1986 25 Hp Merc dies @ half throttle

[Case450]

My 86 merc ser. #6008686 sputters out at half throttle on both acceleration and deceleration.

I took it to the mechanic and he said they couldnt finger the problem and suggested I throw parts at it until it's fixed. He said to change the cdi which costs more then what i paid for the motor.

I was under the impression that a cdi either works or it doesn't. So I'm wondering if it's a trigger switch or something. Other then this sputtering the motor is in perfect shape. I have new plugs, coils, gas, fuel lines, cleaned carb and it has over 120 pounds in each cylinder.

There has to be someone in the world here that has an idea or a similiar thing happen to them. Can anyone help me please?

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

2 stroke or 4? sounds like a fuel delivery problem. the water pump supplies presure to run the fuel pump on most small mercs. do you have a good stream of water out the pisser? a bad waterpump will effect the fuel pump giving less than needed presure to suply fuel also check the fuel pump check valves if there shot you will lose fuel suply as well rebuild the fuel pump and possibly the water pump./ thats my guess. If the engine is 2 stroke and has been sitting you may want to check the float to see if old oil gas mixture has hardened and made the float stick. My engine i bought a 40 HP 1979 merc had the float almost cemented to the floatbowl

 

[Case450]

Re: 1986 25 Hp Merc dies @ half throttle

it's a 2 stroke...the carbs are cleaned (had done at the merc dealer)...the pisser seems to working well...good pressure in the stream...lol..not too sure how else to tell tho....but it's a full blast comin' out the hole.....is there a video or something somewhere where i can check this float?

i'd like to check it myself to know for sure that it's working fine so i can rule it out....i hate having to trust the mechanic...most mec's are good but some are shysters.....like i dont think it's electrical like the last mechanic told me because it idles perfect, runs at wot fine...just half throttle is the issue....i just wanna get it running right!

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

no video just another joe trying to help out here. do you have a way to check the compression? lets revue new plugs clean carb water pump good guess that leaves the fuel pump by process of elimination. or a clogged jet in the carb. Hate to think the walls are scored that would be a nightmare. see if you can loan a tool and get the comp tested if all eles fails. I would think a mechanic would at least check this above all else though. the diapham in the fuel pump steches after some time and needs to be replaced like a baloon thats been filled to many times rendering it next to dead. I would in any case make sure the fuel pump got rebuilt before i set out on the gulf of mexico. long row home

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

also 1 more thing do you have a tiller handle or a throttle. if it,s a throttle can you start your engine in drive? may mean nothing but the kill switch for reverse and foeard by a chance in hell may be shorting out at half throttle i,m just throwing possibilities at you here though

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case 450. what you really need is a OEM MERC MANUAL for that motor..
That would be your best bet.
I almost cried when I read SHYSTERS. My mom was from Ireland. and used the word all the time.
Hell I forgot what your problem was.

 

[Case450]

Re: 1986 25 Hp Merc dies @ half throttle

lol...shysters...well i'mma country boy and we use that all the time..haha

back to the motor..yeah, the compression is good...i think there was over 120 pounds in each cylinder... the more i think about it i think your right...it has to be a fuel delivery issue...because if it was electrical or something it just wouldn't work. I mean, electrical is either on or it's off.....this motor is a tiller....but see, it starts fine, idles fine, kill switches off fine, and if you fight with it and get it to wot it will run at wot fine.....just that halfway point....now if i can find a manual or diagrams online i'd like to take that carb apart and have a look-see

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

I think if i were you I wouldn,t mess with a carb you say they rebuilt I would changet the diaphram in the fuel pump as i said it becomes warped and lose after a while and my mistake was the water pump doesn,t give it the pressure the exhaust does. it pulse against the diaphram that pumps the fuel through the line to the carb. when worn it loses flexability and therefore pumps less pressure. a fuel pump repair kit it relatively inexpensive and a definite + toward feeling secure when it comes to fuel delivery. I also think they would have checked the gap on the plugs if they have a gap mine don,t there just a circle spark plug gaps are crucial to performance. get the specs and check

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case you are wrong about elec on or off when it comes to 1984 and above motors.
Let me read your post again Shyster.

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

lol shyster I know alot of them

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case450.
Have you had the fuel pump rebuilt?

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case.
Ok . runs WOT No problem?
Idle/ trolls No problem?
Mid range sucks Yes/NO or is transition to slow/fast/ stumble.
Don't Make ME come up there and fix that motor without catching some BIG PIKE.

 

[Maxz695]

Re: 1986 25 Hp Merc dies @ half throttle

I told him several times to rebuild the fuel pump but he,s avoiding the problem so i,m giving up tonight talk some sense into him

 

[Case450]

Re: 1986 25 Hp Merc dies @ half throttle

lmao....i'll give you all the pike you want...i can guarantee 40" plus.....at any rate...the fuel pump hasn't been looked at....and i figured the mechanics would have looked at the carb first...but i've been wrong before, more times then i care to admit! haha

this motor runs perfect at idle and if I fight with it it'll get to WOT and run there. I'm not sure for how long though because I obviously get farther and farther away from the dock at this point and when i try to throttle down to turn back around it'll sputter and die at half throttle. I let it sit for a minute and pull it and she firest back up and idles perfect. I throttle up and as soon as i hit half throttle it'll konk right out but if i fight with it i can feather it through to open her up again to WOT. Again I dont know for how long because now i'm pointing at the dock...soon as i throttle down again it happens again.

I have since taken the carbs right apart (I forgot my camera sorry) and everything looks fine, I blasted carb cleaner into every orifice I could find. There were no tears in the diaphragm....not sure whats going on with this rig.

I really do appreciate the help though. It could be an intermittent electrical problem on 84 and newer? I just sourced a good used cdi...should I get it then and try that?

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case. you have to rebuild the fuel pump. You can't see minor pinholes in the diaphram.
It's like 15 bucks american

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case don't throw money at it. Usally a motor won't run wot with a bad cdi.??
Ice out Big Pike

 

[Case450]

Re: 1986 25 Hp Merc dies @ half throttle

Okie dokie...i'll pick up a fuel pump this week and try it again...will keep yall posted

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Just the rebuild kit, Not the entire pump. OK. (AYE )( canuck spelling?)( It's just a diapram basicly(SP?)

 

[Mi duckdown]

Re: 1986 25 Hp Merc dies @ half throttle

Case I just reread your first post. Although I would still rebuild the fuel pump.
My manuals are at my cottage. the hesitation in mid range has got me thinking.
How bad is the hesitation. My 1980 25 does the same for like a second. between idle and full power. But i'm on plane in 10 seconds in my 16 foot sea nymph.??

 

[Case450]

Re: 1986 25 Hp Merc dies @ half throttle

I have this motor on a 15.5' mirrocraft deep and wide aluminum tiller. This one planes out fairly quick but the sputter will actually kill the engine if your not ready for it. You need the reflexes of a mongoose (and know it's coming) to fight through the sputter to get to the high end of the throttle range. Same when throttling down.

I mean, what has me is the marine mechanics I took it too for them to have a look at are no slouches. I mean, i took the motor to those guys because those guys are the guys you go too when no one else can fix your ****. On the other hand, it is a mechanical motor, there should only be so many things that can affect the motor the way it does right? - i mean, eh?