1973 85 horsepower Evinrude reduced power under load?

[Catfishcabin]

So me and a friend of mine picked up a 1973 four-cylinder 85 horsepower Evinrude for his 18 foot pontoon boat it had been sitting for probably about five years we did a little bit of work to it and seems to run really strong out of the water but when we put it in the water it runs great until wide open throttle. We get wide open throttle for about eight seconds and then the motor begins to Surge and bog until we dial back the throttle lever and then it Smooths out. We've replaced the fuel pump fuel lines clean the carburetors replaced the coil packs replace the rectifier replace the vacuum line and we're still having the problem. The only thing that I've noticed looking at the motor this time is that there's a crack in one of the stator coils? Could this be our issue or maybe even timing or points?

 

[Faztbullet]

Replace/rebuild the fuel pump and check to see if it has a anti siphon fitting in fuel tank,if so remove and replace with brass nipple.

 

[Catfishcabin]

Fuel pump was replaced 2 weeks ago and there is no anti-siphon in the tank

 

[jimmbo]

The last 20% of the throttle require far more than a 20% increase in fuel demand, the symptoms you describe are most likely fuel starvation. Have you tried a different fuel tank?

 

[Catfishcabin]

No we have not tried a new fuel tank but we're in the process right now of rebuilding the carburetors and discovered that the carburetor kit contained a larger orifice on the needle jet so we're hoping that that improves the issue that were having

 

[Catfishcabin]

No Title

 

[F_R]

Fuel pump pressure gauges are not very expensive. Get one and temporary tee it into the line at the fuel pump outlet to monitor the pressure as it runs. Note: the pressure gauge you are looking for is usually combined with a vacuum gauge. The pressure gauges for new fuel injected cars are for reading a much higher pressure. You are looking for only a few PSI

 

[Catfishcabin]

So just got back from the lake after rebuilding the carburetors. We left a no wake zone got on the throttle and everything was good again for about 10 to 15 seconds and then it bogged out started misfiring and loss of power we went ahead and dialed back the throttle got to shore spot beached it did a little bit of swimming and then went back to the doctor pick up a friend of ours got back on the throttle everything was good and then it bogged out again and then maybe 18 to 20 seconds later all the sudden the throttle came back two full speed and didn't have any more issues that trip and we ran it quite a bit. Once the throttle came back we were still getting some misfire backfire kind of sounds coming from under the cowling but that all seem to smooth out towards the end of the trip. We're happy that it's running well now but still have no idea why all the sudden it decided to smooth out