Stalling when hot Mercruiser 5.7 2 Barrel Carb

[DangerMouse1]

Wondering if anyone can point me in the right direction. I have searched around and can't find a similar problem to this.

I have a 5.7 Mercruiser, with a 2 Barrel Carb TKS, from 2005-06. I have no problem starting, cruising, accelerating etc. But when the engine gets hot, after an hour + of use, when I take it back to idle I will see the RPMs start to go below idle range of 700 and then eventually the engine will cut out 5 secs or so. I have no problem restarting it, but not great when coming into dock.

I thought it was the ignition coil, so have replaced that, but didn't fix it. Carb is recently rebuilt, by previous owner. I sprayed Seafoam in the carb and added to the fuel. Fuel filter doesn't have any water in it. I have tuned the idle mixture by ear, trying to find a place to attach the vacuum gauge (can't find where though?).

My only thinking is this:
1) change spark plugs
2) change water fuel separator filter
3) adjust idle mixture screw (just one on this carb), might be running too rich when hot causing a stall?
4) go into the rest of the ignition electrical system (this feels like chasing the unknown)

Anyone experienced similar to this, or ideas on a fix?

 

[Triangleboater]

I had this issue on several on my vehicles including outboards. Look into the ignition module. In your case the thunderbolt or similar. Only happens when warm tells me it isn't in the fuel system and in your ignition system.

 

[Scott Danforth]

first, its never the coil

if the idle is just dropping to a point where it stalls, its not ignition, its your carb. most likely dirt in it. cant fix that by pouring magic snake oil from a can, need to pull the carb apart and fix. your idle RPM should be 650 in gear. the motor should run fine down to about 500 RPM with a properly cleaned and adjusted carb.

if the engine just quits like the key was turned off, look at your ignition system. if it is the ignition trigger, it will progressively get worse until you are stranded.

 

[DangerMouse1]

Thanks for the replies. Will look into both the tks ignition module and carb and give feedback.

The engine is not quiting like I turned off the ignition. It is a slow and progressive die with falling rpms over around 20 seconds or so. I will try taking out the idle mixture screw and blowing some compressed air through, before a complete carb rebuild. --- What I don't get though is it is idling low and nice when warm. Stalls only after being run for some time and then taken back to idle. Why would this be a carb issue?

Going to looks at testing the TKS module, think there is a way to do this in the manual.

 

[Scott Danforth]

The engine is not quiting like I turned off the ignition. It is a slow and progressive die with falling rpms over around 20 seconds or so. I will try taking out the idle mixture screw and blowing some compressed air through, before a complete carb rebuild. --- What I don't get though is it is idling low and nice when warm. Stalls only after being run for some time and then taken back to idle. Why would this be a carb issue?

because its a classic symptom of a dirty idle circuit. the idle circuits are the lowest in the bowl. any sediment or water will collect to the bottom of the bowl. also, make sure you empty the contents of your fuel filter into a clear container and look for contaminants. and take a fuel pressure reading. you could be running low on fuel pressure.

Going to looks at testing the TKS module, think there is a way to do this in the manual.

yes

 

[DangerMouse1]

Thanks for reply Scott Danforth

Couple of questions.

1) Are you talking about the water/fuel separator filter, or the filter in the carb?
2) Fuel pressure reading is something I have been struggling with. No vacuum valve attachment point on this carb only a pcv going to the manifold. Nothing in the carb manual either about where to attach for readings. Any suggestions?

 

[Scott Danforth]

fuel/water filter
you dont need a vacuum attachment. you need to get the in-line adapter (per the manual) and plumb between the fuel pump and carb to take a pressure reading.

part number 91-18078