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Author Topic: '93 Mercury 25hp tuning?  (Read 8892 times)

Jethro

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'93 Mercury 25hp tuning?
« on: Jul 22, 2013, 07:00 AM »
I recently picked up a boat with a 25hp Merc, 1993 vintage. It doesn't seem very smooth at all when at trolling speeds. Shakes pretty good. Is this normal for this motor? It also stutters a bit when it's at speed, it will run perfect for 30-60 seconds at a time then looses power for a few seconds (not completely, just seems to slow down a bit) then it comes back into it no problem. I've ran for 10 minutes like that, so it doesn't seem like a huge problem. I'm pretty good with motors and such, I've built a few motorcycles and work on my sleds and some very small 2 and 3 hp outboards but this one looks completely foreign to me when I take the cowl off. Is there anything that I can try to adjust? Are there 2 carbs on this 2 cyl motor that I can synch or is it one carb? It doesn't even look like a carb that I am used to. I didn't see anything under there that looks like it's serviceable. I have yet to do a compression test on the motor but probably will tonight. Anything else I can check? Any suggestions? Thanks.

OTIS

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Re: '93 Mercury 25hp tuning?
« Reply #1 on: Jul 22, 2013, 07:18 AM »
Ethanol wreaks havoc on outboards; you could look at the fuel filters and lines.  You may want to install a fuel filter/water separator.
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Dispy

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Re: '93 Mercury 25hp tuning?
« Reply #2 on: Jul 22, 2013, 08:46 AM »
"Meet the new with your ears, eyes and mind wide open, and your mouth tightly shut."

Jethro

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Re: '93 Mercury 25hp tuning?
« Reply #3 on: Jul 22, 2013, 09:06 AM »
Nice, once again with the good links. Thanks!

TunkTroller

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Re: '93 Mercury 25hp tuning?
« Reply #4 on: Jul 22, 2013, 12:27 PM »
I have owned a number of the Merc 25's. The motor itself has changed very very little since the early 80's. Mostly just cosmetic changes to the cowling. They are good motors and they really turn up however ethanol really messes with this particular motor.

The bottom cylinder in particular (can't remember why) is prone to blowing on these to begin with and the ethanol really enhances the possibility. I would guess you have some serious build up in the carbs causing the rough idle and low speed. You can play with you low speed idle to see if it helps but I would seriously advise getting the carbs cleaned anyway. If you keep up on it you will not have a problem.

Also try and use up your fuel and not fill the tank with too much so that it will sit for periods of time without being used. The longer the fuel sits without mixing the more likely you are to have problems. So in other words only put in the tank what you think you will burn within a week
 

Jethro

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Re: '93 Mercury 25hp tuning?
« Reply #5 on: Jul 22, 2013, 12:41 PM »
Is the low speed idle the choke pull as well? The cowl says to spin the choke pull to set idle, but when I turn it, it doesn't change anything at all. It only seems to turn a half a revolution as well.

I'm overwhelmed by the motor, which is funny. I have rebuilt dozens of Suzuki 4 bank CV carbs on motorcycles which are very complicated carbs. But this outboard doesn't even look like a carb, just a block? I am a little scared of working on it, but I'm looking into finding a Mercury factory shop manual and once I have that I'll be good to go.

TunkTroller

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Re: '93 Mercury 25hp tuning?
« Reply #6 on: Jul 22, 2013, 07:24 PM »
There should be a low speed screw on the carb as well. I know what you mean by spinning the choke knob and honestly I dont remember that ever doing anything even on brand new ones.

Have not looked at one in awhile because I switched to 4 stroke after having ethanol problems but if I remember correctly the choke turning actually just adjust the amount of choke applied when pulling it out.

Jethro

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Re: '93 Mercury 25hp tuning?
« Reply #7 on: Jul 23, 2013, 07:02 AM »
if I remember correctly the choke turning actually just adjust the amount of choke applied when pulling it out.

Oh, you may be right there. This motor barely needs any choke, it fires right up and the choke can be pushed right back in. Installed new plugs last night, the old ones are not the recommended plugs, maybe that will help my cause. Old plugs did look to have a pretty good burn, maybe a little rich, but certainly not lean.

fishlessman

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Re: '93 Mercury 25hp tuning?
« Reply #8 on: Jul 23, 2013, 12:47 PM »
i had to change the lines on my 92 merc 115 a few years ago, they were leaking and collapsing, take an end of the line and see if the insides smear off on your hands. they could be leaking or letting air in. i had to change the main tank lines and the small lines under the cowl

joefish1

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Re: '93 Mercury 25hp tuning?
« Reply #9 on: Jul 23, 2013, 03:20 PM »
Check this link out  http://www.marinepartsplus.com/catalog/category/outboard/2_stroke/mercury/1-25HP

Find your motor and look at the parts.  The problem I had was the seal was to thin just before the reads causing an air leak by the small hole on the bottom of the seal.

Jethro

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Re: '93 Mercury 25hp tuning?
« Reply #10 on: Jul 24, 2013, 07:23 AM »
Changing the plugs seemed to make a big difference. Put a half can of SeaFoam in the tank and went for a good long run. Didn't stumble at all. Very responsive now too.

eyedoktr

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Re: '93 Mercury 25hp tuning?
« Reply #11 on: Jul 24, 2013, 01:32 PM »
Changing the plugs seemed to make a big difference. Put a half can of SeaFoam in the tank and went for a good long run. Didn't stumble at all. Very responsive now too.

It will continually do this if you use it for trolling. Running at that low a speed will start to foul the plugs and the motor will begin to bog down. Can the motor be used to troll ? Yes. Should it be ? No.
Pete Lewis

threeten

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Re: '93 Mercury 25hp tuning?
« Reply #12 on: Jul 25, 2013, 09:11 PM »
Changing the plugs seemed to make a big difference. Put a half can of SeaFoam in the tank and went for a good long run. Didn't stumble at all. Very responsive now too.
was going to ask if you tried any additives! seafoam is a godsend haha.used to run two 5 gal buckets with holes cut in bottom and sides-like a drift sock but cheaper ;D-so I could keep the idle a little higher at trolling speed on my little boat.it helped keep it from loading up so much.probably harder on the motor though. make rope handles for the buckets if you do this though!
still run two drift socks on my boat now too.help control speed better.

 



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