Clutch dog or something else??

Here's the problem with my '99 70 hp Johnson. About two months ago, after fishing in the morning, I put the boat on plane and started to head home. 2 minutes running and I feel a bump on the engine as if I had hit something. 15 seconds later and again the bump. It continued during the whole way back, like clockwork, exactly 15 seconds between jerks. Next time out it didn't happened, it was a very short trip about 7 miles. The next time out and ran 26 miles to my fishing destination, and nothing, but after 3 hours fishing I started to head back and the problem came back again. The higher the RPMs, the more frequent the bumps. For example, around 4700 rpms the engine jerks every 12 seconds or so, around 4000 RPMs it jerks about 20 seconds or so.
I read somewhere that this could be a high speed misfire. Should I take apart the lower unit and check for a damaged clutch dog?

Thanks in advance.

Re: Clutch dog or something else??

it seems to me if it were the clutch it would get progressively worse not hit and miss. How long since gear case oil change?? Have you checked it recently?? chunks of steel on the magnet of the drain screw?? is your linkage set properly??..I am leaning toward some water in your fuel..check and clean/replace your fuel/water separator or install one. this is the most simple thing to try..also some isopropyl alcohol will evaporate water out of your fuel..also try adding some sea foam fuel additive at about triple the regular rate and spraying some sea foam or deep creep into your carbs with motor running.
If it is in your gear case I would fix it ASAP..those parts are pricey as heck..most of the time its cheaper to buy a used one than to repair..just the clutch dog is about $100 and each gear around $300..ridiculously high prices..I need to open up a gear fabrication shop..got to still be making crazy profit at half these prices.

Re: Clutch dog or something else??

Thanks Sparkie. The gear oil was changed earlier this year, as a matter of fact I had the mechanic replace the seals on the gearcase, he told me that everything down there was good. I had this work done because I had just bought the boat and wanted to know it's condition. I'm thinking on rebuilding the carbs, the last work I did on the engine was taking apart a carb and cleaning it due to a float valve getting stuck.