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Merc 90 query

Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Topic: Merc 90 query
    Posted: 06 Nov 2019 at 7:14pm
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Wondering if somebody nay have some suggestions.
My Merc 90 2 stroke has developed an odd problem. The logical thing is to take it to the service agent - except;
a) the motor is in Vanuatu and I have had some experience of the local agent. If it was Firmans Marine (my NZ agent) no problem.
b) the motor is only doing this after extended running. It would likely run perfect in a test tank all day long.
I can go 20k out, fish 3-5 hrs then run 20k back home. A number of times lately 3-4k from home (virtually same spot each time) it starts dropping revs and surging. My guess was a fuel starvation problem. I have a filter/water separator between the motor and fuel tank. Changed that. Replaced some fuel line and re kitted the pump. Next run I thought problem solved. Perfect. Nope. Back again. The fuel bulb has pressure when this happens - squeezing it doesn't alter anything.
I have a spare virgin coil/HT lead. Just fitted it to one cylinder - will swap over til done all 3 if problem persists. Checked tank breather vent, and cracked the filler cap. No change.
In the back of my mind I wonder about a crack in the block/head. But seems weird to be so repeatable. Once running first time, runs/starts perfect all day. Until 3-4k from home. I wonder about fuel level in tank - never been an issue before - but will add fuel to tank next time. Not sure if related - but has become a bit of a pig for first start of the day - then purrs/starts like a kitten all day long - until 3-4 k from home.
Anyone got any suggestions where else to look?
Thanks
Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 7:48am
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Are you sure its not going into limp mode ?

Pull the spark plugs, keep them laid out in order.
If a head gasket then one will be very clean.. check inside the bore/ top of the cylinder with a torch.

Remove cowl/ air'filter' , take rpms up to fast/very fast idle. put hand over the carb inlet, pull a big vacuum, and just before it stalls, pull hand off fast. This is likely to clear any crap partly blocking a jet.

Check float levels...

Is there spray around the engine while traveling.. ie coming up from a transducer.. this sort of spray over long term is bad for powerheads

And electrical issue is generally consistent, thu with packs these days one getting hot may give symptoms.
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 8:00am
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Thanks Steps. It doesn't have a limp mode - that I am aware of. Altho it is limping. If I give it more throttle when it does this it 'clears' briefly and the revs shoot up - and then it bogs down again.
I should say I pulled the carbs apart this time last yr - clean as inside. I always drain them when storing the motor. Reset floats (slightly out).
The plugs are gapless and generally always clean. But will check.
Engine doesn't get spray when travelling.
Yes I wonder about a coil breaking down - but at a guess, the issue seems to be across all 3 cyl rather than one bad one - but prob hard to be certain.
I will try the air intake trick.
Thanks
Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 9:32am
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Here is one other thought that seems to fit the symptoms - altho a bit weird.
I have an underfloor tank, custom made. Wanted to clear the floor area (tote tanks) for game fishing. I generally fish on 35-40l /day. The tank holds around 45l. I can see fuel in the tank when it has this issue, and generally I can run the tank to where I see no fuel and it hasn't run out. So I usually have 10l or so spare in the tank when back and can run it dry at times. (Yes there is spare in the boat). I am wondering if the pick-up tube has some corrosion in it and I am sucking air at around 35l used. Altho the squeeze bulb says it has pressure. Near impossible to verify the state of the pick-up tube - sealed Al tank. Will dump more fuel in next time and see what happens.
Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote MacSkipper Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 9:58am
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Can you run on a tote tank?  Borrow a locals - Try this when symptoms appear and this rules out fuel lines, primer bulb tank etc?
Good fishing trip nothing breaks, great trip catch fish.
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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 1:15pm
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I do have the original tote tanks. Just a matter of re plumbing it when it happens . Not simple - but a possibility to try.
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Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote CrayZfish Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 3:10pm
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Partially blocked Carb is my guess, exact thing happend to my 75Hp Mariner i just got it back and that was the issue. When it surged at cruise on test run, Outboard mech asked to bring back into neutral or slow idle for a few mins for carbs to fill up, took off again fast and problem went away for a min until carb drained again.
Ended up being a small piece of degraded rubber from a small hose section after the inline filter. Carbs cleaned, hoses x3 replaced, went without issue yesterday.
 
Why choose either diving or fishing when you can do both. Besides crayfish tail is very good bait!!
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Post Options Post Options   Likes (0) Likes(0)   Quote krow Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 9:42pm
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If your bulb isn't sucked flat then not a fuel blockage before the bulb. My advice get a section of clear tube same size as fuel line put it in near the motor and see if AIR bubbles are getting into fuel line. If you have a crook connection somewhere in the line it might not leak but under vacuum suck air. A bit is ok and the system can deal with it but too much and you get fuel starvation. Might explain why intermittent too. If this is the issue you will see it even on the muffs. 
Hard to start first time may also indicate that the fuel is falling back to the tank because air is getting into the line. Fuel issues are sometimes tricky problems. Another thing is how old is your bulb? They have been known to perrish internally and you could have a partial block on the top side of it. Bits of rubber or the spring jamming. 

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Post Options Post Options   Likes (0) Likes(0)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 07 Nov 2019 at 11:31pm
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Yes Krow - agree. Had another look for leaks today - pressurised the system waiting for fuel. Nothing. But may be prior to the bulb. did look but found nothing - but that doesn't eliminate it. hould mention I changed the bulb 2-3 months ago. But yes - clear line may work. the odd thing is next day it will run fine for several hrs - no sign of y/days problem. So I doubt a blocked carb etc - should still be blocked next day. but I will keep woring on it.
Mate arrived from NZ 5 mins ago. Up at 3am, hitting the water. Stand by.
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Alan
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Post Options Post Options   Likes (0) Likes(0)   Quote offcut1 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Nov 2019 at 8:03am
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I had a 1998 75hp Merc that was doing the same thing. After lots of messing around, cleaning carbs, new plugs, tune etc, we ended up replacing the entire fuel line, from tank to bulb to bayonet fitting. That did it. Even a tiny pin prick will let in air under pressure.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 08 Nov 2019 at 8:21am
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The clear temp section fuel line is good idea..
Using another/ tote is also and excellent idea
Crazyfish post onto it also.
Make sure you have the correct std fuel line for your fuel type..if not you are likely to find the inside skin of the line swell (restricted fuel supply) and also break up/off cause issues with the motor fuel filter (there are normally 2 filters.. the water trap one and one under the engine cowl on the engine)



Couple side snippets

when back and can run it dry at times

This can cause all sorts of problems, picking up crap in the pick up filter and doesnt 'wash' out on next tank fill

Also never run a engine out of fuel.. yeah we used to do it in the old days on lawn mowers etc with leads etc... you will notice in the last 30 to 5 sec before the engine cuts, the rpms go up.. this is because the mixture gets very lean. very lean and std spark timing... means some very high temps in the cylinders.. enough to melt piston head alloys.. burn valves etc.

I am wondering if the pick-up tube has some corrosion in it and I am sucking air at around 35l used.

Where is the pickup?
 should be, towards the read of the tank if possible, at an angle pointing towards the stern.
Should be stainless.

 The bulb pressure...
The bulb is only to pump fuel into the engine fuel pump to start it..from there the engine fuel pump takes over...
most fuel pumps are not self priming. therefore the slightest air leak before the pump and at the pick up will cause fuel supply issues
The slightest air leak before the pump will cause fuel supply issues.
The most common issue with bulbs is they are not mounted vertical.... There are 2 non return ball bearing valves in it that relay on gravity to fall back and seal.. hold the fuel in lines and carbs etc between starts.
 Many a bulb has been replaced simply because it is not mounted or hangs in the near vertical position.
If y stop anchor up 20 mins or few hrs, go to fire up, no fire, pump the bulb, fires then the chances are it is not hanging vertical.

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Post Options Post Options   Likes (0) Likes(0)   Quote Snapper12 Quote  Post ReplyReply Direct Link To This Post Posted: 08 Nov 2019 at 12:02pm
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Check ur fuel tank pick up tube.
If it's got crack at the bottom then when fuel is low in tank its sucking air.
When tank is full u wont have that issue.

Just a suggestion as this happened to someone I know.
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Post Options Post Options   Likes (1) Likes(1)   Quote Alan L Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2019 at 2:48pm
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OK - sorry - couldn't keep up with the thread. Lost internet - just back now.  Most of the suggestions are on track.Steps/Offcut close. Found the issue at 4am this morning - altho woke early and thought I sorted it at 2am.
On the way to fad at 4am, it was worse. So under torchlight in the briney - decided to hunt further. Found the culprit. 
My fuel line - as supplied by the Firmans Marine (Profile boats) is a double layer thing. I don't like it. What happened to simple fuel hose? The inside had collapsed at a bend. Wonder it got any fuel. So all I could do is pull the inner core out and fit the outer as a temp line. Problem fixed. Purred all day long - 8 hrs on water today.
Bit of a relief to find the cause .
Thanks
Alan  
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 10 Nov 2019 at 9:32am
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as supplied by the Firmans Marine (Profile boats) is a double layer thing. I don't like it. What happened to simple fuel hose?

 Maybe not... vacuum rated.... 91 petrol grade... hose As against say pressure rated diesel or leaded petrol hoses

This is not an outboard or marine thing.. real common with boy racers cars, resto of old classics... and hot rodders usually get it right.

Nor is the fuel issue..issue principle applies to everything from a line trimer to a Model A ford....only exception a EV Wink
 Hmm thinking about that.. cables not upto std would do same thing and catch fire.. just like a poor fuel hose...
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