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Old 28 May 2012, 10:59   #1
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Mercury 75 2 stroke, 2000 hesitant starting

Hi

My Mercury 2 stroke 75hp, year 2000 has been giving me some trouble starting up. Seems to start ok when cold with the choke, but its a bit hesitant to restart when its warm.

It seems to be running a bit lumpy on idle too with the engine slightly jerking around every now and then which i believe is it misfiring.

I have the workshop manual so can do everything my self i just need to know what to trouble shoot.

I synced the carbs the other day, based on the manaul instructions but they seemed to be correct. The plugs were not a tan brown colour though which to me indicates a fueling problem.

I'll get some pictures later on but they were black with small amount of crusty carbon deposits, so its not burning the fuel correctly.

Where is my next point of call
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Old 28 May 2012, 12:25   #2
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Have you checked the fuel filter on the engine and if fitted water seperator external to the engine? Id check all filters and to eliminate it replace the plugs you may have a duff one there. Cheap enough to replace them to eliminate it just keep the old ones to refer to the color of the plugs if further fault finding is necessary.

How recent is the fuel? May need a new HT lead to the plug / connnection if one has deteriorated this could result in misfires. Were all 3 plugs the same color?

Few ideas to chase up there. I assume the oil is still mixed by the engine and not premixed?
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Old 28 May 2012, 15:31   #3
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ignition timing? ive reset mine and cured cold starting problems
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Old 28 May 2012, 15:55   #4
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What plugs are you using? are they surface fire plugs without the sticking out electrode?
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Old 28 May 2012, 17:00   #5
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they should be BUZHW-2 and yes surface fire
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Old 29 May 2012, 09:40   #6
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There is a thread on here Mariner 75 problems starting from warm - my warm starting issues were cured by a) idle screw adjustment (was not the same on all 3 carbs) and b) fuel pump service kit
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Old 29 May 2012, 11:55   #7
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when i used to repair OBMs on the 75hp i used to set all three carb mixture screws at one and a half from lightly seated (I know what the service manual says), is the choke solenoid stopped working properly ,to check clamp of the two small pipes and try,have you dirt in a carb ,are all spark plugs the same colour?are the carbs sychronized?have you tried it when it is straight up and down ?
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