1971 Evinrude "Thumps" at WOT update

[alderman]

1971 Evinrude model # 25102 just converted from long shaft to short shaft. Best I can describe is like a "thump" at WOT. Almost feels like hitting a underwater obstruction. Just had this motor converted from long shaft to short shaft and lower unit resealed, shop that did the work said all the gears and shift dog looked good. Does not seem to be trying to jump out of gear, just running on lake wide open and all of a sudden you get a loud thump and the boat jumps. Prop is tight and seems to be no play in it when in foward or reverse. Any one got any ideas?

 

[R.Johnson]

Re: 1971 Evinrude "Thumps" at WOT update

If all else is correct, your shift linkage is out of adjustment, This should have been done at the change over.

 

[alderman]

Re: 1971 Evinrude "Thumps" at WOT update

Thanks I was afraid it was going to be a major problem. Is this a complicated adjustment or can it be done by a novice like me? Any and all help will be appreciated.

 

[Solittle]

Re: 1971 Evinrude "Thumps" at WOT update

This is a job made for novices. In fact I saw it in a post that professional wrenches are not allowed to do this - only novices. Detach the gear shift linkage at the motor. Put the gearshift in neutral. Align the linkage and adjust the adjustment wheel & rearrach. Teat & readjust if necessary.

 

[alderman]

Re: 1971 Evinrude "Thumps" at WOT update

It dosen't seem to be the shift linkage followed advice above and checked owners manuel. All seems to be in adjustment. Could this be a prop problem?<br />It has the factory 9x10 on it. Today while trying it out for the first time 3 full size men and all gear is when it started. I guess I should mention it's on a 1960's vintage 14' starcraft aluminum fishing boat. Could it be a cavitation problem or trim setting? Thanks for all the help.

 

[fireman57]

Re: 1971 Evinrude "Thumps" at WOT update

Are you sure that the "thump" is not a miss in the engine? If you can do it safely while at WOT look back and make sure that the engine is low enough in the water that it can't draw air. Was there a jackplate on the boat for the long shaft?

 

[Joe Reeves]

Re: 1971 Evinrude "Thumps" at WOT update

If you changed the lower unit when you converted to short shaft, I'd suspect a damaged shifter dog and forward gear.<br /><br />However, if the gearcase is the original and no problem existed previously, the shift linkage would be incorrect. Try the following.<br /><br />Engine not running. Spin the prop and engage forward gear. Adjust the shift linkage so that you can actually feel the slight friction generated when the shifter dog is sliding against the forward gear when you move the propeller in a rotating movement... back and forth. That would guarantee that the shifter dog is fully engaged in the forward gear.

 

[alderman]

Re: 1971 Evinrude "Thumps" at WOT update

It's the same lower unit Joe just took the 5" extension out and replaced drive shaft, water tube, and yes shifter rod. Sure sounds like you guys nailed it again, Thanks for the help. I will adjust this week and let you know how it works. Thanks again. Ray

 

[alderman]

Re: 1971 Evinrude "Thumps" at WOT update

It turned out not to be the shift linkage after all. Adjusted linkage and still had same problem. Motor was not trying to jump out of gear, the prop is slipping on the hub, but only at wot and only with full load in boat. I have ordered a new prop. from iboats and it should be hear next week. Once I have it swapped I will try her again. Thanks again for all the help.