Mercruiser 5.7 shuts off when idling when warm

[AShipShow]

I have a 96 bayliner with a merc 5.7, 2bbl mercarb, and alpha 1 gen 2.
It seems to start up and run great, but I've noticed the last few times out, that after cruising around for a while, when I stop and idle, the boat has a tendency to idle for like 20 or 30 seconds, and then will just die..

My guess is its something related to the carb... Maybe choke? idle mixture?? just guessing, but wondering if anyone has any thoughts on what it could be?

It will typically restart right away after it dies if that makes a difference.

Appreciate any thoughts.

 

[Scott06]

I assume based on your other posts timing is ok ? Is the throttle cable set up to allow it to return to the idle stop screw and the screw is controlling the idle throttle plate position? There is a screw on the side of the mercarb holding a half moon linkage that connects the choke to the throttle. This is what the idle speed screw seats against mine got loose and the idle speed screw wasn’t holding the throttle open any more causing it to die at idle

other than that look at idle mix adjustment

 

[AShipShow]

I assume based on your other posts timing is ok ? Is the throttle cable set up to allow it to return to the idle stop screw and the screw is controlling the idle throttle plate position? There is a screw on the side of the mercarb holding a half moon linkage that connects the choke to the throttle. This is what the idle speed screw seats against mine got loose and the idle speed screw wasn’t holding the throttle open any more causing it to die at idle

other than that look at idle mix adjustment

I'm pretty sure everything you mentioned should be good to go since I did a carb rebuild during my restoration and rechecked all my adjustments last season... That being said, I will definitely recheck everything...

You did remind me of something though... I remember when setting the idle speed screw that rides on the fast idle cam, It seemed kinda difficult for the cam to turn, like the choke wouldn't be strong enough to actually move it because there is too much friction between the cam and the idle screw... I'll have to recheck that and make sure its operating correctly... Makes me wonder if the choke isn't opening the whole way...

 

[Kubaat3lover]

That’s fine, you release it when engine is cold and you add throttle I had same question in the past regarding this screw and the friction, guys explained that to me:
Thread 'Mercarb idle speed screw and adjustment'
https://forums.iboats.com/threads/mercarb-idle-speed-screw-and-adjustment.745271/

 

[Scott06]

I'm pretty sure everything you mentioned should be good to go since I did a carb rebuild during my restoration and rechecked all my adjustments last season... That being said, I will definitely recheck everything...

You did remind me of something though... I remember when setting the idle speed screw that rides on the fast idle cam, It seemed kinda difficult for the cam to turn, like the choke wouldn't be strong enough to actually move it because there is too much friction between the cam and the idle screw... I'll have to recheck that and make sure its operating correctly... Makes me wonder if the choke isn't opening the whole way...

If choke is sticking closed your plugs will be carbon black and probably won’t run until plugs get cleaned

 

[Kubaat3lover]

Do you know how to check If choke is sticking?

 

[tpenfield]

My thought is fuel pump. Not sure if you have a mechanical or electric pump. If it is mechanical - check to see if the mechanism is worn or the diaphragm has developed a perforation.

 

[AShipShow]

Do you know how to check If choke is sticking?

The plan was to take off the spark arrestor and watch it until it gets up to operating temp and make sure it fully opens...

My thought is fuel pump. Not sure if you have a mechanical or electric pump. If it is mechanical - check to see if the mechanism is worn or the diaphragm has developed a perforation.

I don't think this is the problem because I think my issues would be under acceleration when the fuel demand is higher.

My guesses at this point is its an idle mixture problem... Thinking it starts up okay when cold and runs around fine because once I'm off idle its no longer an issue... Then when it gets up to temp and the choke is open, I'm no longer getting the extra fuel during idle and its stalling out...

 

[Scott06]

The plan was to take off the spark arrestor and watch it until it gets up to operating temp and make sure it fully opens...



I don't think this is the problem because I think my issues would be under acceleration when the fuel demand is higher.

My guesses at this point is its an idle mixture problem... Thinking it starts up okay when cold and runs around fine because once I'm off idle its no longer an issue... Then when it gets up to temp and the choke is open, I'm no longer getting the extra fuel during idle and its stalling out...

Simple to adjust the idle mix screw although yours may have the anti tamper plastic piece on there cut it off usually you will be 1.5-2 turns out and be ok

 

[AShipShow]

Simple to adjust the idle mix screw although yours may have the anti tamper plastic piece on there cut it off usually you will be 1.5-2 turns out and be ok

Nah, I've adjusted already when I put the carb on the motor after a rebuild before splashing my boat last year... I remember setting it at 1 1/4 turns out to start, but no clue if I went too far one way or the other screwing with the idle speed and idle mixture and whatever else lol...

The only other thing I can think about is if its ignition system related... Last season while getting ready to splash the boat, I was doing some tune up stuff in the driveway and my thunderbolt V ignition crapped out on me and found out it was the module which is a small fortune to replace, so i ended up replacing the ignition with the Delco EST system... Its been working great, I'm just wondering if theres something goofy going on there...