Re: 1988 90 HP Johnson shutting off
I just read over everything listed pertaining to this "shut off, cut back on problem" and frankly I've never heard of such a problem.<br /><br />This has me wondering if the "off/on" scenario might be, as "Walker" suggested above "Skipping" rather than having the engine shut off, than back on. If so, the sensation would be that every so often the engine is hitting something underwater... a jerking sensation. Should this be the case, the problem would only be evident only when in the water and at one of the higher rpm settings.<br /><br />Should this be the case......<br /><br />(Jumping Out Of Gear - Manual Type)<br />(J. Reeves)<br />This pertains to lower units on all OMC manual shift outboard engines, or any OMC engine with lower units defined as a Shift Assist or a Hydro Electric Shift unit which incorporates a "Shifter Clutch Dog".<br /> <br />Within the lower unit, splined to the prop shaft is what is most often referred to as a clutch dog, hereafter simply called dog. The dog has at least two lobes protruding from it on both ends, facing both forward and reverse gear. The forward and reverse gears also have lobes built into them near their center area. When the engine is running, in neutral, the gears are spinning constantly via the driveshaft being connected directly to the powerhead crankshaft, but the propeller does not turn due to the fact that the dog is centered between the two gears, and the dog lobes are not touching either of the gear lobes.<br /> <br />When the unit is put into either gear, shift linkages force the dog (and its lobes of course) to engage the lobes of the the gear. The lobes of the spinning gear grab the lobes of the dog, and since the dog is splined to the prop shaft, the propeller turns.<br /><br />The lobes of the dog and gears are percisely machined, most with right angled edges that could be installed in either direction, and some with angles slightly varied that must be installed in one direction only (one end only must face the propeller). Dogs that can be installed in one direction only, if reversed, even if the dog and both gears were new.... would jump out of gear almost immediately. Keep in mind that the lobes are percisely machined with sharp angles!<br /> <br />Due to improper adjustment or worn shift linkages, but usually due to improper slow shifting, those percisely machined sharp edges of the lobes become slightly rounded. Now, with those lobes rounded, as the rpms increase, the pressure of the gear lobes upon the dog lobes increases to a point whereas they are forced apart (jumping out of gear), and due (usually) to the shift cable keeping tension on the engines shift linkages..... the unit is forced back into gear giving one the sensation that the engine has hit something, and the cycle continues.<br /> <br />Some boaters have the mistaken belief that shifting slowly is taking it easy on all of the shifting components..... Wrong! Shifting slowly allows those percisely machined sharp edges of the dog and gears to click, clank, bang, slam against each other many times before they are finally forced into alignment with each other..... and this is what rounds those edges off! The proper way to shift is to snap the unit into gear as quickly as possible.
<br />I also think that a dyno diagnosis (dealer) does not always replicate a real world issue. It's a great tool, but not magic.<br /><br />I still think he has a fuel issue or a gear case jumping. My thinking says that the carbs very temporarily run out or are low on fuel. Thus he feels a miss or surge. Don't know for sure. I'm not there.<br /><br />That is why I asked about the fuel situation. Let's look at all angles. Eliminate the cheapest and easiest to fix first. 
