Mercury outboard 100hp 2 stroke 4 cylinder around 1990 starting and idling problems

[Tommyuk1]

Recently bought boat and having engine trouble any help would be much appreciated!

When starting the boat from cold it does start with choke and fast idle on sometimes but on a few occasions it wont start at all and will just turn over and over without a pop or bang. If it does start I can then run it on fast idle to get it up to temp and then reduce idle and will sit happily at 1000 rpm for a short time but if left in neutral will cut out.

If i then turn the engine off (change fuel tank etc) it really struggles to start and sometimes it will just drain the battery to nothing and then im in the brown stuff! I prime the bulb of course.

I have tried to think of what the problem could be I have changed and tested lots of things I will list my findings and what i have changed.

Spark plugs
Fuel pump/Diaphragm kit
Fuel filter

Checked Carbs and floats there clean
Checked spark it jumps 8mm and looks stong on each cylinder

Checked compression

Cylinder One (top) 130 psi to 140 psi

Cylinder Two 130 psi to 140 psi

Cylinder Three 100 to 110 psi

Cylinder Four 130 to 140 psi

Engine Ser No 09660421

I realise the No 3 cylinder is low compared to the others but that is still good amount of compression!?!

Any help greatly appreciated thank you for taking time to look at this thread

 

[Barnacle_Bill]

Re: Mercury outboard 100hp 2 stroke 4 cylinder around 1990 starting and idling proble

How long has you 1991 motor been sitting without being ran? If gor a couple years or more you should read this.
http://forums.iboats.com/showthread.php?t=158086

Also check the FAQs for doing a decarb. It might bring that low compression up some.

 

[Tommyuk1]

Re: Mercury outboard 100hp 2 stroke 4 cylinder around 1990 starting and idling proble

Thanks for your reply, boat was not used for 6 months. When it starts the power output is good easily takes me to 35 - 40 knots acceleration is fairly smooth it just doesn't like idling or re-starting if it cuts out! I have checked the carbs there immaculate, I did take a look at the tops of the pistons and there pretty dirty looks like there covered in black grit?!?! I thought detonation?