could be due to the float chamber having a low level due to incorrectly set float height or gunged up fill valve. Works ok maybe just about when engine is vertical but when you put it on the back of a dinghy the engine may no longer be vertical and will exacerbate the problem. Clean the valve and reset the float height. When in neutral, fuel use is minimal. In gear needs more fuel flow and if level is already low it runs out fast. I had exactly this problem with a Tohatsu 3.5. Took a while to find.
 
greeny said:
Consider the angle the engine is operating at. If the problem is due to fuel shortage, that could be due to the float chamber having a low level due to incorrectly set float height or gunged up fill valve. Works ok maybe just about when engine is vertical but when you put it on the back of a dinghy the engine may no longer be vertical and will exacerbate the problem. Clean the valve and reset the float height. When in neutral, fuel use is minimal. In gear needs more fuel flow and if level is already low it runs out fast. I had exactly this problem with a Tohatsu 3.5. Took a while to find.
Click to expand...

Thanks for those thoughts. In this case the carb should be OK as it came from a donor engine which was working fine (until I ruined it) so not suspecting the barb/bowl/float.
 
I had this - ran fine in a bucket but not on the Dinghy - and the problem was the fuel was low and the engine angled such that the fuel was pooling at the back of the tank away from the pipe.

It would start again because once stopped the tank was angled forwards again.

You should test brim full of fuel just in case.
 
When next on the dinghy will ensure it's near to full, but believe it was when I had the problem on Saturday. There's no filter in the tank that I can see, but would have thought there would be a filter between tank and carb - need to find a workshop manual.
 
Mark-1 said:
Yeah, there's only one filter and it's in the tank above the Fuel Tap.

More of a sieve than a filter.
Click to expand...
Remove the tap and the filter comes with it. Normally a cylinder of ss gauze pushed over the top of the valve assembly. I took mine out and fitted an inline cheapo unit off Ebay meant for mopeds or lawnmowers. Easy to remove and blow through to check and clean.
 
I removed the tap from the spare tank I have, but there was no filter in the housing immediately above the tap. However, it did all look a little manky and I do have an unused tap recently ordered from Ebay so will check that later.
 
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