Merc 115 Thunderbolt - Electrical Miss?

Booyah Bass

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Have 1 Mid 70's Merc 115/1150 Thunderbolt In-line 6. Have had carbs rebuilt, new timing belt, timing set. Will start and run with WOT. Appears to have a electrical miss. When idled down it will stall out before I can get into gear. Will restart but will do same thing again. Have changed all hoses and removed fuel tank and cleaned. Have run in the dark at house to see if there is possibly a wire shorting. Any suggestions!!!
 

Faztbullet

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Re: Merc 115 Thunderbolt - Electrical Miss?

Will it run/idle with the warm up level at about half way???
 

Booyah Bass

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Re: Merc 115 Thunderbolt - Electrical Miss?

Yes! But it still appears to have a miss. I can idle it down when it is out of the water and put it into forward and or reverse without it dying. As soon as it is in the water it is almost like the RPM's are too low.
I just had a mechanic work on the carbs, new timing belt, brushes on the starter, set timing and he couldn't figure it out.
This is a boat and motor I am fixing up. The wires from the stator were dry rotted and cracking or insulation coming off. I placed heat shrink tube over wires and covered bare wires. Could this be the problem??
 

Faztbullet

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Re: Merc 115 Thunderbolt - Electrical Miss?

What are the compression numbers?? Low compression can sound like a miss and added backpressure for being in water hurts it more
 

75TowerOfPower

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Re: Merc 115 Thunderbolt - Electrical Miss?

Long sot but there could be a smal break in the stator wires and at low idle since the motor normally bouces hard to the right and left it could jar it loose.

You sure the mechanic is a good on for old inline 6's? They can be a hard beast to tame. Make sure he does have the timing set right, with the fast idle it advances the timing so since it runs then your timing may be off.
 

Booyah Bass

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Re: Merc 115 Thunderbolt - Electrical Miss?

He said that when he set the timing it would be ok one minute and the next it would jump around.
The compression is as follows:#1-120,#2-123, #3-120, #4-130, #5-125, #6-130.

I wonder if I advance the RPM's just a little with it in the wanter because of the back pressure if that would help it not stall out also?
 

Chris1956

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Re: Merc 115 Thunderbolt - Electrical Miss?

The timing should not jump around.

The stator simply charges the battery on that motor. It does not affect ignition at all.

Normally modern (read young) mechanics set the carb idle mixture too lean. You might richen it a bit to help the stalling problem. No more that 1/8 turn CCW, should be necessary.

Idle pickup Timing s/b 4-6 *BTDC. Max timing s/b 21* BTDC.


Check lower plug wires for arc to ground. If you find it, replace wires. Check all spark plugs for too clean condiiton. If they look brand new, suspect water intrusion.
 

Booyah Bass

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Re: Merc 115 Thunderbolt - Electrical Miss?

Do you think I could Have a bad spark plug? I do not know how old the plugs are. Should I change the plug wires & plugs??
 

Chris1956

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Re: Merc 115 Thunderbolt - Electrical Miss?

Plugs are cheap. Change them, but don't expect too much.

The wires are not cheap. If they arc to ground, you need to change them, if not. leave them alone. Clean the ditributor cap. They are $200+ so dont break it. The rotors are non-removeable. They are $200 so don't break it.
 

Booyah Bass

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Re: Merc 115 Thunderbolt - Electrical Miss?

How will I know if the wires are arching to ground?

Can any type of wires be used from automotive? I know that many wiring kits kits now come to where you have to cut wires to desired length and then place end fittings.

I have already cleaned the cap & rotor with emory cloth.

I also noticed that the plugs that are in it do not have the end tab for setting the gap - they are flat on the end - is this the normal type plugs for this motor and are they better? If memory serves me, they are NGK.
 

Jacket4life

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Re: Merc 115 Thunderbolt - Electrical Miss?

How will I know if the wires are arching to ground?

Can any type of wires be used from automotive? I know that many wiring kits kits now come to where you have to cut wires to desired length and then place end fittings.

I have already cleaned the cap & rotor with emory cloth.

I also noticed that the plugs that are in it do not have the end tab for setting the gap - they are flat on the end - is this the normal type plugs for this motor and are they better? If memory serves me, they are NGK.[/QUOTE]

Yes, those are the right type of plug. And I, personally, prefer NGK. Also, I personally, would change the plugs first. Unlike Chris, my experience with these age motors has been that new plugs make a huge difference, all other thiongs being equal. Most of the ppl on this forum reccommend changing them annually. Of course, all other things may not be equal in this case. But my first trip to this board was about a Mercury with similar issues you are describing, and plugs were an instant fix. Plus, they are cheap, esp, compared to almost everything else having to do with a boat! :D

If I remember correctly, there are two different plugs reccommended for this age motor, do some thread searches on here, I've read several threads in the past where this was discussed. Mine runs well on the ones I have, I always just take one to the store in the spring and get 4 new ones.
 

Booyah Bass

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Re: Merc 115 Thunderbolt - Electrical Miss?

I appreciate all of the help! I am very impressed with this site and the quick response from everyone! This is my first time and I have already picked up some good tips just by reviewing different threads!
 

Chris1956

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Re: Merc 115 Thunderbolt - Electrical Miss?

Bass, The plug wires are stranded stainless steel. If you see, hear or feel arcing to ground, they are bad. Automotive wires will not work well.

Plugs for the motor are gapless. No other plugs are specified. I have never seen any difference between Champs or NGK. Take your pick. I have used Autolite as well.
 
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