1997 Johnson 150. Compression is even in all cylinders at around 90.... The engine starts up quick and runs well for awhile.... Then at some point (usually between 30 minutes and 1 hour) the engine often dies.... When I restart it, the RPM's go up to over 2000 and if I put it into gear, and give it throttle, it dies if I move it much beyond idle.... If I let it cool down completely, it will run for another 30-60 minutes before acting up again. I've had the boat in the shop twice, they changed the power pack and said I was good to go.... I took it out this past weekend and had to limp back in after an hour.
Is this engine still a VRO model or converted back to pre-mix?
Does the warning horn sound when you turn the key to the ON position?
With the key in the ON position (engine NOT running), does the warning horn sound steady and constant when the Heat Sensor with (usually TAN) protruding from the cylinder heads is grounded.
Does the warning horn sound when this problem raises its ugly head... and if so, what kind of warning does it emit?
Your description of the problem sort of indicates a fuel restriction BUT that should give a constant steady horn warning... which you do not mention. The restart high rpm could result due to a lean setting (rare but it happens), and the bogging out when throttle is applied would result from a low quantity of fuel in the carburetors. It makes one wonder if you're encountering a anti siphon valve problem as follows.
(Fuel Anti Siphon Valve)
(J. Reeves)
Many of the later OMC V/6 engines incorporate a fuel restriction warning via a vacuum device attached to the powerhead. If the engine overheats, or if you have a fuel restriction, the warning is the same.... a steady constant beep.
NOTE... Only the V/6 & V/8 engines have the above "Fuel Restriction Warning". The warning horn will not sound on the other models.
The fact that a engine is not overheating, but the warning horn sounds off with a constant steady beep, and that the rpms drop drastically would indicate that the engine is starving for fuel due to a fuel restriction. Check the built in fuel tank where the rubber fuel line attaches to the tank fitting. That fitting is in all probability a "Anti Siphon" valve which is notorious for sticking in a semi closed position. It will be aluminum, about 2" long, and the insides of it will consist of a spring, a ball, and a ball seat. If this valve exists, remove it, knock out those inner components which will convert it to a straight through fitting, then re-install it. Hopefully that cures the problem.
The above procedure will cure a restriction problem with the anti siphon valve as stated. BUT, it may also allow fuel to drain backwards to the fuel tank when the engine is not running (siphoning backwards) due to the fact that the carburetors/fuel pump etc are higher than the fuel tank. This condition is not an absolute as the valves in the fuel primer bulb usually prevent this backwards siphoning problem. However.... if this does take place, the cure would be to install a new anti siphon valve.
NOTE: There has been cases when the output valve in the fuel primer bulb would come apart, and the inner portion of the valve would actually reverse itself and be drawn back into the primer bulb's output valve body. This in effect would create a shut off valve and result in a fuel restriction. If this is the case, you should be able to feel something laying in the bottom of the primer bulb when held horizontally.
