115 stalls out at high rpms

vegetto

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Mar 6, 2017
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Have a 1985 115 Mercury, inline 6, on a 1985 astroglass 175v fish and ski. I've just purchased this boat before Thanksgiving, so I've never ran it before. I have, however, performed maintenance items and have replace: starting battery and both trolling motor batteries, water pump, spark plugs, fuel water separator, and cleaned the carbs. The fuel was treated before putting it away for the winter. Starts ok when cold, maybe turns over for about 1 second when warm. Engine seems to run great in the driveway, although this is my first boat. When I put the boat in the water, starts up great (I've warmed it up just before going in) and I can start going. Out of the hole, excellent. Shoots up to top speed in about 10-15 seconds, running at around 4500 RPM. Does great until about 3-5 minutes into a trip and engine will start to lose engine speed. If I try take it back to idle and then go back to full throttle, it acts like its bogging down. If I keep doing that, and keep it at full throttle, it will just eventually die. If I take it back to idle and then I just keep speed up enough not to flood the stern, I can take it back to idle once its safe and it may or may not die, usually doesn't die at this time. Although, if I keep trying to throttle up, it will die. If it dies at any time, it will become hard to start. My first thought was I was sucking air, but it doesn't do this in the driveway, only when underway. So I kind of went a different direction and decided to chase down the electronics. I've been reading so many posts on here and most seem to think that most of the time it is a spark issue.

I haven't tested for spark during the time this is happening, but I've gone through some of the troubleshooting from CDI Electronics website and have a few things that came up out of spec. First, the resistance readings from the Blue, Blue/White, Red, and Red/White are all good, but the voltage readings were not. Checking the voltage for the Red and Red/White wires came up with a negative voltage readings (I am a diesel mechanic by trade and I know how to use a meter). I check battery voltage at the battery positive post to the previously used ground (starter case) and came up with 12.5 volts. The spec for this is 20 volts or more.

Second, I also tested the rectifier. It was putting out about 12.9 volts at the battery wire the short time I had it running. Testing voltage from the post with the Yellow AND Gray wire on it was 6.7 volts. Spec says I need to be over 8 volts. Tachometer is working.

The troubleshooting really isn't clear what to replace or what is bad with these readings. I'm also not 100% sure these specs are accurate and I am still suspecting a fuel issue. As far as troubleshooting the fuel system, bulb is not hard when this problem occurs, but I've read that it isn't hard when engine running. Bulb is not collapsed as if I have a fuel restriction. Fuel tank have open vents and are clear. Fuel water separator has been replaced.

I'm not really sure where to go on the electrical side, and I don't want to just throw parts. As far as the fuel side goes, I think I will remove the tank and check for intermittent debris, maybe a possibly cracked suction tube, loose fittings, etc.

Any other suggestions are very much welcomed and appreciated.
 

merc850

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Jul 7, 2010
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"The spec for this is 20 volts or more" if you are using a 12v battery how can you expect to get 20v or more? What manual are you using? 12.9v is kinda low should be 14 or so. Is that fuel water separator before or after the bulb and any Merc from that era doesn't have one so maybe the fuel pump is being starved by it. Check the max rpm in the manual - I think it should be 5200 or more.
 

Whaler672

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Mar 6, 2017
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Make sure there is no water in the tank. Siphon some out into a clear container and let it sit for a few minutes. If there's water in the tank you will see it separate in the container. Had the same problem and there was water in the tank. Every time I would apply the throttle it would draw the bad gas and start stalling. Slow it down and it would draw the good fuel. Something to check out....
 

vegetto

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Mar 6, 2017
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"The spec for this is 20 volts or more" if you are using a 12v battery how can you expect to get 20v or more? What manual are you using? 12.9v is kinda low should be 14 or so. Is that fuel water separator before or after the bulb and any Merc from that era doesn't have one so maybe the fuel pump is being starved by it. Check the max rpm in the manual - I think it should be 5200 or more.

The specs come from CDI Electronics website. I realized now I made a mistake, the spec is not supposed to be in volts but DVA. I'll have to recheck. But with that said, that makes the rectifier test within spec.

The engine was running that long, so I wasn't too concerned about the 12.9 volts. I'll run longer tomorrow and retest.

The fuel water separator is before the bulb. I'll check the micron rating and get back with that. I wasn't aware that they weren't built with a filter. When did they start using the filter?

Not too concerned with max RPM for now, just want it sustain running. I'm sure I just need to retune the carbs and go from there.


Make sure there is no water in the tank. Siphon some out into a clear container and let it sit for a few minutes. If there's water in the tank you will see it separate in the container. Had the same problem and there was water in the tank. Every time I would apply the throttle it would draw the bad gas and start stalling. Slow it down and it would draw the good fuel. Something to check out....

That's going to be the next thing I'll do. I'll just go ahead and remove and drain the tank, reseal all the fittings and level sending unit, check lines for debris and cracks. New and clean fuel is just the way to go.
 

Silvertip

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Sep 22, 2003
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28,758
Are your alarms working? Reason I ask is the engine may be overheating and losing power. If the alarms are not working you would never know until you seize the engine. Proper fuel/oil mix? Hit the choke/enrichener when the engine begins to die. If the engine picks up then you have a fuel delivery problem. The primer bulb is not supposed to stay firm with the engine running. Fuel is being drawn through it -- not forced through it. If the bulb collapses then yes, there is a restriction between the bulb and the pickup in the tank.
 

vegetto

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Mar 6, 2017
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I've never tested the alarms. I use the mixing bottle for the fuel as with any 2 cycle.

I have tried hitting the fuel enrichener valve and has no effect.

I've been going over the service manual for the engine and it doesn't say anything about having a fuel water separator for it. I see that there's a filter screen at the inlet of the fuel pump and that's it. I think that may be the problem, may be causing a fuel restriction. Has anyone else had that problem after installing a fuel water separator for an outboard? I just wouldn't think that those types of filters would be much of a restriction.

Once the weather gets back to abnormal, I'll get her out again with clean fuel, clean tank, new lines and bulb and a bypassed fuel water separator and we'll see what happens.
 

jimmbo

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May 24, 2004
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12,944
Are your alarms working? .

Alarms? It is a 1985 inline six, there are no alarms on it. Cooling system has no thermostat, only a tell-tale and provision for pressure gauge.



Vegetto
To me it sounds like a fuel starvation issue. Is the fuel tank venting? Try running the engine with a the fuel cap loose. Also pull the pickup from the tank and look at the screen on the bottom of it. Is the screen clean, dirty, or has a jelly on it? Running fine in the driveway means very little, as the engine is not under load. You can have 3 of the cylinders not firing and that engine will still 'run fine' on the muffs.
 

Texasmark

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Dec 20, 2005
Messages
14,544
With what you have my first inclination would be to attack all the fuel system and bring up to current standards; everything in the fuel path. That means everything including new fuel lines, bulb, tank inspection, fuel pump diaphragm, fuel filters (you said you have been into the carbs)....everything including new fuel mix. If I read you right that you have a fuel line water separator in the line between the tank and the engine which is just another obstacle in the way for the fuel pump to suck fuel up to the "tower" and feed everybody at max rpms.

Since you bought a used boat and it is of the type it is, and the hole shot is "excellent" tells me the prop is not the reason for your 4500 rpms and to leave it be. With an excellent hole shot you should be running at the upper limit or so of the "recommended operating range". Rpm max for that engine is 5500. I ran my '88 115 on my 17 ? ft Ranger Fisherman (pad hull) at 6000 when on plane and trimmed for the sweet spot every chance I got for the 7 years I had it and for about that many years my SIL had it of which I am aware. I ran a "ported" Laser (before the advent of the Laser II) SS prop that gave me the best of both worlds, good hole shot and good top end.

If you "warmed it up" before you launched as I think you said you do/did, you might as well drop the lower unit and insert a new impeller in the water pump. The water pump is not made to be run dry sitting on the trailer at the launch pad. While there drain and refill the lower unit with Quicksilver HD gear lube and new gaskets on the drain and fill screws.

Get all that done and come back to this post and up date it with your current conditions for more support.
 

racerone

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Dec 28, 2013
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36,033
Test run with a good portabe fuel set-up.------------If you were starting it on the trailer and dry, you need a new impeller and likely a housing too.-----------Big mistake.
 

vegetto

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Mar 6, 2017
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Replaced fuel lines, bulb, and pick up tube. Started over with clean fuel, removed the spin on fuel water separator. Runs great, I get up to 5000 and did great for the whole trip. Only problem now is that is a ltitle hard to start after warm. I'll get it out again and go over the electrical when hot next time.

Also, when I start the enine at boat launch, I just crank on it until it picks up, never runs for more than 5 seconds. Water pump tell tale just fine.
 
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