Merc 50 suddenly lost half revs at WOT

[Nos4r2]

Was out trying it out for the 1st time on the water yesterday.<br /><br />Ran fine for 2 hours or so of intermittent use-maybe 20 mins of that at WOT. Running 40:1 premix with a bottle of Redex in the tank. BUHZW-2 plugs.<br /><br />On the way back in at WOT the engine suddenly lost power. Felt like it was only at 2/3 throttle. No nasty noises at all and sounded like it was running on all 4-just at lower revs. It wasn't a gradual fade. It was like I'd partially shut the throttle.<br /><br />Plugs were clean but oily (as I'd expect with that amount of 2-stroke and redex). Found what looked like a carbon flake on one plug on inspection but nothing else-but it wasn't between the electrode and earth.. Peehole appears to be partially blocked-constant drip of water from it but it's hard to blow through. Plenty of water out of the prop when running on muffs and from vents above prop.Fuel bulb is hard.<br /><br /><br />I'm GUESSING it's gonna be fuel related but if anyone has any ideas?

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

BTW, compression figures are (from top)<br /><br />135<br />130<br />128<br />130

 

[Laddies]

Re: Merc 50 suddenly lost half revs at WOT

It sounds like you lost 1 or 2 cyls. have you cked. the spark

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

Yes-checked with a gunson timing gun. Got spark on all 4. It's too bright out to see sparks at plugs themselves at the moment.

 

[Laddies]

Re: Merc 50 suddenly lost half revs at WOT

As long as you have spark on all 4 then its got a problem with a carb plugged up

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

Thanks Laddies. <br />Just finished stripping/cleaning carbs. Got an awful lot of goo out of the lower one.It'll have to wait a week or so til I can test her on the water tho <br /> Just gotta sort the problem with the peehole being partially blocked now!

 

[Laddies]

Re: Merc 50 suddenly lost half revs at WOT

The 50 HP that the telltale hose comes out of the bottom of the exhaust plate tend to plug up unhook it from the telltale and try to back flush it with water, the other thing they do is kink where they hook up to the telltake outlet

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

Cheers again! I wasn't sure about backflushing it with water-was going to do it with compressed air but I'll hook it to the hose now, it's easier. It's not kinked at the outlet.<br /><br />I'm tempted to raise it up and exit it out of the top cowling with an extension so I can see if I'm getting cooling water to the top cylinder-or will this not work? It'd be easier to see that way anyhow-it's hidden by the stern at the moment so I can't see it properly from the helm.

 

[Oats05]

Re: Merc 50 suddenly lost half revs at WOT

Same thing happened to my 35hp Mercury. The CD box or switch box had to be replaced. It took me months to figure out what was causing this. Part costs 125.00. If everything you try doesn't work this is probably the problem. Check out my other post Mercury Adjustment.

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

Won't be a major one if that is the problem-I've got a spare good switchbox anyway. Cheers all.

 

[Laddies]

Re: Merc 50 suddenly lost half revs at WOT

Water pressure will tend to move more than air, as it tends to move the object rather than go around it--Bob and yes I have moved them on engines that were completly pluged

 

[Nos4r2]

Re: Merc 50 suddenly lost half revs at WOT

Thanks Bob you're a mine of extremely useful information.<br />I actually meant once I've cleared the blockage I'll extend the hose up to the top cowl (I figure if there's enough water pressure to raise the pee stream that high then it'll be getting to the upper end of the water jacket so all 4 will be cooling ok-never got round to doing it after asking the question in a previous post) but if it's possible to drill and tap to move the peehole take-off then that's another very handy option I have.<br />It's all very well me being able to theorise about this stuff but someone who's done it saying it works ok is a massive bonus.<br /> Very much appreciated.