1991 Evinrude 115 Intermittent THUMP at speed

[alshilling]

I love my Sea Pro 190 CC and the now "antique" motor that makes her move.  So much, I had it completely restored, bored out, new seals, LU rebuild.  And she has been running just terrific!  Yesterday, I was heading out with some friends to do some water skiing, when the engine mad a "thump."  At first I thought I had hit something (Manatee?) so I shut her down, tilted motor up, checked prop for crab trap, log, blood, whatever, and nothing.  Started back up and everything seemed fine.  It did it again, but then I had to go idle between some bridges, and it didn't happen.  Once on the other side, lit it up, took off great as always then all the sudden it did it again... seemingly random.  I have attached a video where you can see this freak occurrence in action.  I would appreciate any suggestions!IMG_6821.MOV.3gp (816.84 KB)

[Juan]

I can't tell anything from the video. Does the thump sound like it's coming from the head or the lower unit? Does the motor jump or jerk when the thump happens? Do you lose or gain any rpms when it happens? Is it happening only at high speed or also at idle? Does it shift from neutral into forward and reverse without making a thump? Need more info at this point.

[freezerfiller]

It almost seems like it's cutting out and the thump is the sound of the prop trying to turn the engine instead of vice versa. My recommendation is to first check your fuel water separator for excessive water. If you don't see any significant water there, I'd get a new power pack. Those things are good for about 5 years and are infuriating to diagnose. If you swap out the power pack and it quits doing it, you've solved the problem. If not, you now have a much needed spare.

[985crabs]

I had similar hiccups when my water separator filter needed changing. But there was definitely a noticeable RPM fluctuation that showed up on the tach.

That filter is a good, inexpensive place to start.

[alshilling]

switched out power pack.  I checked the stator and it ohmed out AOK on all sets of windings (I bought the shop manual last year).  Was out again this weekend, and the thumping, which is most prevalent in the "high low speed" range (1800-2200 rpm indicated) but still random.  Only does the thump under a load... Gets up and goes, and then, after about two miles, a really horrible whining sound... Dead in the water... my ex-wife/girlfriend/roommate (it's complicated) pointed out an oil film... tilted motor up, and, with bait net beneath, carefully removed vent screw... pumped in some lower unit fluid and carefully cracked the lower plug to allow it to fill... didn't look good... limped back to boat ramp, took her home and when I drained the fluid, it was a grey, nasty color with horrible odor.  Some big chunks on the magnets... so I am thinking all the time and effort on the motor was wasted.  So I am taking her to get the LU rebuilt (2nd time in a year).  I didn't see any fishing line on the prop shaft... so who knows what happened.  I am thinking I might be ready to move up to a new sea pro and bite the bullet with a monthly payment... maybe with one of them fancy 4 strokes and all .  We did catch a bunch of snapper and a really nice sized trout though... I will post what degree of damage they find... it was about 500 last time I had it rebuilt...