35 h.p. merc stumbles @ high speed

doug mccann

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Jan 24, 2003
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My 1986 35hp Merc runs great until you go past about 75% throttle. Then it flutters/stumbles and doesnt go any faster. It smoothes back out when you back off the throttle.<br /><br />I put new plugs in and it ran fine for one trip down the lake. Then back to same old problem. It has always been like this in the four years I have had it. Marina replaced fuel pump diaphram, checked carb, tried different prop, ran up the lake with voltage meter on it, checked compression, and cant find anything that causes it. I have also replaced bulb and hose to fuel tank. <br /><br />Because new plugs make a difference one time leads me to think it is something electrical. Coils maybe? Someone told me I should try swithcing to plugs with the j post instead of the flat/no post type called for. I'm wondering if the j post might hit the piston.<br /><br />Does anyone have any idea what is wrong with this motor?
 

alcan

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Dec 14, 2001
Messages
2,505
Re: 35 h.p. merc stumbles @ high speed

Hi Surefish<br /> It sounds to me like fuel starvation. Have you tried squeezing the primer bulb while the problem occurs? How about putting some premix fuel in a spray bottle and squirting some in the carb during the occurance? Maybe the syncronizing is incorrect? Choke working properly? The plug change would only help under extended low speed operation (a lot of trolling) and thats iffy.
 

doug mccann

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Jan 24, 2003
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Re: 35 h.p. merc stumbles @ high speed

I have tried squeezing the bulb but it didn't help. I will try squirting fuel into the carb (got to wait until the lake thaws out). I think the choke is working ok, at least it works properly when I choke it to start it. What do you mean by "syncronization"? Is that the same as the timing?<br /><br />If it is starved for fuel, it seems like new plugs wouldn't have temporarily cured the problem. <br /><br />If spraying fuel into the carb doesn't make a difference, what should I try next?
 

alcan

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Joined
Dec 14, 2001
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2,505
Re: 35 h.p. merc stumbles @ high speed

Hi Surefish<br /> The timing or spark advance is syncronized to the throttle. So when you operate your controll, you advance the timing and at the appriopiate place begain to open the throttle. I could be that your throtle isn't opening full up. The reason I asked about the choke was to see if it stayed in the off position during opperation. some times the leaver says off but the plate says on or partly on. These are just some ideas to check out. Of course you could be dropping a cylender do to an electrical problem. It just sounds more fuel related sitting here. Darn that ice.
 

87Stratos

Cadet
Joined
Feb 13, 2002
Messages
20
Re: 35 h.p. merc stumbles @ high speed

I had the same problem with my Merc, I checked a list of possible causes for months on my own until finally I had my mechanic check it out. Mine had the same symptoms and the only thing that I did that made a slight difference was changed the spark plugs and to me this did not make since. Finally the mechanic found the problem was a bad stator. My total bill was $288.<br /><br />Good Luck.
 

doug mccann

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Jan 24, 2003
Messages
3
Re: 35 h.p. merc stumbles @ high speed

Thanks for the info guys, I will follow up on these possibilities.
 

mellowyellow

Vice Admiral
Joined
Jun 8, 2002
Messages
5,327
Re: 35 h.p. merc stumbles @ high speed

might want to check the fuel pump diaphram while<br />you're at it.<br />good luck,<br />M.Y.
 
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