Mercury 402: Can only turn flywheel with spark plugs out

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eldplanko

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I have a 1975 Mercury 402 SN: 42777XX. It's been in storage 2 years. The flywheel will only turn (either by the pull-start or by the startor motor) with the spark plugs out. Any ideas on what is wrong?
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Thanks for your quick reply and helpful link.

The part I still don't understand, is why can I not rotate the flywheel (by pulling on the pull handle) with the spark plugs in.
 
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Chris1956

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Ed, Is it possible that the tips of the spark plugs are hitting the top of the piston?

You should verify that the plugs are the correct ones. A lot of mercs used the 1/2" reach, gapless plugs(like Champ L-76V) in the older motors. If someone put in some 3/4" reach, gap plugs you might have clearance issues.
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Thanks for the great adive. I'm using NGK BUHW spark plugs. I've used these ever since I got the engine, with no problem.

I have to admit that I'm unfamiliar with the internals of this engine. Are there valves? Could an exhaust valve be stuck closed?
 
saumon

saumon

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Re: Mercury 402: Can only turn flywheel with spark plugs out

No valves.
 
This_lil_fishy

This_lil_fishy

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Eat yer wheaties lad....

Like a snowmobile, good compression means lotsa armstrong starting. The sparkplug thing was a good thought, but if it turns over fine without plugs it should be fine.

Ian
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

I'm convinced it's an airflow issue. I can put my finger on the spark plug holes and feel a vacuum as I turn the flywheel.

Does this means it's an intake problem? I'll look at the carb...
 
saumon

saumon

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Re: Mercury 402: Can only turn flywheel with spark plugs out

If you can't turn it, it looks to me that it's stuck in gears. Maybe the shifter has slipped out of neutral. Can the prop be rotated freely in either directions?

BTW, vacuum on the down stroke is normal.
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Thanks for the great ideas, I appreciate all the help.

It's definitley not in gear. I know the lower unit on this, as I previously did a total teardown and rebuild on it.

Would the intake or the exhaust being plugged cause this?
 
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CharlieB

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Pull starter does NOT work with the plug IN?

But the pull starter does work with the plugs out?

I've seen many poor connections/starters/batterys have this problem but NEVER with a pull starter. Well, there was one just yesterday that I had to use BOTH hands, (I'm getting weak, I think).

If the pull starter works without the plugs, you (and I) maybe need to start working our arms a bit more to get in shape for fishing season.
 
saumon

saumon

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Did you try the elct. starter with a fully charged battery?
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

With the plugs out, it will turn over, VERY difficultly with the pull start (and I'm not light in the arms). It is definitley more difficult than it should be, and no where near fast enough to fire up. With the electric starter (jumped via the boat battery wires to my truck's battery... truck running), it turns maybe once per second. I'm thinking poor electrical connections also (and will try jumping directly to the starter), except for the fact that it is so hard to pull with the pull starter.

Again, thanks all the advice, you guys are great.
 
BR

BR

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Your original post stated that motor was stored for two years. Was it fogged before storage? Maybe try spraying some sea foam or 2 cycle oil in plug holes. Cylinder walls may be completely dry or worse...rusted.
 
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Scoots

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Hi,

I don't mean to "piggyback " on your issue, but, I am having a similar(apparently compression) problem with a 1970 20hp. The motor turns over nice and easy out of water and wants to start but doesn't. If I put it in water it is next to impossible to pull. There was some debris thatg shot out of the exhaust ports under the powerhead and I could kind of see where some mud wasps had been in these same ports. Could this be the problem perhaps for both of us??
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Not flogged, butI don't think piston binding on the cylinder wall is the problem. I turns fine with the plugs out. I pulled the carb, and the intake sideof the engine was nice and shiny. I'm thinking exhaust side might be plugged.
 
daveswaves

daveswaves

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Re: Mercury 402: Can only turn flywheel with spark plugs out

You said you overhauled the lower end. Did this problem start after that? The lower end is not that hard to take off, I would suggest that you drop the lower end and then see if the powerhead is free to turn over. Something is "bound up" and it is not a blocked exhaust or intake, you still have difficulty turning it over with the plugs removed, according to one of your posts. That leads me to believe the lower end may be the issue.
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Lower unit was 4 years ago. Worked fine for 2 years after that.

Thanks for the good advice though.
 
daveswaves

daveswaves

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Re: Mercury 402: Can only turn flywheel with spark plugs out

eldplanko said:
Lower unit was 4 years ago. Worked fine for 2 years after that.

Thanks for the good advice though.
Click to expand...
Yes but then it sat for 2 years. If nothing else you should be changing the water pump impeller anyway, every 2 years is the norm. So, as long as you have the lower end off to change the water pump impeller you can check to see if it is any easier to turn the power head over. If not, you have work to do inside the power head.
 
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eldplanko

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Re: Mercury 402: Can only turn flywheel with spark plugs out

Thanks for the great advice. I was thinking the same thing, but didn't want to waste the time on the lower untill the upper was working.

BTW, not sure I ever said it was difficult to turn with the plugs removed. It turns quite freely with the plugs removed.
 
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