'77 Merc Thurnderbolt 500 won't idle

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 he changed the plugs, 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 and 4 gallons of fresh 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?

I ran a compression check (1)=110/115 (2)=100 (3)=110 (4)=can't get my tester down in there. Today I discovered if I start it initially with the fast idle 50% or more, it fires right up. When I idle down to where I think that I should be at idle, it doesn't miss or anything, it just slows down until it just quits. I also noticed today that I have a few bubbles in the clear fuel line going to the top carb, is this normal? I also have a slight drip coming from the bowl nut on the top carb.

Any ideas?

Re: '77 Merc Thurnderbolt 500 won't idle

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.

UPDATE: Part number is still good #53719A2 (Pickup assembly bracket), ordered and shipping tomorrow!