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Mercruiser Knocking

S

seymour064

New member
I own a '85 23 foot Renken that was sitting for a couple years. I replaced the motor with freshly rebuilt mercruiser 170 and got it in the water about 4 weeks ago. When putting it in the water I ran it for a few minutes then it died and was hard to start. I took off the distributor cap and the rotor was fried and the distributor was in bad shape. I spoke to a mechanic that happened to be in the marina yard and he said the bearing in the distributor went bad. Luckily I had the old one and replaced it and the problem was solved. I then took the boat out for a cruise and as soon as I got it on plane it died. The best I could do was to get it to idle and if I gave it any gas it would die. I started troubleshooting my flow problem and when I took off my fuel water separator there was water and debris in the fuel. I need to drain the tank but in the meantime I hooked up a potable tank and the fuel problem appears to be solved. I did notice the motor sounded a little loud and didn’t have the power I’d expect from a fresh rebuild. Today I took it out and got it to about 3k RPM and the motor sounded like it was rapping and got worse the longer it ran. I killed the motor and idled back to the marina. If it’s not in gear it sounds great and revs just fine. Obviously I need to have a certified mechanic look at it but they are booked up for a while and I was wondering if I’m missing something. I check the timing and the wires all appear to be on correctly.
 
No. The sterndrive has been rebuilt and the gimball bearing replaced and aligned. I got down to the boat today and swapped out the carb with a rebuilt one from the old motor and it idled and ran smoother but still had the same issue when under load. After hearing it today it isn't really knocking but rather sounds like one of the cyclinders is not firing. I've heard the water shutter can burn up if the motor backfires which it did when I had the distributor problem so I plan on checking that next. Will also do a compression check.
 
I'm having a very similar problem. The engine runs great on Idle or under 2000RPMs, but when placed under load, it seems like its missing, choking and then there is a loud knock (not a backfire), but only once in a while. Not consistantly. I have currently replaced the distributor cap, rotor and the coil. No luck. It seems like the timing is off, but it's not. Points are suspect and I'll look at thise next. If anypone has ideas or if you solve the problem, I'd like to know what it was. Thanks,

Craig
 
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