Mercruiser 3.0L starting but stalling

[Burner21X]

Hi all,

Just inherited my father-in-law's 2007 Bayliner with a MerCruiser 3.0L TKS Alpha One. Had it out at the lake last weekend, and it ran "ok" though had very poor hole shot performance. It's never been a quick accelerator, but this was considerably worse than normal, struggling to get to plane towing one teen skiier. Also had some starting and dieseling issues (would keep firing for 30-45seconds after killing engine).


Due to this, I decided to check and change the spark plugs, and pulled and cleaned the flame arrester. Boat had ACDelco Mr43lts, replaced with NGK BPR6EFS. I don't have a spark plug guage, but the gap on the ACDelco's looked wider than the NGKs.

When starting again, boat fired up quickly the first time, but stalled after a few seconds. Had trouble getting it going again, but got it running. It wanted to die if I let it get under 2000 RPM. Was able to keep it running by continually revving the engine, and eventually, was able to get it to keep at neutral idle speed 1100-1200 rpm, but would die after idling for a few minutes. This has all been on the muffs, not in the water.

I've pulled the flame arrester and checked that the accelerator pump is spitting in fuel properly on throttling, and that seems good. I will try reverting to the old spark plugs to see if that makes a difference, but that puts me back at square one with the original issue.

FYI... My father-in-law had it at a boat repair shop last year with similar issues and I think the guy spent some time messing around with the idle screw, which is (I think) why the idle RPM is set high. I'd try to take it back in to him, but all the boat repair places here are closed on weekends and backlogged. We're heading out with the boat for a vacation in less than a week, so it looks like it's up to me (with the assistance of you fine people of course)

​​​​​​​Any ideas what the issue could be?

 

[Bondo]

Any ideas what the issue could be?

Ayuh,....... Welcome Aboard,...... My 1st guess is, the idle circuit, +/ or the TKS needs attention,.....

But, I'd start by checkin' the fuel filter for anything but clean fresh gasoline,.....

 

[tpenfield]

I'm thinking fuel system problem and would probably point my middle finger at the carburetor (TKS is a carb'ed engine, right?)

If I were me, and didn't have much time to fiddle with it before needed to use it, I'd do a de-carb (Mercury Power Tune stuff) and then run a high concentration of fuel system cleaner through the engine for about 5-10 gallons worth of gasoline. See if that improves things.

 

[Burner21X]

Thanks Ted/Bondo. Yes, the TKS is a carb'd engine. Thought we had done away with carbs??!

I pulled the new spark plugs and tracked down a spark plug gauge. My inititial thought was correct. Turns out the NGK plugs come gapped at .31 and engine spec is .45 (found it printed on the flame arrester). I regapped the plugs and took it on the water and it's no longer stalling... but back to the original problem of poor performance (poor hole shot and top speed of ~32mph, where it typically should be ~40mph) and stubborn starting.

I'll try your suggestions... fuel filter was going to be my next stop, and I'll check in to the Merc Power tune stuff. Likely could use a carb rebuild, but don't have time for that before we hit the road.

 

[Burner21X]

Problem solved (for now). Picked up some Power Tune and a replacement fuel filter. Swapped out the filter (not fun based on location on engine), and she fired right up, no starting issues, no dieseling, and the rpm at idle was down to about 900. Probably still needs and idle speed adjustment, but will do that once I get it on the water. Fuel filter was gummed up and appeared to be installed poorly. Not sure when last changed, if at all. Pic attached.

Cheers.
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[Rick Stephens]

With a vacation coming up, take the time to run it to some water and test it. No fun to have your boat run poorly with kids making faces at you.