1979 70hp Evinrude WOT towing issues

[saylor4202]

I have a 1979 70hp evinrude. The boat is an older boat but worked fine for skiing and wakeboarding last season. There was a small fuel leak in the bulb and hose so I replaced the primer bulb last year. At the start of this year the boat starts right up and seems to run great. We started skiing and it would only rev up to 3500 barely able to pull a skier. When we are not skiing the boat will rev up to around 4300rpms but it still doesn't feel like I have full power.

Some things I tried and checked.
-Checked spark at each cylinder
-Squeezed bulb at max speed and had no affect
-checked fuel lines and they look to have been recently replaced

-Ideas
I noticed that the vent on the gas was only 1/2 opened.
The primer bulb stays hard for about 2 minutes then slowly gets softer (Engine not running)
The gas was fairly rich with oil could that give these symptoms?
How can I check to see if I have the correct prop?

Thanks for all the help, I know a little about motors but am new to Evinrude.

[Solarman]

If prop did fine previously, don't mess with it.
If mixing oil and fuel in tank, it is very important. Pre-mixing has to be consistent. Too much oil, motor runs sluggish, lacks full power. But too little oil can burn your motor up. Best rule of thumb, buy good oil, and maintain strict measuring. If using a 12 gallon tank of fuel and want 50:1 mix, add 1qt of oil (for the full 12 gal tank). if one of the older 6 gal tanks, add 1/2 qt, or 1 pint (1/2 qt) for each 6 gal tank.

I use this as an example, most manufacturers back then were mixing at 50:1 ratio. If your motor manual/ manufacturer suggested a different ratio, then mix accordingly.

Try mixing fuel properly, can also do a compression test of each cylinder. The compression test can tell you if each cylinder is running equal. If too big of a difference in compression then other problems may exist, reason not getting good power and RPMs.

Good luck. Post back when able, if you find anything.

[saylor4202]

Thanks a lot for the suggestions. I will be a little more careful when adding oil, its a 6 gallon tank. My problem is that I usually only need to add 2 or 3 gallons to top it off. So I am usually trying to measure per gallon. I can't find my compression gauge but I am hoping that a cylinder is not the problem. I will take it out again this weekend and make sure the gas vent is fully open. I also cleaned the plugs good since they were slightly black with oil. I will probably also get a new set to try out on the water. I did also pull the airbox off and notice the carbs had a little gas coming out below the butterflies. It was just wet and did not seem like a stream of gas. This was of course after I was pumping the primer bulb with the engine off in the garage. I was trying to see if their was a fuel or vacuum leak somewhere.

Anything else I can quickly test when I am on the water?