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  Engine/prop shaft jerks consistently and regularly - why?

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Author Topic:   Engine/prop shaft jerks consistently and regularly - why?
ShrimpBurrito posted 07-28-2003 02:32 AM ET (US)   Profile for ShrimpBurrito   Send Email to ShrimpBurrito  
We were out for a leisurely boat ride today on the river, (with company, of course) and things were pleasant and uneventful for about the first hour. Then, suddenly, the engine (1987 40 HP Johnson, 15' Super Sport) jerked the boat as if we had hit something small in the water. We didn't see anything, so I just wrote it off as a fluke. It happened again maybe 30 minutes later, and then again maybe 15 minutes after that. The frequency increased over the course of an hour or so until it eventually was happening about once every 10 seconds. The noise/jerking motion was similar to what would happen if the engine tilted away from the transom about an inch or two while under power, and then suddenly slammed against the bracket.

Despite the problem not occuring at idle or with increased throttle in neutral, I thought one possibility was that there was water in the fuel. So, I added Heet to the tank, but that did not solve the problem. I haven't purchased fuel at a marina in about the last 10 tank fulls, but it's not unheard of to get water from a land-based gas station.

The engine does not jerk in reverse, nor does it usually jerk at speeds less than ~3.5 MPH. Once you go faster than that, you hear some kind of click, and then it starts. You can slow down to 3.5 MPH or less again, but the jerking persists even at these speeds until you first go in reverse, and then forward again.

The engine RPM's are not interrupted during the jerk, so I don't think it's a backfire. Also, once the jerking starts, the frequency does not increase with speed. The intensity of the jerk, however, does appear to increase. I'm thinking it's some kind of mechanical issue (gears or one of the shafts), but I've poured over the assembly diagram and I don't see any of those parts that, aside from maybe a shifting lever, would not cause the jerking frequency to increase with an increase in RPMs.

Does anyone have any ideas or suggestions? I'm guessing I'm not going to avoid dropping the lower unit, but I'd at least like to have some parts at my side and ready to go.

Clark Roberts posted 07-28-2003 07:41 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
ShrimpB, sounds like the shift dog is the culprit. I'm not sure of the specific configuration on your engine but most have two small "ears" that hold in sockets and when these round off there is a 180 degree slip/bang/wham and ears re-seat in opposite holes. Each slip/re-seat removes more metal and the slips come more frequently. Don't hesitate to drop that lower unit 'cause it will just get worse and do more damage (replace water pump inpeller while you're at it). A shop manual will depict all and maybe even give part numbers... I wouldn't attempt without a manual myself... good luck.. happy Whalin'.. Clark.. SCN
Peter posted 07-28-2003 08:59 AM ET (US)     Profile for Peter  Send Email to Peter     
Could the shift cable require adjustment?
raygun posted 07-28-2003 11:06 AM ET (US)     Profile for raygun  Send Email to raygun     
My 86 Evinrude 90 did that until it spun the driveshaft. The splines sheared off where it fits into the crankshaft. ouch. 5k+ to fix.(new crank and new driveshaft) Hello new engine. You are just going along and it feels like you hit a bump, right? I don't know if the driveshaft was the cause or effect. I "think" it was the end result of the "bumping". My thinking is check the shaft now before it's too late(it never hurts to check that water pump anyways) and if it looks fine, follow Clark's advice, then go through the ignition on the motor. It's missing a beat for some reason and I'd bet it's electrical, like a corroded wire to powerpack, etc.
Sal DiMercurio posted 07-28-2003 12:28 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
It's the shift dog.
Unless your real familiar with changing it, better have it done.
Theres more to it then just removing & changing it.
ShrimpBurrito posted 07-29-2003 01:22 AM ET (US)     Profile for ShrimpBurrito  Send Email to ShrimpBurrito     
Thanks for the advice everyone. Looks like the lower unit is getting dropped. I've never done it myself before, but I am fairly mechanically inclined (like replacing the water pump on my car). That being the case, Sal recommends having it done. Is that the general consensus here?
Clark Roberts posted 07-29-2003 06:02 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Bigshot posted 07-29-2003 12:03 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     

Or get one from E-bay.

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