3.0L Mercarb rough idle

[havoc_squad]

The first lake test after de-winterizing my boat 1.5 months ago, my idle is mostly crap at load. It gets worse as the longer you stay at idle under load, which is making me suspect flooding at idle.

Also, I believe after a run at speed, it seems to idle slightly better at load until you get past the 2 minutes or so, then the idle starts to break down slowly over time.

The venturis are NOT leaking, I checked that myself.

Last year I installed a clean remanufactured mercarb of the exact model. It was adjusted properly and the boat could idle in all aspects fine.

Do I say bad gas? No, its 1 year or less old. I don't suspect water intrusion since its 100% non-ethanol gas.

Do I say trash in carb? Possible but not likely, have water seperating filter, the fuel pump's stone filter, and the filter in the carb inlet.


That leads me to suspect the fuel got varnished or non-trash based obstruction in the carb.


On most carbs, how much removal does the Mercarb generally require to clean out idle passages 100%?

 

[Bondo]

Re: 3.0L Mercarb rough idle

The first lake test after de-winterizing my boat 1.5 months ago, my idle is mostly crap at load. It gets worse as the longer you stay at idle under load, which is making me suspect flooding at idle.

Also, I believe after a run at speed, it seems to idle slightly better at load until you get past the 2 minutes or so, then the idle starts to break down slowly over time.

The venturis are NOT leaking, I checked that myself.

Last year I installed a clean remanufactured mercarb of the exact model. It was adjusted properly and the boat could idle in all aspects fine.

Do I say bad gas? No, its 1 year or less old. I don't suspect water intrusion since its 100% non-ethanol gas.

Do I say trash in carb? Possible but not likely, have water seperating filter, the fuel pump's stone filter, and the filter in the carb inlet.


That leads me to suspect the fuel got varnished or non-trash based obstruction in the carb.


On most carbs, how much removal does the Mercarb generally require to clean out idle passages 100%?

Ayuh,.... 100% disassembly, soakin', blowin' out, 'n reassemblin',...

Other wise known as Rebuildin' the carb,...

 

[Silvertip]

Re: 3.0L Mercarb rough idle

Choke working properly and not staying partially closed? Loading up at idle is not a lean condition so I would not suspect a clogged carb but one that is running rich at idle. Check for an inlet needle and seat that is not is not closing tightly. A stiff dose of Sea Foam in the fuel may solve a sticky needle and seat. It is not possible to use too much of this stuff so a full can of Sea Foam in 5 - 10 gallons of fuel may fix the issue although the engine may smoke a bit during the cleaning process. If that doesn't work then a rebuild is in order.

 

[havoc_squad]

Re: 3.0L Mercarb rough idle

Choke is wide open when it should be.

The carb is barely 1 year old from fresh crate re-manufactured with warranty. Varnish/gumming should not happen this fast with non-ethanol gas fully stabilized.

Can't even get in a full season without some superficial issue making my boating season miserable.

If I have re-build it every year or two on a regular basis, I'm going to soon need to be committed to a mental institution.

Sigh....

 

[havoc_squad]

Re: 3.0L Mercarb rough idle

Info update:

Discovered either someone messed with timing last year or wormed off time quite a bit, retarding the firing order badly. I think it was retarded back to 4 to 5 degrees.

This crapped up everything: idle, acceleration, and top end.

There was nothing wrong the carb. It's doing a bloody good job being that chef mixing air and fuel.

Timing was fixed and factory idle adjustment specs got it ball park good on first lake test. Idle at load while not fully warmed up seemed a little bumpy, needed to be smoothed up. Ran fine in all tests once fully warmed up.

Second lake test made small richer idle air mixture adjustment to smooth out idle during transition from cold running engine to fully warmed up.

Bout time, a boat that idles well and 4 banger that hauls butt fast out of the hole when I punch the stick down.

 

[Jonnybbad]

Re: 3.0L Mercarb rough idle

If you have ignition points, you may want to consider checking your dwell with a dwell meter. If your timing has retarded, the points have probably worn some and could possibly use adjusting to correct the amount of dwell. If you do so, you will need to adjust ignition timing again afterwards. Congrats on figuring out your issue.