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Mercury 7.5 Difficult Impossible to Start

O

Oil Gypsy

New member
Posted this earlier but it showed up in the wrong section Hello, I’ve recently acquired a early 80’s Mercury 7.5 outboard, the unit is in immaculate condition having been in storage for many years, unfortunately it wasn’t properly prepared for long term storage, the gas inside the had evaporated and left a yellow deposit through the fuel system. Since owning the motor I’ve cleaned and rebuilt the carb with all new gaskets and a new float, replaced water pump and housing with all new gaskets, replaced gear selector seal in the lower leg, refilled the leg oil. I’ve metered the coils and have 960 ohms on each coil and 5 ohms from spark plug cable to earth. Inspected the rear ground cables which are good with no corrosion. The compression is 120 in both cylinders. The spark plugs were changed and gapped at 30thou and I have a good spark on each plug. I have a new tank and fresh fuel. The motor is impossible to start normally, I can get it going with starting fluid quite easily and it will then run and idle perfectly, it will restart after 15 – 20 minutes but after that it requires starter fluid again. I’ve run Seafoam top end cleaner through the carb while its running, this produces plenty of smoke and bogs the engine down, shortly after the seafoam spray is stopped the motor runs normally. When puling the cord turning it over it does appear slightly stiff, not as easy as my 15hp, I’ve put this down to the high compression. I’ve more or less run out of ideas to make it start reliably and any suggestions would be appreciated.
 
Does spark jump a gap of 3/8" or more , yes or no?----Are you opening throttle far enough for starting ?---Put it up against the neutral stop.-----When you pull choke on , is it still fully closed after you pull the recoil ?
 
Thanks for the response.. Checked the spark plug specification and changed the gap to 1.0mm (Champion QL77JC4) as recommended. When you say jump 3/8 or more do you mean hold the plug 3/8 from the head when checking for spark? Throttle is on the high side of the Start Marker and the choke flap remains closed.
 
You test to see what the ignition system can do.----This does not involve sparkplugs.------You can make a simple device with a 3/8" gap.-----Or buy a spark tester about $10 I hear.-----Have used a test device since about 1968 to do a proper spark check !!
 
Thanks, Ive ordered the spark testers should be here monday. I also took the carb apart again, found two things, the venturi had a damaged side fin and wasnt sitting on top of the main nozzle properly and there was a gasket missing from the idle tube, I have always assumed it was stuck in the orifice. Have to wait until Tuesday for new parts to arrive before I can test again.
 
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