1974 20hp blueband merc

[doryfishin]

hi , i took my boat out yesterday with my merc 200 on which ive been doing up . she was running sweet at 1/4 throttle through the 6 knott limit so when i was clear i opened her up and god she was shifting after about 5 minutes there was a bit of a slight surge in power so i throttled off only to leave it with a miss at idle speed to half throttle and its white smoking which it wasnt doing before . i think it will rev to wot fine but sounds rough at lower revs with a miss and a pinking sound every now and again . it was running great up until then and its left me stumped at what it could be , as i have replaced the ht coils both hi / low stator coils cleaned and adjusted the carb including a link and sync overhauled the gearbox new impeller etc etc would the timing slip and ifso is there an easy way to check it as the manual says its got to be done at full throttle in forward gear ? any pointers / advice and help would be much appreciated thanks in advance .

 

[doryfishin]

Re: 1974 20hp blueband merc

i did a compression check since last post and results were pretty dire 5psi on no1 and 25psi on no2 so i stripped her down bores look ok no1 piston ok no2 has a slight scuff and middle ring is pinched were the scuffing is . the spark plugs were black with oil and my test tank is full of black oily water is there anything else that would cause it to run crap as ive seen alot worse pistons that still would run ok . any ideas why plugs would be oily and black and obviously oil is blowing out the exhaust into the water thanks in advance .