Stalling, sudden low idle.

[ShoalSurvivor]

Mercruiser 4.3 warmed up fine.
Dropped it in the bay and she kept stalling in nuetral. Just seemed “cold” at first so I let it warm a little longer. Finally behaved as expected.

Didn’t even get out of the harbor and I noticed my throttle was high, but she was barely moving. Then she stalled again.

Towed to dock. Restarted and ran in nuetral, rev’ed reliably, but idling at 250. She’s not overheating. Temp is at 160°

What could cause sudden low idle? Last time out, no issues.

Is this an adjustment I can make at dock? Or is my weekend adventure toast?

Is this my idle adjustment screw on the fore/port side of carburetor?

 

[Scott Danforth]

Carb? EFI?

 

[ShoalSurvivor]

Carb? EFI?

Carb. 4bbl

 

[ShoalSurvivor]

Carb. 4bbl

I turned the screw until the tach raised. I thought “that was easy”. (Not sure I did the right thing). I don’t have a separate reach gauge to hook up. Now it’s idling at 850, bit when I put it in gear, it wants to lurch. Makes me think that’s not the idle, but something else. Thanks.

 

[ShoalSurvivor]

I turned that screw back down a little. It’s not lurching anymore but still idling at 420rpm.

Is that the right idle adjustment for a 4bbl Weber carb? Is it possible that my tach is just not reading accurately?

 

[harringtondav]

That is the idle speed screw. But if it revs in neutral but doesn't idle or deliver power, I suspect a dirty carb or fuel supply problem. Verify a clean fuel filter and that your electric fuel pump is good. Checking the fuel pump can be tricky. It doesn't start until the engine is running and building oil pressure. You can jump the harness leads to the oil pressure switch on the lower rear port side of the engine. The pump should start at key on. Checking pump flow and output involves disconnecting the carb supply line and collecting the gas. I don't have the flow spec, but it should be a steady 3-7 psi flow.
If all good, its time to disassembly and clean your carb. You didn't state your engine year, but you probably have a 3304-9565A6 or -95692, Weber 9600.
The Merc carb kit is 8M0120196, Sierra 18-7749.

 

[Fun Times]

I turned that screw back down a little. It’s not lurching anymore but still idling at 420rpm.

Is that the right idle adjustment for a 4bbl Weber carb? Is it possible that my tach is just not reading accurately?

Seemingly If your engine was idling and running fine for a long time prior to any other service jobs or adjustments done, then all of a sudden you have both a high and low idle and engine dying situation, then unfortunately chances are to the higher side there’s going to be other issues vs a simple idle RPM adjustment from what does look like the screw in your photo though there are others on that carb that helps with some better idle and running too for fine tuning purposes that’s sometimes a bit better to do with the throttle cable disconnect as the cable could cause issues too should it have gone bad.

Thus far from your descriptions, it sounds more likely you could have something such as say water and or debris in the fuel, an internal carb problem, or even possibly a small amount of water getting into the cylinder's are again just some things of additional things out there to consider thinking about for inspection… < all this if you’ve been keeping up with the tuneup items like spark plugs, distributor, cap, and rotor but they may need re-inspection too.

 

[ShoalSurvivor]

Thanks for replying.

You didn't state your engine year,

2004 4.3 mercruiser repower. I replaced the fuel filter and carb 2 years ago. Looks clean. Only issue I had with fuel was that mfg listed 12-15psi on fuel pump but carb was limited to 7psi. i could not find a lower psi pump, so I added a pressure regulator. Worked perfect after that for 2 years.

I didn’t even have to adjust the idle when I replaced it.

no other drive or throttle work has been done since.

more below.

 

[ShoalSurvivor]

sometimes a bit better to do with the throttle cable disconnect

some more detail:
ran last night relatively well with the minor adjustment that I made to carb screw.

i had a low throttle issue, but only high issue after I made an adjustment. I turned it back down 1/2 way after that And no more high issue.

the big difference i noticed, and I hope this is relevant info, is that my throttle position is more sensitive. before yesterday, there was some play at nuetral. (Throttle between 11:30 and 12:30, would be in nuetral And no gear engaging). Fwd and rev required moving throttle beyond those positions to engage.

after idle adjustment, it’s 12:00. Period. When I “click“ into nuetral, from either direction, is stays in gear until I put a little extra pressure on the throttle lever, and it engages very quickly. Feels too “tight”

nothing else changed that would suggest a cable adjustment, but I can do that. It only seems to have changed with idle screw adjustment. does that help?

thanks for input.

 

[Lou C]

The symptoms you mention suggest internal carb problems like a failing needle valve or bit of dirt holding it open. What do the spark plugs look like (the porcelain insulator of the center electrodes)? White = lean, black =rich that will give clues to point you in the right direction.
When you say you replaced the carb, what did you replace it with? New OEM, or reman? I am very suspicious of reman carbs sold by internet sellers. Often they are not set up right or are combinations of different carbs, none of which is to the specs for your boat.

 

[ShoalSurvivor]

The symptoms you mention suggest internal carb problems

Thanks! I replaced the stock carb with an:
Edelbrock 1409 Performer Series Marine 600 CFM Square Bore 4-Barrel Air Valve Secondary Electric Choke New Carburetor

I replaced the fuel pump at that time, too, but struggled to find a compatible pressure. Pumps were over 10psi and the carbs req max 7psi. As mentioned, I added a pressure regulator and dialed it in to 6 or 7 psi.

I will verify output pressure, as that could be a cause, and check the spark plugs. Oddly, now I’m seeing “low pressure” pumps for the 262 that i couldn’t find before.

I appreciate your help and will report back.

 

[harringtondav]

I replaced the fuel pump at that time, too, but struggled to find a compatible pressure. Pumps were over 10psi and the carbs req max 7psi. As mentioned, I added a pressure regulator and dialed it in to 6 or 7 psi.

I will verify output pressure, as that could be a cause, and check the spark plugs. Oddly, now I’m seeing “low pressure” pumps for the 262 that i couldn’t find before.

I appreciate your help and will report back.

I wondered if your pressure regulator may be restricting flow.
3-7 psi electric fuel pumps are common and relatively inexpensive. The challenge is finding one with pipe fittings so you can adapt it to the SS fuel plumbing in your boat.
Below is a Carter pump that I installed on my 4.3LX. With the correct brass flare adapters it was a perfect fit replacement with the Merc expensive OEM pump. I had to splice into the original pump wires.

Electric Fuel Pump

carterengineered.com

 

[Lou C]

This is carb was bought new but now it's 2 years old correct ? Have you contacted Edelbrock tech support? The only thing I have ever heard of with these carbs is that one of the floats can be too close to the side of the float bowl and cause the float to stick. One thing you could do is remove the carb, drain out all the gas, let it dry and see if you can using an appropriate sized hose, blow through the fuel inlet with the carb upside down, that's an easy test to make sure the needle valve holds.