Engine stalls at idle

[Clifton]

This spring I started the motor on the hard: it started easily and ran smoothly at idle once it was warm. It did stall once (on idle) on the way from the boat lift to the slip. Now it starts, but even after it's warm, it stalls as soon as I reduce the throttle to idle.
Plugs are a little sooty. I cleaned them and there was no improvement to the stalling. Fuel filters are clean. The motor runs cool, has worked well for the years I've had it. I ran it for awhile at about 1/5th throttle and it seemed to be surging Increase in speed or a different vibration) a little, but it was very minor. That mostly settled out after awhile, I assume it was still warming up. Stalling persisted.

Any help will be greatly appreciated!!
I think I'm going to take this opportunity to install an electronic ignition, perhaps change the plugs, check the exhaust hose for obstructions, possibly replace the plug wires and coil. All are quite old now.
Thanks!
Clifton

[zellerj]

Sometimes a piece of dirt can momentarily block the jet to the carb. Make sure you have filter between the fuel pump and the carb, and take the time to remove the carb and clean out the bowl, and the jets. Most stalling issues are with fuel.

If it is not fuel, then you might have an issue with the ignition system, but I doubt that. 90% of the time it is fuel.

[JOHN COOKSON]

Have you tried adjusting the idle mixture adjusting needle? Turn the adjusting needle in until it lightly seats then back it out 1 1/2 turns Maybe with small adjustments after that you will get a satisfactory idle. Also try varying the idle speed while adjusting the idle mixture.
There are a couple of small idle jet holes near the throttle butterfly (in the carburetor) that are very susceptible to blockage. As noted this could be the problem.
Ignition system maladjustments can goof up the idle. If the advance is stuck in the advance position the idle will never be correct. If the dwell is not correct or if the timing is off because the distributor has been rotated the idle will be affected also.
This said blocked idle jet holes are the usual cause of no\poor idle.

ex TRUE GRIT

Edit: Noticed you reported sooty plugs. Check to be sure the choke is opening all the way.

[Sam]

John's advise is on target. I would also consider emptying the fuel tank. If you set is that you can place a hose or stick pump down to the bottom probably emptying half the tank will clear gunk, water, emulsion etc. A test using an auxiliary tank might give some insight beforehand.

[JOHN COOKSON]

Blocked idle jets can happen sort of randomly. (for lack of a better term)
If after cleaning the idle jets (assuming this is the problem) they become gummed up again soon you will need to get rid of the contaminated fuel and\or improve your filter system. A fine mesh polishing filter just before the carburetor might help - depending on the level of fuel contamination.

The A4 carburetor demands clean fuel to operate correctly.

ex TRUE GRIT

[Clifton]

Thanks All! And a follow up question

In order to clean the carb, do I need to replace any gaskets? I will try suggestions in this order:
1. Adjust carb
2. Dismantle and clean
3. replace fuel with fresh gas.
I will let you know how it goes, thanks again!

[Sam]

Your "surging" comment was the tip off for me - had similar issue few years back. Emptyied the tank added fresh gas and used the opportunity to clean carb [ off engine but not disassembled except for main jet & mix screw] with spray carb cleaner, changed fuel lines and added fuel pressure gauge and polishing filter. As preventative measure changed out 27yr old electric fuel pump. Problem solved but the main driver I feel was the fuel change. The bottom two gallons were water laced and very murky. I mention this only suggesting you may want to try this first and see if it helps.