Mercruiser 3.0 Rough idle then stall after use

[JerseyBoater]

Hi all,

I have a Mercruiser 3.0L 2005 with TKS Carb. Boat has been running well and been out for several trips this month. Yesterday after about 2 hours of use it suddenly developed a rough idle. Normal idle is around 700/800rpm, but now it jumps between 500 - 1000 before stalling out. I assumed it was more of a fuel supply issue so I removed and cleaned the inline filter in the carb body. There was some small red bits (maybe fuel dye?) but nothing big or any obvious restriction in flow. Started it back up and idled fine no problems.

Today I went back out, started fine, went out wakeboarding and after an hour or so back to the rough idle. Fuelled up again and the engine almost wouldn't stay running but made it back to the dock. Once you're in gear and putting any revs on the engine appears to be fine and go all the way through to WOT without any issues. (I felt like it was lacking a bit of power but can't quantify that in any way!)

Background on the engine:
Start of summer service plus extras (new plugs, dist cap, oil filter, new coil and HT leads, fresh fuel) all performed by me.
Outdrive serviced by Mercruiser dealer, reported engine sounded good and there were no issues with the drive so just a maintenance job
Carb removed and cleaned about a year ago

Things I've done so far:
Cleaned inline filter in carb
Put fresh petrol in

Things to do:
Fuel / Water separator
Fuel filter on fuel pump if I can get to it / get it off as I painted the whole thing without realising a while back!
Change plugs and check condition, lean / fouled etc

Does this sound like I'm on the right track, anything I'm missing or should check too?

Thanks

 

[alldodge]

Fuel filter on fuel pump if I can get to it / get it off as I painted the whole thing without realising a while back

Ya know, just from that statement makes me think it has not been replaced in a long time. If this is the case we might have a winner, as to how crud go it the carb

Either way, you do have a fuel issue from my point of view. If all that checks out, pull the carb and clean it real good, sounds like your idle circuit

 

[JerseyBoater]

Yip that's my thinking too. I'm still trying to sort it out, broke the bolt head off the bottom of the pump filter so going back tomorrow for take 3! Will let you guys know how I get on. Then carb and a general tune up.

 

[JerseyBoater]

All up and running!

​Changed the filter, it was not in great shape, and I didn't seat it properly so engine didn't run initially (once the fuel in the carb ran out) but on reseating all ok, original idle issue returned.

Changed plugs, looked ok.

Started engine and put the revs to around 1600. Used the mixture screw to do 2 full turns, then 4 out, and 2 back in (so in theory back where I was). It then started idling ok!? So I did the mixture setting as per the mercruiser service manual just to be sure. (in gear in water). Took the boat out for a long run, lots of different RPMS, idling etc all ok.

I'm not entirely sure what fixed it, but at a guess if there was something small stuck in the idle circuit of the carb could changing the mixture screw flush it out as it were?

 

[alldodge]

A speck of crud can cause a carb running and idle issues, same with a clogged fuel filter. Either way, if it starts running rough again, then some of the crud from the filter may have made its way back to the carb. Wouldn't be a bad idea to flush out the fuel line, but may be OK in any case.