75hp Merc Low Power

[dalton565]

Hey guys

Need some advice. Boat for some reason is not producing enough power to get up on the skiis, basically the holeshot is not as good for some unknown reason. Top end is also down from 35 to 25mph for another unknown reason.

The boat was fine at the end of last season and has been stored in a garage.
I cleaned the carbs using carb cleaner thinking it might be that but no still the same. Tried new fuel but its still the same. Spark plugs changed at the end of last season.

Only thing that has been done is service by myself, impeller gear oil.

Any idears ???

 

[Barnacle_Bill]

Re: 75hp Merc Low Power

What year motor? Could be high speed jets clogged or fuel pump. See if this applies to you.
http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/1999/EN_05.PDF

 

[dalton565]

Re: 75hp Merc Low Power

thanks for the reply, i dont have a vacuum gauge or the the required T connection but found this on ebay, just wondering would it do the job

http://cgi.ebay.co.uk/ws/eBayISAPI....Track=true&ssPageName=VIP:Watchlink:middle:uk

thanks

 

[Barnacle_Bill]

Re: 75hp Merc Low Power

I think that gauge would do it but if you have one of the square type fuel pumps, just rebuild it to be sure. Kits are cheap and its an easy job. Just be sure to use a Merc kit so you are sure to get the new style check valves.

 

[dalton565]

Re: 75hp Merc Low Power

Bought the fuel pump tester today and connected it up. Ran the engine at normal idle and it was giving a reading of between 0.15 and 0.2 bar.

I gave it a few revs and it rose slightly but not much. Is this normal, cant really tell from the diagram as its saying take readings in in's

Anyone any idea

 

[Barnacle_Bill]

Re: 75hp Merc Low Power

Here is a conversion site. You should see 3.5 inches at idle at 6 inches at WOT. You need to know your altitude above sea level for the conversion.
http://www.dragsource.com/index.php?navselect=calculators&calctoview=8

 

[dalton565]

Re: 75hp Merc Low Power

Bhile

Thanks for the quick reply. am i right in assuming that an inch is equivilent to a pound per sq inch.

Converted the numbers and
idle = 0.18 bar = 2.6PSI
WOT = 0.31 bar = 5.0PSI

This agrees with what it says here
http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/1999/EN_05.PDF

So am i right to assume there is no problem with the fuel pressure.
Have changed the fuel filter also.

 

[Nam]

Re: 75hp Merc Low Power

A quick check on compression may help!

 

[Barnacle_Bill]

Re: 75hp Merc Low Power

Bhile

Thanks for the quick reply. am i right in assuming that an inch is equivilent to a pound per sq inch.

Converted the numbers and
idle = 0.18 bar = 2.6PSI
WOT = 0.31 bar = 5.0PSI

This agrees with what it says here
http://www.marinepartsman.com/Mercury-Marine-service-bulletins/Mercury-Mariner/1999/EN_05.PDF

So am i right to assume there is no problem with the fuel pressure.
Have changed the fuel filter also.

No, inches is a vacumn reading (inlet side of pump) and psi is pressure (output). reading. Your readings are just a little low according to my manual.

 

[Silvertip]

Re: 75hp Merc Low Power

You said you cleaned the carbs with carb cleaner. If all you did was spray them you did absolutely nothing. Carbs need to be disassembled to be cleaned. You also said you replaced the plugs last season. Did you pull them so find which plug is not firing. You are very likely running on two cylinders. A spark check is in order. When it bogs out, push in on the key momentarily. If the engine picks up, you have a fuel problem.

 

[dalton565]

Re: 75hp Merc Low Power

Hey, thanks for the reply silvertip. I will take all this advice into consideration going to do some more work on it tomorrow. Thanks for the advice on the carbs, i sprayed alot of carb cleaner into them, also removed the drain plug to remove any water.

I will remove them and clean them manually if it turns out not to a fuel issue.

I honestly dont think its only running on 2 cylinders, i have a good ear for engine, and other people have heard it, and it sounds as tho its running pretty much perfect.

Will look into it and update post
thanks

 

[dalton565]

Re: 75hp Merc Low Power

solved the issue today !!!

Turned out that when i was draining the carbs for the first time. i dislodged the throttle arm (that opens and closes the carb as you throttle it), reconnected it and took her out for a spin.

Absolutly flying was at 45 in the blink of an eye and planeing in seconds. Thanks for all the helps guys. Just knew it would be something stupid in the end.