9.9 gradually loses top end RPMs when hot

[dcleveng]

Engine is a 1977 Evinrude longshaft, electric start, freshwater motor. It is used on a 24' pontoon on a 10hp limit lake. Not sure what propeller is but not the stock one but a heavy load version. When checked years ago motor topped out about 4500 RPMs.<br /> The problem is when run at WOT after about 5 min the engine gradually and almost impercipably loses top end RPM over about a 2 min time frame. It is almost as if the throttle cable is slipping but inspection by many pairs of eyes shows that not to be the case. Nor does the timing plate seem to move with respect to the throttle "roller". Once the motor slows down it seems to be run like it was on 3/4 throttle. It will run this way fine for "hours", also will idle and start well. If the motor is shut down for a minute or two and restarted the "lost RPM's" will not come back. If the motor is shutdown for an hour or so and restarted, then it will run WOT with all RPMs for about 5 min and then gradually lose them (guess about 500 or so RPM's) over a two min time frame same as before.<br /> This problem has existed for about 3 years during which many things have been changed with no observable effect on the problem. We tried another gas tank, another hose, new spark plugs, a new fuel pump, running with cover off (to cool electronics?), pumping the tank hose bulb, carburator rebuild kit (including a new high speed jet), and new water pump impeller. The motor seems to be getting the proper cooling water flow, a new impeller was tried sort of "what else should we try". Nothing appeared to wrong with the old impeller. Also removed and tested the thermostat (boiling in water pan on stove) and it seemed to operate fine. The thermostat was reinstalled. As I said the surprising thing was changing all this stuff seemed to have absolutely no effect on the problem. The motor has not had any ignition stuff changed. Like I said when it loses these RPMs it is a very gradual process taking about 2 minutes. <br /> Anybody have any problems similar to this? Any experience on what we should try next before we put it in the water (mid May?).

 

[Kev]

Re: 9.9 gradually loses top end RPMs when hot

I'm no expert, but I would do a visual inspection of your coils. See if one or both coils looks old, sort of "exploded" or cracked from the outside. If so, the coil could be "breaking over" when it gets hot. That will cause your spark to get weaker and could possibly cause the loss of power you describe.<br /><br />Also check your spark plug wires for small cracks in the insulation, or for general poor condition. Bad spark plug wires could also cause you to lose spark as the ignition system heats up.<br /><br />Good luck,<br />Kev

 

[OBJ]

Re: 9.9 gradually loses top end RPMs when hot

Welcome Dave!<br /><br />First....what is the compression like on the cylinders? The higher the better but both cylinders should be with in 10>15% of each other.<br /><br />Take a look at the starboard side (drivers side) of the carb. You will see a small shaft coming out with a spring pin in it. When the engine starts to die as you described, check the position of the spring pin. At WOT, the pin should be verticle. If it's back or forward, something is causing the shaft to rotate.<br /><br />Which brings me to my next suggestion. Take a look at the cam follower roller. This will be toward the back of the carb on the port side. It rides on the cam under the flywheel. The cam pushes against the roller. The lever attached to the roller has a link from it to the throttle plate shaft. If this roller has deep grooves in it, replace it. Can you tell us if the roller and cam are in sync?<br /><br />Take a spray bottle of fuel mix with you. When the engine dies, shoot some fuel mix into the carb cover and see if the engine picks back up. If it does, you have a fuel flow problem. If the engine wants to choke and slow down more, likely the problem lies elsewhere.<br /><br />Just for grins....look under the flywheel and see if you see any goo laying about under there.

 

[CFronzek]

Re: 9.9 gradually loses top end RPMs when hot

The big picture is of a small motor pushing a big boat. If the boat was pushed much beyond displacement speed there was a big load put on that motor. That means excellerated wear. A compression check should tell the tale. <br />Two strokes take fuel in on the underside of the piston. When piston/ring/cylinder wear gets too bad cumbustion gases get down below the piston and lean the mixture out. Lean mixture=more heat=loss of power.<br />I wish that your trouble was something on the outside but I don't think so.

 

[Chinewalker]

Re: 9.9 gradually loses top end RPMs when hot

I would think you'd want that motor spinning another1000 RPM or so, peaking at 5500, not 4500. It's possible the motor is overheating at wide open due to heat build-up, which can cause it to lose RPM. As it is you're trying to drive uphill in 5th gear while towing a trailer. You have to get a prop on there that will allow it to rev up a bit, .ie like dropping down a gear while towing uphill...<br />- Scott