'88 Evinrude 15hp lost thrust

I haven't seen this one addressed yet. Hopefully some advice before I spend $40 on gas and launch for a failed lake test.

Last weekend I took my boat out for 3 days of fishing and used my kicker (this one) for trolling approx. 8 hours or so over the course of 2-3 days. Typically a 1 1/2-2 mph speed. I'm pushing a 19ft fiberglass boat - very normal stuff. The prop is 9 1/2x10 (stock).

On the 3rd day, about 4 hours in, I started to notice lack of speed for the rpms. I gave it throttle and the motor continued to operate very well, but seemed a bit on the warm/borderline hot side. There was plenty of exhaust through the hub but I didn't get any torque on the blade.

I immediately thought I spun the prop so I took it to the outboard shop today for a new one and he put it over a spindle in a vice and the prop was still solid on the hub.

Here's the caveat: I recently tore it down to replace one of those stupid plastic grommets at the power head because it was blocked. I put it all back to together and she ran great until this incident.

When I got home I checked all my bolts and realized I hadn't done the "break-in" check and the 6 base bolts were were all a bit loose (3/4 - 1 full turn loose). I didn't have any water coming into the motor but I had noticed a small amount of exhaust in the cowling throughout the the day. The tell-tale was normal. I checked the compression last night and still got the same reading as before the rebuild (can only get 70 since I have to pull start and hold the tool over the plug hole).

I just re-torqued these and and all other bolts back up and put it in the tank. It runs just fine, still, and idles right down to the very bottom end. I engaged the gear with the prop on and with a bit of throttle it acted like it wanted to empty the barrel with very good thrust.

Would those loose base bolts be the cause of loss of power at the prop? The performance of the motor gave no indication anything else was wrong outside of the prop thrust problem.

I'm ready to take it to the lake in the morning but would like some opinions before hand.

Re: '88 Evinrude 15hp lost thrust

maybe motor smoked into cowlings, sa under was only smoke and just a litle air.motor is getting air to burn with fuel mix int cylinders.cant burn smoke instead air

Re: '88 Evinrude 15hp lost thrust

Good advice above. If not that it may be running on one cylinder. They run very smooth on one but lack power. I would check for spark on both next time you suspect things aren't right. In a barrel they always seem to have lots of muscle but on a boat is where the power loss really shows up. Best of luck. Rick.

Re: '88 Evinrude 15hp lost thrust

My best guess now is that it was ventilation caused by exhaust escaping the the lower mid-section.

Tom in Olympia (Tom's outboards) told me this morning it sounds like ventilation from the motor being too high and then mentioned the base gasket (after I said I had pulled the powerhead) - which I guess could have been causing ventilation by forcing exhaust down the mid section cavity (outside the exhaust tube) in front of the prop. By re-torquing the bolts the exhaust seems to have been redirected back out the hub.

Both cylinders are firing. I cleaned the plugs after a timing check and minor idle adjustment. Even when I was having thrust issues the engine "acted" completely normal with the throttle - hence my immediate assumption of a spun prop.

I ran it in the tank again. There was no more exhaust coming into the cowl. When I was running it on the lake I took the cowl off while I was having problems but it didn't change anything. I pulled the thermostat tonight and reseated it (it was new 3 weeks ago). The head still gets pretty warm but not even close to steam water and the water coming out of the tell-tale is pretty warm too so I know it's circulating well (with slight temperature fluctuations). I might be running the idle a bit lean too so it would naturally be a bit warmer. The stream is consistent with engine speed - constant but less at 200+ish rpm barely running low idle to strong and steady from anything much over idle. This motor has been a flawless runner now that the carbs were thoroughly cleaned and the water blockage removed.

Does this theory of exhaust coming out the mid-section make sense? I guess I'm gonna give it a shot in the morning chasing kokanee.

I welcome any other input before wasting a trip to the lake.

BTW- I highly recommend Tom's Outboards in Oly if you need service. These guys have a passion for what they do. They are a bit farther for me to drive than the big "A" north of me but well worth the trip.

Re: '88 Evinrude 15hp lost thrust

You might be right about the rebreathing exhaust gasses. However i have looked your post over & am at a loss as to which 6 bolts you left loose? The head bolts? If so then yes that would explain things.

How are the Kokanee hitting at American Lake? Figured you would be fishing salmon around Tacoma.

Re: '88 Evinrude 15hp lost thrust

It's the base bolts coming up from the bottom cowl that attach to the bottom of the block.

American lake gets a bit crazy at the launch so we're chasing the Kokanee in Clear Lake at first light (the one by Tanawax).

I was actually rigging up mooching leaders last night but the reports are still that it's slow in area 11 (a few blackmouth here and there) and when combined with unfavorable tides + half tide at sunrise I just can't see enough working in our favor this early in the season to justify the extra time and expense of running saltwater. Not to mention that Puget Sound isn't the place to be if you only have one engine in case my kicker isn't working.

My next door neighbor is hitting your area in the morning - Columbia. One of my buddies that's joining me tomorrow just pulled two 18+lb steelies out of the Cowlitz Saturday. It's really hot down your way!

Re: '88 Evinrude 15hp lost thrust

Problem solved! I've seen other other posts with similar issues so I hope this will assist.

It was, indeed, exhaust escaping the front end of the lower unit/mid section drain holes. After tightening the bottom bolts back up she ran like a top all day on the lake.

We didn't catch any kokanee today but we also didn't get to the lake until 8:30 and they were already holding 20ft above the bottom at 60ft and the lake is a bit too small to spend much time effectively in the pocket.

Tonight I'm going to change out the fuel line from the water separator to the kicker to 3/16" (from 3/8") as well as shorten it by 24" since it has a bit of a tough time on restart initially keeping a fuel line vacuum on slow idle. It ran for 45 - 75 minutes at a time like a new motor at various trolling speeds.

And "Machinist," your website is excellent! It was so much more helpful than that garbage manual I bought. I hope you'll let me know when you're up this way so I can thank you in person. I've got a whole bunch of "silver slayers" to give you that I tied last year for last fall's river run. Everyone around me was wasting their wet line time catching pinks and I found what worked again and again to limit on Coho.

Re: '88 Evinrude 15hp lost thrust

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