TOPIC: Evinrude 70 power issue

I have a 78 model 70 evinrude with a problem after it is running for several minutes. It starts fine and runs smooth at idle. It takes about 5 minutes to get out past the no wake zone and then it planes out great and runs up to the mid 40's mph. After 5 minutes give or take it will bog down and only run around 32mph. If it is idled back down it takes a lot longer to get back on plane and will only accelerate back to around 32 mph. If any one has any ideas what might cause this issue please let me know your thoughts. I have checked all the wires for the coils, power pack and changed the plugs. All help is appreciated.
Thanks
Ron

I had a very similar issue this summer Ron. It turned out that the vent on the gas tank was partially restricted, even though it was opened all the way! Same symptoms, I just knew is was having motor problems, but a new $10 gas cap fixed it. Might be worth checking out?

Thanks Mark. I should have mentioned it happens with two different tanks and I checked the vent on both. It also has a new inline fuel filter and primer bulb. Gas lines look good and flow freely. I'm thinking a sinking float in one carb or possibly trash in the bowl.

wrangler86 wrote:

I'm thinking a sinking float in one carb or possibly trash in the bowl.

I think that's what I would try next (carb cleaning and rebuild kits) as well. Don't forget to do the fuel pump too, just good "insurance" IMHO.

Ron, sounds like you may have a heat-related ignition problem.

One way to check for this is to take it out and get it to fail.

Then, take a timing light and check each cylinder for good spark while the engine is running.

If you have a "firing indicator" instead of a timing lite, that will work, but an inductive timing lite is plenty bright and easier to see in daylight.

At any rate, if you can eliminate the ignition system as a problem, it'll help get you to a final diagnosis.

HTH & let us know what you find.......ed

Thanks Mark and Ed. I had planned on putting kits in the carbs sooner or later so now is as good a time as any. Fuel pump is a good idea too.
Thanks for the timing light idea. I have one and that will be easy to do before the carb kits get here. Not sure how soon I will get the chance though working in retail going into the holiday season but the lake is only 15 minutes from the house. I'll post what I find out when I get it checked out.

wrangler86,

It is highly possible that you have COMBINATION a heat-related electrical problem and a car/fuel-pump/crud in the tank problem.

IF it was me,I would FIRST clean the tanks/lines, then test the electrical center both cold and after 20-30 minutes of normal operation and finally I would test the pump (and if that's OK) i would see about cleaning/rehabbing the carbs.
GOOD LUCK.

yours, satx

Thanks satx, I was thinking about a heat related ignition issue and I will check it out cold and hot before I tear into the other fuel related possibilities. Sometimes you don't know what combination of issues you might get when you pick up an old motor. At least the compression is excellent and the lower unit is solid. Thanks for every ones ideas.
Ron

Just thought I would update with the results. It turned out to be the power pack loosing spark to one cylinder when it got to operating temp. Thanks to everyone that had suggestions. One easy way I found to check for spark while on the water is to get a set of engine plug lights some custom car guys use. They plug between the plug wire boot and the top of the plug and flash lights when the engine is running. No light no spark. You still need to diagnose the cause but checking for spark is quick and easy.
The motor runs like a champ now. I can play with the motor height, trim and prop to see what it will do. A springtime project.