1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Did a search and came up with some relevant threads but none that specifically address my concerns. So here goes. Sorry if I'm a little long winded but I wanted to present all facts.

Looking for some input on my 1981 Evinrude 15 hp outboard. I recently purchased this motor and 12' aluminum. Here's the problem.

First time I took the boat and motor out, I started up the motor, and left the dock area. It went to wide open throttle no problem, motor made good RPM's (based on sound) and got the boat up on plane, and got to about 16.5+ MPH (GPS). However, I noticed that the longer the motor ran (starting after about 3-5 mins), it would begin to bog down, lose RPM's, and I would eventually lose speed down to about 12.5 mph and barely keep it on plane. After the bogging down began, if I let off the throttle, came to a stop, and cracked it wide open again immediately, there was a noticeable lack of power and it struggled to get up on plane.

I then shut the motor down, let it sit for a few minutes and then fired it up. Again, up on plane immediately, ran good, got back up to 16.5+ MPH, but eventually began to bog down. Then, after bogging down, it regained some power (without shutting down), I got back up to about 14.5 mph, and then eventually back down to 12.5 mph. It never got worse than this. I shut down a second time and waited for 5-10 mins, then fired up and again back to full power for the last 5 mins of the trip back to the dock at the end of the day.

More info:

Fuel was fresh and fresh oil was mixed at 50:1
Water pump was running, can touch the head for 3-4 seconds easily when the bogging occurs. Overheating likely not the cause IMO.
When the motor was running and bogging down, I squeezed the ball on the fuel hose and this did nothing. The ball does not completely firm up like I'd expect but does hold pressure. Tank is vented and was vented for the entire time.
The motor starts right away, runs very well at low throttle, no audible misses, no sputtering. When bogging down, it simply sounds/feels like someone was gradually closing the throttle, it does not sound like the motor is missing at all.
The flywheel is not perfectly tight. When the motor is cold, at a certain point in the flywheel rotation, It wiggles very, very slightly. Not measurable. Not at all points in the flywheel rotation. No oil under the flywheel, if a bearing is going, it hasn't leaked yet.
The prop is not spun.

Had two trips like stated above. So I did the following:

Re-built the carb with genuine OMC kit. Noticed that the welch plug had a leak and there was thick black reside all around the plug and it had dripped down the side of the carb body. Replaced and re-sealed welch plug.
Replaced spark plugs
Checked compression - both cylinders are 110-115 PSI cold. Did not check when motor was hot out on the lake. Assuming that if I'm getting great compression cold, it will be good warm. Maybe this is bad thinking.
Ran the motor in a dark area. No noticeable errant sparking.

These repairs were done and I took it out yesterday. The motor started a little easier, I generated a little more power (got up to 17.8 MPH but this was with the wind) and 17.0 MPH into the wind. Seemed to last slightly longer with full power, but not a huge difference. The one thing I tested that I did not test previously was: when the motor was generating full power, I cut the throttle down to idle. Then I opened it back to full throttle and immediately did not have full power. There was no long bleed off of power. This may or may not be new, but it was the first time I tried this. I think this is a good clue, but I can't think what to do next.

I was going to do the fuel pump but I received the wrong pump kit, new one is on the way. At the same time I'll do the fuel hoses with ethanol resistant ones. My last suspects are:

1) Carb - is there an adjustment I can make?
2) Fuel Pump? Going to put the new kit in and try again, but figured I'd post here to see if there were any other ideas in the meantime.
3) Thermostat problem?
4) Spark - could this be a power pack heating up and slowly failing? I don't think it is, as I said when it is losing power, it is not missing, it just sounds like the throttle is gradually closing.
5) Wobbly fly wheel - could this be causing some binding when the motor heats up? I think it's doubtful - due to the immediate loss of power described just above, and due to the fact that the power came back on the first trip without me letting the motor idle or shut down.

If you've made it this far, thanks for reading!! Looking for any and all ideas. I was reading another thread where there was a motor with similar symptoms and it ended up being a head gasket. Would have never guessed that. Any and all ideas are appreciated.

Thanks!

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

When you overhauled the carb did you remove the high speed orifice and make sure it was not restricted/plugged?

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

clean the jet holes in the carb with welding tip cleaners and run carb cleaner through it.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

The jets were both very clean, no plugging or fouling. Carb body and other orifices very clean and unplugged as well. I think I'll run some sea foam in it when the new pump kit goes in.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Use a flashlight. Put it into gear and twist the throttle fully. Check the CAM wheel and look at the plastic lid ontop of the carb and see if you get any lift due to a hairline crack. This made the butterfly twist open more than 100% + probably made extra air leak into the carb on my Johnsson 15hp.

My lid was cracked and really hard to see. But at full throttle the CAM wheel put such preassure on the lid that it made it crack. And when running the engine it worked fine until about 95% throttle. To fix the problem I had to drill and tap a extra screw and epoxy glue the lid to make sure it was not leaking ontop.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Mullerhawk, what year is your motor? I'm guessing it's 1987 or newer? Mine does not have the plastic top and plastic float bowl, they only used that style from 1987-1992 I believe.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

I would be more concernd about the flywheel, is the nut torqued to spec?

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

I don't know if it's torqued to proper spec or seized on but I couldn't remove the nut so I know it's not loose. I know it's a concern and I'll monitor it but I want to eliminate the simple things first

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

I guess I should add that there is zero visible wobble in the flywheel regardless of the RPM or throttle position.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

inductive timing light on each plugwire at problem rpm can verify spark is ok...spray bottle with fuel mix , mist into air intake at problem rpm can help with fuel delivery diagnoses...some possibilities...filter for intake hose in the tank is getting partially blocked, air leak in a hose fitting, coil/powerpack is heating up and failing, etc.....the "wobbly flywheel" how much end or lateral play does it have?

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Thanks for the suggestions lots to think about. As for how much wobble, impossible to measure. I'd be pretty confident in guessing less than 20 thousandths.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

I had a similar problem on my 77 15hp and it turned out to be that I had to set the float DROP to the full 1 1/2" before it would quit bogging and now will run strong as long s I want it to. Just a thought. Good Luck

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

iliketrout said:

Mullerhawk, what year is your motor? I'm guessing it's 1987 or newer? Mine does not have the plastic top and plastic float bowl, they only used that style from 1987-1992 I believe.

Click to expand...

Ahh - yes! I stand corrected. Your carb is the older model cast in metal instead. As another member wrote - check the float so the level is correct.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Will Bark said:

I had a similar problem on my 77 15hp and it turned out to be that I had to set the float DROP to the full 1 1/2" before it would quit bogging and now will run strong as long s I want it to. Just a thought. Good Luck

Click to expand...

When I put the new carb kit in, the float was slightly above horizontal. I put the new float on and adjusted it to as close to horizontal as possible as per the repair manual. Have you adjusted yours past horizontal? At horizontal, we mesured and confirmed that the float would drop all the way down until it touched the float bowl, and this was 1-3/8' drop without the bowl on. The manual said anywhere between 1-1/8" to 1-1/2" but I didn't think adjusting any further would make a differrence since it was touching already. I'll double check all my numbers though, thanks for the input.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

No, with the carb inverted I set it exactly horizontal with the lip of the carb and then set the drop to 1 1/2" and it worked for me. Since my tank is up in the middle of the boat I went to 1/4" fuel line as suggested by THE Machinist in another thread. You might install a clear fuel line between the pump and carb and watch for any bubbles going to the carb; if you see any you have an air leak somewhere and that will bogg your motor down. Good luck

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Check your fuel lines and fuel pump for air leaks. Once it starts sucking air, you'll end up with foamy fuel in the carb and you'll get a slow degradation like you mentioned. Let it sit, and you'll let the air settle out of the fuel.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Thanks for all the tips. I'll be working on it this weekend but I won't get it out until the last weekend of August, at which time I'll be sure to post results.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

iliketrout said:

Thanks for all the tips. I'll be working on it this weekend but I won't get it out until the last weekend of August, at which time I'll be sure to post results.

Click to expand...

I was wondering if you ever resolved your problem. I have had the exact same thing happen with my 1981 15 hp Johnson. I have cleaned the fuel tank and lines, put in an in line fuel filter, did a carb rebuild with a carb rebuild kit. rebuilt the fuel pump (because the guy before me put it together wrong and it didn't work), I have good compression, good sparks, and the engine runs smoothly at all speeds. But over the course of a minute or two at full tilt it powers down to like half throttle. The only difference is that about 20% of the time there seems to be a small dead spot when I throttle up and I can pop the choke for split second to get past it, then the problem wont happen for a while.

I wanted to share this next info with you once I read your story because your problem is so similar to mine. I had the boat on the water the other day and it was doing its typical power down thing so I popped the cowling off at full speed and low and behold I started to gain speed again back up to what seemed like full power. If I put the cowling on I slowly lost power again. Every time I did this I had the same result. Also that dead spot that sometimes happens when I power up was rectified if I took the cowling off when the problem came up. It was like the engine could all of a sudden get air when the cowling was off. I assume that there is something wrong with the motor seeing as how the motor was designed to run with the cowling on. But I was wondering if anyone had any thoughts as to what the problem could be. All of the motor casing drain holes seem to be clear.

Could it be poor vacuum pulling through the carb throat, if it is what could cause that, stuck reed valves, bad gasket head gasket or something. There are no visual signs of leaky gaskets or cracks in the motor anywhere.

Anyone's help would be greatly appreciated. Thanks.

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

snapgator, you should start a new thread to keep your posts seperate.
It sounds like you have an exhaust leak filling the cowling with exhaust and starving the engine of oxygen. It is most likely the base gasket leaking, #106 below.
http://www.ishopmarine.com/ishop/js...CatFilter=BRP&eCatSupFldr=supplier-100-005-EJ

Re: 1981 Evinrude 15 HP E15RCIS - Losing Power at Wide Open Throttle

Daviet said:

snapgator, you should start a new thread to keep your posts seperate.
It sounds like you have an exhaust leak filling the cowling with exhaust and starving the engine of oxygen. It is most likely the base gasket leaking, #106 below.
http://www.ishopmarine.com/ishop/js...CatFilter=BRP&eCatSupFldr=supplier-100-005-EJ

Click to expand...

I posted my question on this thread because I wasn't sure how to reach the other guy who had a nearly identical problem. The only difference is he says nothing about trying the motor with the cowling off. But if he shut it down for a few minutes then the cowling would have time to fill with air again if in-fact this is an exhaust leak. I think my motor issue is still relevant to this thread.

I want to thank you for your idea, it is something I would never have thought of until I found it myself by working my way through the motor changing items out along the way. This, I think is a great place for me to look to solve this problem. I have never seen any smoke come out from under the cowling, do you think I would see smoke under the cowling if this base gasket was the problem?

Thanks again, and if this doesn't work and if no one else responds to me here I will start a new thread.

P.S. I am still open to other Ideas.