2002 25HP Great spark cylinder won't fire

redmist

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I have a 25 HP 2002 Merc SN 0T557192 Mod 1025201ZD I have lost power and motor runs rough. I remove the lower spark wire and there is no noticeable difference in how the motor runs. Also, the top plug gets warm quickly and the lower barely gets warm. I have an excellent spark from both plugs/wires but lower cylinder does not seem to be firing. I have swapped coils and plugs but the same problem persists. I am thinking my switch box may be bad and is timing the spark incorrectly. Part number on the switch box is 855713-A2 but I can only find the A3 (Non-Commercial) and the A4 (Commercial/Jet 20) on this site. All help is greatly appreciated!

Thanks ahead for your help, Dave
:confused:
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

I will try a compression test this morning. Any input is appreciated. Thanks, Dave
 

Laddies

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Re: 2002 25HP Great spark cylinder won't fire

Make sure the flywheel magnets have not come loose.
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

I will go and check but could you give me an idea of how to find them please. Thanks again, Dave
 

Laddies

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Re: 2002 25HP Great spark cylinder won't fire

You will need to pull the flywheel, use a puller and don't strike the flywheel with a hammer just the puller bolt, lf you hit the flywheel it can cause the magnets to break.
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

Can see magnets clearly. Small piece of metal stuck to one magnet. 3mm x .5mm. All magnets seem to be clean well attached and alligned correctly. Do you know what the timing shoud be set at for this unit???? I noticed that the idle adjust on the choke will not increase the idle because the rod that connects it to the timing advance linkage will not bottom out and pull linkage forward. When my dad took the boat to a mechanic he was told that it was a problem with the switchbox. I found both fuel tanks and sediment bulb had over a quart of water in them. I am wondering if a not so honest mechanic may have adjusted the timing to make motor run rough. Besides wouldn't the first thing to check on a poorly running boat motor be water in the fuel???
 

Laddies

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Re: 2002 25HP Great spark cylinder won't fire

25 degrees at WOT. The only thing that may have been done to the timing would is to retard it and that would not make the engine run rough only slow the RPM down. Use a spark gap on the ign and see that you have spark on both cyls. If not swap the leads to the coils and if the spark changes cyls. then the box is bad if not the coil is bad on the cyl. without spark.
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

Thanks again for all your help Laddies!
I swapped green wires on coils with no change. Both coils will jump over 1.5 inches to ground and very hot/blue across the plug as well. I even swapped plugs again. For some reason, the spark seems to be firing too early or late to get correct timing. Could the pickup be partally bad and how do I test it???
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

Laddies, if you are still out there, I'm begining to think that I may have too much fuel fowling the lower cylinder. Although it dosn't look like it, I think it could explane the problems I'm having. Any thoughts are appreciated, Dave
 

Laddies

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Re: 2002 25HP Great spark cylinder won't fire

With good spark on both cyls. that only leaves compression or fuel and there is a possibility of flooding. Normally if the engine is flooding you will see quite a bit of smoke from the exhaust
 

j_martin

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Re: 2002 25HP Great spark cylinder won't fire

Check the compression. Ignition and fuel make no difference if you do not have compression.

Compression + fuel + spark = fire.

hope it helps
John
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

Did a compression test last week. 100psi +or-. I ordered a carb kit. Other threads have said a bad fuel pump can pump too much fuel and flood the engine. I ran the motor in a barrel of water last night and now have a thick oil slick on the water.
 

sschefer

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Re: 2002 25HP Great spark cylinder won't fire

If you have good strong spark on both plugs, and the initial timing on # 1 while cranking is within specs then # 2 is within specs give or take a few degrees for crankshaft variance and bias circuit deviations. The later is not enough to cause you the problems you're having.

If the spark is in fact good but the initial timing taken on #1 cylinder is overly advance by 180 degrees then check the spark on # 2. If it is at spec,(just use 0 as a close enough reference for this test), then your spark plug wires are crossed or your coils lead wires are swapped either at the switch box or the coils themselves.

Your engine is a single carbed engine and fuel is delivered to the crankase simultaneously to both cylinders. If # 2 is not firing then the raw fuel and oil mix will flow out the exhaust and into the water. If the plug or coil wires are swapped what you may be experiencing is your engine starting and running backwards. This can and does happen under the right conditions.
 

redmist

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Re: 2002 25HP Great spark cylinder won't fire

Thanks sschefer. I did try to switch coil wires but couldn't get motor to start. Motor is running in correct direction. I will give an update after I get carb kit installed on Friday. Thanks again to everyone. This is one of the best forums I have ever used!
 

sschefer

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Re: 2002 25HP Great spark cylinder won't fire

I thought about that John, single reed block, single carb, split manifold. If it were reeds, I would think it would be sneezing badly and blowing fuel out the carb. I didn't read that in his posts so I discarded the idea.
 

sschefer

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Re: 2002 25HP Great spark cylinder won't fire

I missed something when I read your first post. The part about the idle adjustment not working properly. When you pulled the flywheel did you pull the stator off and check the trigger to see if it had any visible damage and was moving freely?
 
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