I need help finding a noise that started on my 1978 115hp Johnson over the weekend. Sorry this is going to be a long post but I dont want to leave out any info. I hadnt had the boat out yet this year and am getting it ready to sell, so I cleaned it up, ran it in the drive, and took it out for a water test. On the water it started making a clacking noise after the prop cavitated and the engine revved up high for a few seconds. It also seemed to lose some power and would idle rough and die.<br /><br />Background info on the motor I rebuilt it about 4 years ago (rings, gaskets, bored 1 cylinder) and rebuilt the carbs. It never really idled good and would sometimes start hard. Also the tach never worked right very intermittent. I replaced the rectifier this year trying to solve the tach problem but it doesnt work at all now. When doing that I found that the bolts on the port side power pack were loose so it wasnt grounding properly probably the source of my starting and idling problems. After tightening them up it ran much better in the driveway, restarting as soon as I hit the key, and ran better for a short time on the water until this problem arose. <br /><br /> I figured something came apart inside so I ran a compression test which was 110-115 on all 4 and checked for rod bearing play by moving the flywheel and watching the pistons mid-stroke, no play detected. The plugs looked fouled so I put in new ones and water tested again, it ran somewhat better but the noise is still there and still not running right. After that run I inspected the plugs and they are all the same with no variation between cylinders. The noise is barely noticeable over the exhaust when I run it in the drive but I can hear it easily when on the water, its a light clacking noise thats erratic and not in sync with the engine, but very consistent. I found past posts here that suggest my noise may be spark knock. If so what would cause that? Im going to test the coils with an ohmmeter this afternoon and check the timing. Anything else I should look for? Would damaged reed valves make a noise like that? Old gas? Is there a way to test the rectifier with an ohmeter? Thanks for any help!
