Mercury 90 down on power

[martin33100]

Hi, we have a very very good condition 1998 Mercury 90 on our rib, the engine hasn't had a hard life and only let me down today. We went out for an hour at about 33 knots and not a problem. We then went up the Beaulieu river at 5 knots and after a while the engine started to run a little rough (like it had dropped a cyl). Went back out to sea and gave it full chat and up 33 knots again, did this for about an our and then with out a warning dropped back to 18 knots at full throttle. Any views on where to look first?

[Turbodiesel]

Could be so many things!, Best to see if it is dropping a cylinder first and identify which one. Once you have got that find out if its lost a spark or is it a fuel problem.

[martin33100]

Originally Posted by Turbodiesel

Could be so many things!, Best to see if it is dropping a cylinder first and identify which one. Once you have got that find out if its lost a spark or is it a fuel problem.

I would say its electrical because I could smell lots of un-burnt fuel when I flushed the engine out. My outboard man that looks after our engine said it may be a plug, coil or CD unit that has gone down.

[9D280]

Not sure what flavour yours is, but also check the HT leads if they are independant to the coils.

My going in guess would be electrical too.

[martin33100]

Originally Posted by 9D280

Not sure what flavour yours is, but also check the HT leads if they are independant to the coils.

My going in guess would be electrical too.

Its a 90 ELPT

[martin33100]

Just to give you an update, I have put in new plugs and swapped the coils around and its still the midle cylinder that isn't fireing, compression is 140psi on all cylinders so it may be fuel related

[9D280]

sounds like you need to dismantle / clean the middle carb...... or check all electrical connetions between the box of electronics & the plug.

[Nos4r2]

Have you got a spark on the middle cylinder?

If you haven't, after the tests you've already done you'll know it's black box related.

[martin33100]

Just found something very strange, when you disconect the ruber pipe that goes from the middle cylinder to the fuel pump the middle cylinder starts to fire, the fuel pump does have a new diaphram in it.

[Nos4r2]

Clean the carbs. You've probably got a stuck float needle.