1996 Evinrude 40 surges at lower speeds

[chopper72]

It trolls fine and goes full speed fine. But when I go 7-12 miles an hour it surges. Any ideas?

 

[user 21074]

Sucking air or fuel pump?

 

[nccatfisher]

My guess if it was sucking air or fuel pump it would be worse full throttle. I am guessing carb ajdustment or cleaning.

 

[RaisedByWolves]

Sucking air is not going to be as noticeable at full throttle as there is no vacuum or at least not as much as at part throttle.

 

[Pappy]

Sucking air is not going to be as noticeable at full throttle as there is no vacuum or at least not as much as at part throttle.

As fuel flow increases so does the vacuum required to get it to the engine.
To let you off "the hook" I am assuming you are talking about vacuum on the induction system not the fuel system.
Hint - just say yes.

 

[Shaugh]

Dirty carb / sticky float is probably the best guess.

 

[RaisedByWolves]

Sucking air is not going to be as noticeable at full throttle as there is no vacuum or at least not as much as at part throttle.

As fuel flow increases so does the vacuum required to get it to the engine.
To let you off "the hook" I am assuming you are talking about vacuum on the induction system not the fuel system.
Hint - just say yes.

Don’t take this the wrong way, but obviously. :x


Sucking air into the pulse line or the pump itself would give the inverse of the symptoms stated.

 

[Pappy]

Sorry, just getting back to this.
Glad you finally clarified you were talking about the pulse line.
However, the pulse line is connected to the crankcase and should have a common signal regardless of throttle angle, RPM, or load.
The piston provides the "pumping" action and reed closure provides the valving for the positive and negative pulses.
A pulse line, therefore, will not see a reduction in vacuum pulse at a WOT setting. Anything in front of the reeds will vary though.
Are we finally talking about the same thing ?

 

[Pappy]

Now...on the original issue.
First thing I would check on the engine is "Sync & Link" to make sure the carb/ignition pick up timing is correct.
Check that one carb is not open at idle as well. Both should be 100% closed. Back off the roller and loosen the link between the carbs while the engine is at idle. If you hear a noticeable change the link was not set up correctly.
You will need a book for the rest of the sync and link.