Mercury 70 idling problem and lack of top speed.

[mcegrb]

Hi Everyone,
I'm hoping some people may be able to help me with some advice. I'm fairly new to boating, havng bought my first boat just last year, a Fletcher 14 GTO with Mercury 70 outboard engine.
It's an old girl, but, to be honest, has been a pretty perfect starter boat, thus far. I've learnt so much so quickly and having so much fun with my friends exploring the Solent.
When I first got the boat it wouldn't idle. In fact, it's never really idled, and stalls when putting the throttle back to neutral.
Even on starting, once the engine has started I've really struggled getting the boat going forward as the engine stalls before I can put it in gear. This has resulted in having to almost jump start the engine with some decent revs on. Obviously not a great situation.
I''ve had the engine serviced recently and it was idling perfectly when the engineer left, but the idling problem has come back and it still stalls, almost straight away when the throttle is eased off.
And very recently top speed has dropped off from what was nearly 30-35 knots down to a top speed of around 19-20 knots. Full throttle does not seem to give full power any longer.
Can anyone help with any possible reasons as to the poor idling and the reduced top speed. It seems a simple enough engine, although my knowledge is very limited so far. I've bought the engine manual (sort of like a Haynes manual) so am willing to give a few things a try if anyone thinks it's likely something simple.

Thank you for reading, and I hope to hear from some of you with some suggestions.

Happy boating everyone

 

[mainshiptom]

Could be a few reasons allthough a sensor failure would be the top of the list, if the engines is either not getting enough cooling water or the sensor is not detcting that?

Also could be fuel problems , dirty filters? old fuel?


Tom

 

[Wiggo]

Tom, I'm guessing it will be an older style 2 stroke, so no sensors to speak of.

For the power problem, I'd start by looking at the plugs (3 cylinder?) for evidence of a misfire - oily/wet would be bad. Dodgy plug leads would be one option, or a failed coil (one per cylinder from memory).

Rough idling can be a million things, including a misfiring cylinder or maybe air leaks round the carb (re-used gasket, perhaps).

 

[spannerman]

If its an older motor with carbs, I'd check there are no airleaks on the carbs, fuel system, and full throttle gives full opening at the carbs
Check the idle speed and mixture as this will give stalling if not correct and that the carbs are balanced, the idle speed should be around 650-700 'in gear'.
Also check the fuel pump as this has a membrane which operates on crankcase pressure fluctuations these can split so you get less pumping effect so it can't keep up with the engine at higher revs.
If these basic checks don't do it then you need to go deeper and check the compressions, and the plugs are OK and correct type, then after that you have the float levels in the carbs that could be incorrect.
Hope this helps.

 

[111KAB]

I would start at the tank checking for dirt, blockages then up to the primer bulb and pipework checking for splits/leaks and finally as part of this exercise check connector to engine. If satisfied the fuel line etc is ok move onto the carbs. Don't just think if they look clean they are clean. Not sufficient to clear out jets by blowing - compressed air required. If all ok move onto pump diaphram. After this fear it may be electrical/timing in which case (depending upon your ability) you may need it 'looked at'.