Evinrude 120 Thunk Sound

[skibrain]

I'm having an intermittent thunk from my outboard that I bought used three years ago. ("89 120 Evinrude V4. with OEM type SST stainless 19" pitch prop. on a "89 Glastron CSS-16 hull). the boat/motor has run flawlessly for three summers and the following just started at the end of last season.<br /><br />The thunk occurs when running sometimes at medium increasing throttle, sometimes at sustained 4,000 rpm range when pulling a skier or sustained lower rpm like when pulling a tube. It is not predictable as to occur through a certain rpm range, nor does it tend to occur more at max. load - i.e. max. throttle hole shot when pulling a 200 lb slalom skier up (although it has happened at that time) Trim is all the way down. Motor is mounted securely to the transom.<br /><br />When it happens, there is a barely perceptabe, quick blip increase in rpm, and then a driveline thunk that feels like a gear slipped, and then the motor continues on at the same rpm and boat speed as if nothing happened. I only get one thunk at a time, but it could happen three times running the length of the lake. (3/4 mile). I'm pretty sensitive to mechanical noises and irregularities, but this is strong enough that non-boaters riding with me last fall would turn and ask . "what was that?" <br /><br />I had a reputable marine repair place look at it last fall and they found nothing wrong. They had it under load through all rpms on a hydraulic dyno connected to the prop shaft and found no ignition or drive train irregularities.<br /><br />Other than paying a mechanic to ride along for a water test, I don't know what next step I should take.<br /><br />Any ideas? <br /><br />Has anyone had exprience with a prop starting to spin the hub inside the prop? (i think these have a rubber damper to help shield the driveline in case of a prop strike) If this happens does it let go and hook up (like slipping one cog on a gear?) Maybe it gets warm and slips more under load. I will have to put on my other prop and give it a try.

 

[DJ]

Re: Evinrude 120 Thunk Sound

skibrain,<br /><br />I think you are experiencing a momentary jumping out of gear.<br /><br />I doubt that condition would show up on a dyno. A dyno gives a constant and smooth load. The water does not.<br /><br />The clutch dog in the lower unit is probably getting mnarginal, or your shift linkage and/or cable is getiing a bit out of adjustment.<br /><br />Have you been "easing" this into gear? If so, stop doing that. That is what rounds off the lobes on the clutch dogs.<br /><br />I do not believe a prop hub would act that way. That's simple enough to test, just switch props for a couple of runs.

 

[BF]

Re: Evinrude 120 Thunk Sound

Hiya & welcome<br /><br />I think DJ's on the money. Sounds very similar to the "thunk" that my 150 merc would occassionally make. I was worried it would end up being the clutch dog, but it was cured by a slight adjustment to the shift cable. I think either 1) the cable wore in / stretched or 2) maybe when we had the LU off one time, we might've misaligned the the shift shaft by 1 spline.<br /><br />Either way, I was really pleased that after the cable adjustment it has never done it again. If you try an adjustment, make sure it shifts into fwd AND rev properly.<br /><br />Brent

 

[skibrain]

Re: Evinrude 120 Thunk Sound

Thanks for the input. <br /><br />I think I'm pretty decisive on getting it into gear, but there is a lot of in and out maneauvering around skiers, so I am the likely culprit. I have no clue how the original owner handled it. It's been a great rig otherwise.<br /><br />A ball park quote from the repair shop is $300-400.

 

[skibrain]

Re: Evinrude 120 Thunk Sound

Two updates. I adjusted the shift linkage so it pushed the shift linkage 1/8" further into gear. Test drove the boat, and was still getting the occassional thunk, so I returned the shift linkage to it's original position.<br /><br />I took it into the repair shop again. They took the lower unit apart and report no wear to the clutch dogs or any metal shavings in the gear oil. Any other ideas what this could be?????<br /><br />Do I pay to have someone do a water test?<br /><br />pay to have them repair the clutch dogs anyway>>

 

[Terry H]

Re: Evinrude 120 Thunk Sound

Have them replace the waterpump and check the shift linkage length while the l/u is off. If you haven't had the waterpump changed it's time, and if you have, then maybe the shift shaft got turned one turn inadvertantly...and that would cause a problem like you are having...if they had to take the gearbox apart the only the only diference in cost is for the parts...you can do the water test or if you want them to, go along... just a thought

 

[skibrain]

Re: Evinrude 120 Thunk Sound

They did replace the water pump. They did a 15 - 20 minute water test last evening and of course, it didn't repeat the behavior for them.<br /><br />I will mention the shift linkage to them. Any other ideas?<br /><br />I appreciate the insights. I'm new to this board and it sounds like lots of years of experience and wisdom. Its a great resource.<br /><br />Brian