1986 Mercruiser 260 idle issues

[system-f]

I have a 1986 Mercruiser 260 (350) that is having a strange idle problem that I cannot seem to figure out. The boat runs fine above 1200 rpm all the way to redline. Performance anytime except idle is absolutely perfect. This is a concern because a game warden told me I need to run at idle speed in no wake zones which is currently impossible. I have to run at 900-1000rpm or the engine will spit and sputter.

-Fuel pump was replaced, I am getting good fuel flow (runs fine at WOT)
-the entire ignition system was replaced except for the Thunderbolt module (no change), this was because it needed to be done and I wanted to rule out ignition
-the plugs are new and plug wires are in the correct spot
-compression is good
-Timing is good 6-8degree advance


Here is the problem: The boat will idle at the rpm I set it to (650-750) and then drop down another 100-200 rpm once the engine is warmed up. If I rev the engine and let it idle it will go back to set idle then drop down 100-200rpm and sputter. Restarting when hot is nearly impossible unless I fast idle the boat. If I don't fast idle it tries to start but runs at 200-300rpm then dies (not enough fuel).

I have ordered a rebuilt marine quadra jet and it does the same thing (numbers matching carb). I did the idle mixture screw setting adjustment but the engine does not respond to any adjustments of the idle mixture screws. This is the procedure I followed:
1. Turn the idle mixture screws all the way in.
2. Back the screws out two turns.
3. Warm the engine up and attempt to adjust the idle mixture screws (one at a time) out until the engine increases in RPM and then bogs.

When I try to turn the idle mixture screws out the engine does not respond, I have gone as much as 1 full turn out.

This lead me to believe that I had a vacuum leak, but when the problem was happening (low idle) I squirted soapy water everywhere and there was no change. I focused the water around the carb but the engine remained to idle slow/hardly run.

Any advice would be greatly appreciated.

 

[Scott Danforth]

a $30 carb kit is cheaper than a new carb

you did adjust the choke first, correct?

idle speed when warm should be about 650 when in gear in the water. on land it will be about 725-750

 

[kenny nunez]

If you look down into the primary side when the engine is idling can you see gas bubbling out of the Venturi boosters? If so that means that the idle stop screw has the butterflys too open and the carburetor is in transition just off idle.
Make sure that your harmonic balancer timing marks are perfectly aligned with tdc. If the balancer has slipped your timing is off and that can cause this type of problem.
I once had a 350 with a weird idling problem which turned out to be an internal exhaust leak that was mixing with the incoming air. Take a really close look on the inside of your manifold.

 

[harleyman1975]

Sounds to me like a leaking intake manifold. Reseal it with new gasket set. Vacuum leak.

 

[NHGuy]

You can also check for intake leaks with carb cleaner. That's the way I do it. When a (good) quadrajet won't respond to mixture adjustments at engine idle speed it is very likely an intake leak.