'98 Evinrude 30hp losing WOT RPM when hot

[MakoNY]

The above engine will run 5400 WOT when I first get out on the bay and it's not very hot. After running a while at cruising speed, around 4200 RPM, when I open it up again it will just barely make it to 5,000 RPM. I've noticed that it's done this the last couple times out, which are the only couple times I've ever been on it as I've only owned it a week.

What would be a possible reason that it loses 400 WOT RPM after it's been running a while? Possibly hotter air inside the cover which is less dense and having an affect?

When I first noticed it only hitting 5K I thought it maybe it wasn't propped right, but the next time out it hit 5400 at first and then 5K when it was hot....same thing yesterday. So it's propped to hit 5400, but loses it for some reason. Personally I'd like to see it higher than 5400, but really don't want it at 5K and have to figure out why it's losing RPM before I play with the prop.


Thanks.

 

[boobie]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Sounds like it's losing a cylinder when its warmed up, Do a cylinder drop test on and see if you can find the bad cylinder.

 

[MakoNY]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Well it's only a 2 cyl so I would assume if it were dropping a cylinder it would run way worse and get nowhere near 5K wide open. Plus it idles perfect when hot, has plenty of power through the RPM range and shows no sign of a dropped cylinder....just drops those 400 RPM at WOT.

Would these little 2 cylinder engines run on one cylinder?

 

[MakoNY]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Bump

 

[MakoNY]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Any other opinions on this? Was just out earlier and it'll run about 5500 WOT when I first get out and then after a few minutes of running it'll only get to 5K.

Really don't want to keep running it if something wrong, but it seems to run perfect except for dropping those RPM's.

 

[Daviet]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Just a couple of thoughts.
Try running it with the cover off and see what happens, maybe a slight exhaust leak.
Have you tried pumping the primer bulb constantly, maybe a weak fuel pump.

 

[Daviet]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Sorry, double post.

 

[MakoNY]

Re: '98 Evinrude 30hp losing WOT RPM when hot

I'll give it a try with the cover off and see, sort of along the lines I was thinking. I was thinking since there is no air box over the carb that maybe when the engine gets a little hotter after cruising for a while that the hotter air inside the cover was affecting it. Not that an air box would make a huge difference, but usually at least they let the carb/s pull air in from the bottom where it may be a tad cooler. Although I doubt it's just a normal design issue causing it to lose 400-500 WOT RPM.

I've hit the primer bulb when running WOT just to see and it doesn't do anything. I figure if I had a weak fuel pump then it would have the problem consistently and would never be up at 5500 WOT.

I'll post back after I run it with the cover off. Today will be a nice calm day out there so it'll be easy.

 

[MakoNY]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Well just for the hell of it I decarbed it twice today. Then went out and ran it WOT for a while and it never lost RPM. It occurred to me that since I just bought it who knows when or if it was ever decarbed so figured it couldn't hurt.

Hopefully that's all it was and it stays ok. I'm gonna decarb it again one more time in the morning.

 

[Daviet]

Re: '98 Evinrude 30hp losing WOT RPM when hot

Glad you got it figured out, now have some fun with it.