Could weak spark be the cause of low power?

bobgritz

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When it runs in a tank it seems to have plenty of power but under load it feels like your towing another boat. It would be perfect if you did nothing but trolling but when you crank up the rpms it struggles to push my 14' aluminum boat.

Compression is 60 lbs. in each cylinder. Obviously 80-110 would be better but I've read that as long as they are similar, even lower compression is okay in these old timers.

Is it possible that an engine can just be tired? Would a ring job even make any difference? What should I look for if I do a spark/gap test and could a weak spark be the cause of low power?
 

F_R

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Re: Could weak spark be the cause of low power?

I see no clue here as to what we are working on, but a generic answer is absolutely, a weak or no spark will make it have a severe loss of power. That is because it won't be firing on all cylinders.

If it is starting and running ok except for no power, and the compression is even, it is not a compression-related problem.

What to look for??? Again, what motor?
 

bobgritz

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Could weak spark be the cause of low power?

My bad ... it's an old 1968 9.5hp Sportwin (2 cylinder)!

I pulled the flywheel and cleaned and adjusted the points to .020".
 

boobie

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Re: Could weak spark be the cause of low power?

Got a model # ?????
 

boobie

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Re: Could weak spark be the cause of low power?

Pull one spark plug wire at a time and see if you have a dead cyl.
 

bobgritz

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Re: Could weak spark be the cause of low power?

bb ... I think we posted at the same time ... it's a '68 9.5hp Sportwin (2 cylinder)!
 

1946Zephyr

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Re: Could weak spark be the cause of low power?

Well, a 1968 Sportwin would be any of these model numbers:

Model MQ-14D
Model MQ-14E
Model MQL-14D
Model MQL-14E

The coils are probably in bad shape and need to be replaced and/or the points are in need of replacement. This motor is 42 years old. It's probably time to recondition the entire ignition system.

You may also have a weak fuel pump too. If you're saying it's loosing power, I would check that as well.
 

bobgritz

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Re: Could weak spark be the cause of low power?

Model 9822-D (From Plate) ... Could that be an Evinrude?

Boobie ... you hit the nail on the head. I came in yesterday running on only one cylinder. I checked for spark and top cylinder plug had none so I pulled the flywheel. Top cylinder's points were not even opening and the other was barely opening. I cleaned and adjusted both, put the flywheel back on, then checked for spark and they were both firing and when I pulled each wire there was a definite drop in rpms.

I go out everyday with boat and I ASSumed that my lack of power today was something new. I just went out and pulled wires and the top cylinder plug is not firing again. I'll go out now and re-pull flywheel and see what went wrong. Stay tuned please!
 

bobgritz

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Re: Could weak spark be the cause of low power?

SOLVED:

Worked on this in my garage until after sundown last night. As it got darker and darker out, and while pulling the recoil starter, I happened to catch sight of light flicker beneath the armature plate. I used a tiny dentist's mirror and every time I pulled the starter there was a discharge of a spark. I pulled the flywheel and armature plate then discovered the problem.

Both spark plug cables were torn and worn where they enter the bottom side of the armature plate. The plate moves every time that the throttle speed is changed and the spark plug cables are flexed back and forth. It's similar to flexing something like a paper clip or metal clothes hanger back and forth until it breaks.

The center feed wire was totally severed in one of the cables and nearly severed in the other. I replaced them with a couple of leftover spark plug cables and the engine fired up. I suspect my lack of power (under load) was due to the lack of integrity of the cables (at the flex-joint) yielding a weak spark.

Thanks to all for responding.
 

aerobat

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Jun 1, 2011
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Re: Could weak spark be the cause of low power?

ah... edit- i see its solved !
 

registeroot

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Re: Could weak spark be the cause of low power?

i love it when people update to say they have fixed it! closure!
 
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