1987 Merc 50 3cyl- breaks down under load

[Scallywags]

In Nov 2012 my 1987 Mercury 50 hp 3 cylinder outboard would not start at sea- Engine was running fine before switching off at key while fishing. When trying to restart, sounded like flat battery, then nothing. Towed in.

Replaced starter motor –engine started and ran well with muffs- but no water from tell-tale.

Took 12 months to separate lower unit but then successfully replaced impeller- result: good water from tell-tale.

Problems remaining with intermittent start up-
• replaced battery- problem remained
• replaced main solenoid- starting problem resolved.

Engine starting and appears to be running well at low speeds, but breaks down at high engine speed when under load.

Tested ignition
Removed HT lead from spark plugs 1 at a time, when idling using muffs.
• Removed HT 1- no change in running performance
• Removed HT 3- no change in running performance
• Replaced 1 & 3 and removed HT 2-engine stopped.
Conclusion – engine starting and idling on #2 cylinder only

Looked at spark strength, 1 at a time
• All spark plugs appear to be firing- .

Concusion- coils and HT leads appear to be okay, so looks like possible fuel delivery issue?

Would appreciate thoughts on situation above. Could remaining problem be reed valves? Worth noting in the 12 months to separate lower unit, engine was lying on side and hanging upside down from time to time.

If so, how do I go about and how difficult is it to replace reed valves in this model of outboard? Any advice greatly appreciated.

 

[racerone]

Re: 1987 Merc 50 3cyl- breaks down under load

Start your trouble shooting with a compression test.---It is the most important thing to do.

 

[aussieflash]

Re: 1987 Merc 50 3cyl- breaks down under load

Check Stator and Trigger if bogging under load.I doubt its the reeds,especially if its idling well.

 

[Scallywags]

Re: 1987 Merc 50 3cyl- breaks down under load

Sorry, had done compression test. 120 each pot.

Thnx for thoughts on reeds. I did replace sheathing around wires coming from trigger plate to switch box, as they had perished leaving some wire exposed. This improved start up noticably but not necessarily on all cylinders.

Its weird, while it appears to be only idling on 1 cylinder, it gets up and planes with ease on the water but breaks down when trying to open her up further.

I have booked it in for a "top end adjustment" (I think thats what the mechanic called it). Perhaps he shares your insight Aussieflash.

Will let you know the outcome.

 

[BatDaddy1887]

Re: 1987 Merc 50 3cyl- breaks down under load

Check the power pack (aka switchbox).

 

[CharlieB]

Re: 1987 Merc 50 3cyl- breaks down under load

When were the carbs last cleaned?

Dirty carbs are the most common cause drivabilty and idle issues. Passages get restricted by crud and fuel flow is reduced, limiting high speed power and killing cyls completely at idle.

Also check fuel pump output cranking, you should have a nice fat stream or it will limit WOT power as the carbs will run low on fuel, seriously causing a lean overheated flame, often high enough to melt pistons.

Order carb kits and a fuel pump rebuild kit.

If fuel hoses are hard, replace them.

 

[Scallywags]

Re: 1987 Merc 50 3cyl- breaks down under load

Charlie, Carbs never been cleaned in 3 yrs of owning and no idea before that. Sounds to me a likely culprit. will share these thoughts with mechanic when they hav look next week. In meantime will check the fuel pump output when cranking.
Cheers

 

[little.boat]

Re: 1987 Merc 50 3cyl- breaks down under load

Dude you can repair the carbs very easily yourself before I took it to a mechanic. You can buy carb kits for $5-10. Just a few screws and you can have it apart clean the jets and bowls with something like sea foam. Put back together with new seal kit and take it out and run it. If it works no need to take it to a mechanic.

 

[little.boat]

Re: 1987 Merc 50 3cyl- breaks down under load

Also running dirty carbs can blow your powerhead. I rebuild mine every 2 years and I drain the fuel from the bowls if it's going to be laid up for a while.