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Old 07-19-2017, 06:26 PM   #1
bigdog
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Default Trouble shooting issue on friends boat

Trouble shooting issue on friends boat.

1986 Four Winns 21' bow rider
OMC outdrive

Boat is running real rough at WOT.
Boat seems to run fine at idle, even at 1/2 throttle.

Coil replaced last year.
Fresh gas
New water separating filter.
New gas filter
Just added gas additive to condition the gas and clean the carb.
Washed Flame arrester in cleaning fluid

With all this, still runs rough as described above.

Only other thing I could think of were spark plug wires loose from plugs or coil. Possibly maybe fuel pump ?

Thoughts?
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Old 07-19-2017, 06:31 PM   #2
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Try new cap and rotor.
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Old 07-19-2017, 06:36 PM   #3
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If there's a filter sock in the gas tank it may be clogged. You can eliminate fuel starvation by running off a portable can but be sure to set it up carefully to avoid an accident.
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Old 07-19-2017, 06:37 PM   #4
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What Homewood said plus new plugs and wires...

Good Luck!

Dan
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Old 07-19-2017, 07:37 PM   #5
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Check fuel pressure at WOT, assuming it's carbureted, it should be around 6 PSI. If it's lower than that, it may be starving for fuel at WOT.
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Old 07-19-2017, 08:30 PM   #6
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We used to have a 23' Monterey. It ran rough and sputtered one day. Pulled cap off and the contacts were all blueish green with some light corrosion. New cap and rotor from Trexlers and off it went.
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Old 07-19-2017, 09:20 PM   #7
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Thanks everyone for the suggestions !

Quick question......

If the issue is NOT happening at low speed, or 1/2 throttle why would a new Cap & Rotor affect WOT ?

Will check all mentioned and circle back once the issue has been solved !
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Old 07-19-2017, 09:47 PM   #8
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Ingnition parts will leak electricity to unintended places. "Arcing" is what it's called.

Under a load they leak more.

Or it could be fuel strvation...

Waaaay back in the old 70's I opened the hood on a friend's misbehaving car.

It was night time and dark.

While he revved the engine I was treated to a light show around the plug wires!

Proper trouble shooting will lead you to the cause but if you wish to be a "parts changer" start with inspecting the rotor and cap. If they're not very clean replace them. If they're looking good, wires and plugs are next.

Or...it's fuel starvation...
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Old 07-19-2017, 10:50 PM   #9
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Quote:
Originally Posted by HomeWood View Post
Try new cap and rotor.
I replaced the cap, rotor, wires and plugs this spring and could not believe what a difference they all made. There was a lot of pitting on the cap. I don't know what the replacement frequency is but it probably should be every year or two.
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Old 07-20-2017, 08:40 AM   #10
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to me it sounds like the high speed jet in the carb is clogged, since you say it runs fine at idle and mid-range.
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Old 07-20-2017, 09:00 AM   #11
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Quote:
Originally Posted by dt5150 View Post
to me it sounds like the high speed jet in the carb is clogged, since you say it runs fine at idle and mid-range.
That would be very unusual. The mains are big enough for substantial crud to flow through and the metering rods will tend to break up the crud that collects.
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Old 07-20-2017, 09:11 AM   #12
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plugs, cap, rotor, wires and check the timing
is where I would start
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Old 07-20-2017, 10:06 AM   #13
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Default Failing fuel pump

I experienced a similar problem and it turned out to be a failing fuel pump. At low rpm the pump was able to provide enough fuel, but at WOT it was failing to push enough fuel. I could still run wot, but you could hear and feel slight hesitations in the engine as fuel pressure dipped.
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Old 07-28-2017, 03:26 PM   #14
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Quote:
Originally Posted by rick35 View Post
I replaced the cap, rotor, wires and plugs this spring and could not believe what a difference they all made. There was a lot of pitting on the cap. I don't know what the replacement frequency is but it probably should be every year or two.
Rick35 and Homewood guessed correctly !

My friend had a local marine tech meet him at the dock, who found that the distributor cap was 'cracked' !

Tech replaced dist cap, Points, Rotor, Plugs, and now the boat runs like new.

Knowledge shared !

Thanks for everyone's feedback, always appreciated !

Bigdog
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Old 07-28-2017, 06:19 PM   #15
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Quote:
Originally Posted by 8gv View Post
Ingnition parts will leak electricity to unintended places. "Arcing" is what it's called.

Under a load they leak more.

Or it could be fuel strvation...

Waaaay back in the old 70's I opened the hood on a friend's misbehaving car.

It was night time and dark.

While he revved the engine I was treated to a light show around the plug wires!

Proper trouble shooting will lead you to the cause but if you wish to be a "parts changer" start with inspecting the rotor and cap. If they're not very clean replace them. If they're looking good, wires and plugs are next.

Or...it's fuel starvation...

You're welcome!

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Old 07-28-2017, 08:04 PM   #16
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This has been my year for engine repairs. Did the full tune up on the boat this spring and replaced a head gasket this week on my lawn tractor. Thank you youtube! Wouldn't have tried either without it.
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Old 07-31-2017, 01:42 PM   #17
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Awesome! Glad it's fixed.
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