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'77 Merc Thunderbolt not idling... now what? I have changed/fixed a few things.


HotRod53F100's Avatar
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06-01-11, 04:40 PM   #1  
'77 Merc Thunderbolt not idling... now what? I have changed/fixed a few things.

OK, last year this motor started right up and ran well in the driveway. It started right away and ran smooth. Since that time I had removed the lower unit, and long story short, I took it to a Marine repair place. He reinstalled the lower unit and changed the plugs, but he asked "has this motor been sitting for a while"? "You need to take this out and run it, the carbs need some fuel cleaner run thru them, once that is done, it will be fine. He also commented that he had a hard time getting it started which was odd, although it had sat over the winter after I had fogged the cylinders and had run some StaBil thru it.

Once I got it back, I had one heck of a time getting it to start. I need to put the fast idle 1/2 or better to get it to idle. With the fast idle at 100%, I could tell that I wasn't getting enough RPMs. I discovered that a plug wire must have been broken when the plugs were changed, so I changed them all. I ohmed from the plug wire to ground and from the primary on the coil, they were all equal. I also replaced the primer bulb, the fuel connector, and the fuel line... and yes, the vent is open.

The motor runs smooth, its almost as if I adjusted the idle up, it would be OK. If I turn the fast idle down to normal, it stalls. Like this, I would never be able to turn off the fast idle and get it in gear. I just don't understand, why I should have to adjust anything, what has changed? With the motor running, I pulled the top 3 plugs one at a time, each had spark but maybe not equal intensity, hard to tell.

Here are the things that have changed since last year:
* plugs are replaced with NGK centerfires, used to have AC regular electrodes
* Added some SeaFoam to the fuel
* at one point with the key off, I accidently touched the wire to the battery terminal with it connected backwards. And I mean just for a second.

Questions:
* could adding centerfire plugs cause this?
* could it be because of the 2 oz/gallon of SeaFoam?

Any ideas?

 
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06-02-11, 04:32 PM   #2  
UPDATE: I checked the compression today (1) 110/115 (2) 100 (3) 110 (4) can't get my tester down in there

When I idle down it just sounds like it gets too slow and eventuially quits. I seem to have a slight bowl leak on the top carb, it's dripping out of the bowl nut onto the bottom carb. Not bad, but should be fixed. I have clear fuel line that TEEs to the top and bottom carb. I can see bubbles in the tube going to the top carb, is that normal?

 
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06-05-11, 03:18 AM   #3  
Bubbles and a leak are not normal. Before you do too much troubleshooting on the idle, fix the air and fuel leaks.

 
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06-05-11, 06:00 PM   #4  
Update: I found the problem but I'm still screwed!

I removed the carbs and found that the top carb had a stuck needle. I removed both carbs and cleaned them and reinstalled them. The motor started right up and idled perfecty..... that is for about 3 minutes.

When I removed the carbs I found that the bottom carb intake gasket was a mess, I made a new one and reinstalled. Little did I know that the original gasket had a vacuum hole that probably runs the fuel pump, I'm guessing that is the impulse line. As soon as the lower carb bowl went dry I was back to square 1. I discovered the missing hole when I tore it all down again. Unfortunately when I took it apart, the syncronizing arm, that weird looking cheap plastic piece of junk, snapped!

So now if I can find a replacement syncronizer and put it back together, I think I will be good.

 
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06-06-11, 04:29 AM   #5  
It is frustrating, but when you get it running like a top the satisfaction for a job well done will trump the frustration. Plus, you'll know exactly how to fix it if it happens again. (Of course, the next time it will be something different!)

 
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06-06-11, 05:20 AM   #6  
Rick, having been in deep with many 2 stroke dirt bike engines, motors are not foreign to me but outboards are. I have learned a whole lot during this boat rebuild, its all good information and it calms some of the fears. I really hope that this is like my past dirtbike engines, you start with a toad, work thru the problems, and the result is a fairly dependable unit.... although everyone is telling me that this is just the nature of the beast.

 
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06-06-11, 11:16 AM   #7  
UPDATE: Part number is still good #53719A2 (Pickup assembly bracket), ordered and shipping tomorrow. Thought it best to buy new since plastic gets goofy when it gets old, especially around fuel.

 
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