1999 4.3 liter Mercruiser stalls at Idle

[mrrickb]

I have a carbureted 4.3 liter Mercruiser. It starts fine runs great up to full throttle. But will not idle more than about 20 seconds then dies. If you try to start, have to apply full throttle to start (like the engine is flooded) When it starts to die if you apply throttle it will continue to run. As long as you keep the throttle above 1K it will continue to run fine, but bring it to idle and it stalls

I have replaced the spark plugs - plug wires - (cap does not look bad but have a replacement I will install today) I replaced the fuel filters - fuel pump - and a new carburetor that is supposed to have factory settings.

Yet the problem still persists. Can anyone offer advice??

 

[BillK2632]

I would recheck / set the initial idle. I have the exact same engine / carburetor and a few years ago I rebuilt the carb myself. I had to tweak the idle adjustment even after I set it initially during the rebuilt. I attached the idle adjustment pages from the Mercarb Manual. I would try that first - there are basically two settings - the idle mixture adjustment and the idle speed adjustment. Another thing, when it dies at idle does it just die or is it flooding and stalling? If it is flooding, it is related to the float setting - another thing that is adjusted initially on a rebuild or new carb and might need to be tweaked - I ended up tweaking that on mine also. But check the idle speed adjustment first - adjusting the float requires pulling the top off the carburetor.

 

[velociraptorfarmer]

I would recheck / set the initial idle. I have the exact same engine / carburetor and a few years ago I rebuilt the carb myself. I had to tweak the idle adjustment even after I set it initially during the rebuilt. I attached the idle adjustment pages from the Mercarb Manual. I would try that first - there are basically two settings - the idle mixture adjustment and the idle speed adjustment. Another thing, when it dies at idle does it just die or is it flooding and stalling? If it is flooding, it is related to the float setting - another thing that is adjusted initially on a rebuild or new carb and might need to be tweaked - I ended up tweaking that on mine also. But check the idle speed adjustment first - adjusting the float requires pulling the top off the carburetor.

This is the answer. I just went through this same issue with mine. Hook it up to the muffs in your driveway, and start tweaking the mixture screw (front and center of the carb) until it starts running properly at idle, and then tweak the idle mixture screw until it idles at 650rpm. Rinse and repeat until it's working properly.

That'll be enough to get it out on the water, where you'll need to readjust due to the backpressure of the water on the exhaust messing things up slightly again. You want to do the final adjustment in gear while at idle. Again, mixture screw til it sounds like it's running good, then tweak idle to 650rpm. Once you do that, then try to slowly bring up the throttle and make sure it still sounds good, if not, tweak the mixture screw a bit again until it's happy through the entire lower rev range (<1600rpm).

I just spent the last 4 times out fixing this. Runs like a top again.

 

[mrrickb]

This is the answer. I just went through this same issue with mine. Hook it up to the muffs in your driveway, and start tweaking the mixture screw (front and center of the carb) until it starts running properly at idle, and then tweak the idle mixture screw until it idles at 650rpm. Rinse and repeat until it's working properly.

That'll be enough to get it out on the water, where you'll need to readjust due to the backpressure of the water on the exhaust messing things up slightly again. You want to do the final adjustment in gear while at idle. Again, mixture screw til it sounds like it's running good, then tweak idle to 650rpm. Once you do that, then try to slowly bring up the throttle and make sure it still sounds good, if not, tweak the mixture screw a bit again until it's happy through the entire lower rev range (<1600rpm).

I just spent the last 4 times out fixing this. Runs like a top again.

 

[mrrickb]

Well I am trying to post a video of what happens -- Also Just to complete the ignition system I replaced the coil - Distributor Cap - and rotor, so I have a completely new ignition. And the boat is in the water.
The first start of the day the boat starts right up and will idle, around 800 in forward, but after a minute or 2 you can hear the carb it starts pushing gas out the venture until it the engine stalls. Then it will continue to pure gas into engine for 10 seconds or more after engine stalls

 

[velociraptorfarmer]

Well I am trying to post a video of what happens -- Also Just to complete the ignition system I replaced the coil - Distributor Cap - and rotor, so I have a completely new ignition. And the boat is in the water.
The first start of the day the boat starts right up and will idle, around 800 in forward, but after a minute or 2 you can hear the carb it starts pushing gas out the venture until it the engine stalls. Then it will continue to pure gas into engine for 10 seconds or more after engine stalls

Mine would do the exact same thing started like a dream and ran great until it warmed up, then refused to idle. I'm telling ya, tune the carb for idle once it's warm and you'll be in good shape.

 

[mrrickb]

First thank you billK your download was very helpful

Velcociraptor thank you also but one question was your carb still dumping fuel after the engine stalled, I thought it was a problem with the float valve, causing the fuel to dump into the carb>>

 

[velociraptorfarmer]

First thank you billK your download was very helpful

Velcociraptor thank you also but one question was your carb still dumping fuel after the engine stalled, I thought it was a problem with the float valve, causing the fuel to dump into the carb>>

Dunno, I never checked

 

[Bt Doctur]

If fuel is dumping thru the carb, the needle and seat are defective, not set correctly, or damaged. Even though your pump is new, verify the pressure at the carb. .My old used one start putting pressure out at 14.5 psi then go back to a normal 4,5 until it decided it wanted to flood out some more

 

[mrrickb]

I guess I am taking a new carburetor apart. Anything I should watch out for? How can I tell if it is a defective part vs poorly adjusted float

 

[BillK2632]

I guess I am taking a new carburetor apart. Anything I should watch out for? How can I tell if it is a defective part vs poorly adjusted float

If you purchased this carburetor recently I would call whoever you purchased it from and get their advice. Adjusting the float is not hard, but you have to take the carburetor apart (at least the top off), certainly would void the warranty (if there is one) on a new carburetor. That aside, I've adjusted mine and here is how I did it (see attached). I would remove the carburetor to do this and cover the intake - it is very easy for parts to fall into the intake, then you have a BIG problem. For example, the pins that hold the float hinge together will fall out when you lift the top off if you tilt it. Another resource - search Youtube for Mercarb carburetor rebuild - Mike's Carburetors, the guy takes you step by step on the rebuild and adjustments. Another thing to check on this is the choke - make sure it is opening completely, if it is not, it could cause idle problems.

 

[mrrickb]

I did call the manufacturer they will take back and fix but it will take 2 weeks. Thank you again BillK for the download. FYI the choke works find. I showed my video that is too large to download here to the mechanic at my marina. He told me the only thing that would cause gas to run after engine stops, it can only be the needle valve. it might have gotten damaged when shipped. I have removed the carburetor and plan on disassembly tomorrow
So do I just replace or how can I tell the needle valve is bad??

 

[BillK2632]

I did call the manufacturer they will take back and fix but it will take 2 weeks. Thank you again BillK for the download. FYI the choke works find. I showed my video that is too large to download here to the mechanic at my marina. He told me the only thing that would cause gas to run after engine stops, it can only be the needle valve. it might have gotten damaged when shipped. I have removed the carburetor and plan on disassembly tomorrow
So do I just replace or how can I tell the needle valve is bad??

The needle valve has a sharp hard rubber tip, if it is damaged it will not "seal" when the bowl fills up. The float rises as the bowl fills and pushes the needle down in it's seat to stop the fuel flow. Check the needle for damage or debris / gunk build up on the needle and seat. If you can easily get one, best to just replace it once you have taken it all apart. Hopefully you just find some debris in the bowl and can clean it.

 

[mrrickb]

Update;

Decided to rebuild my old carburetor and send the new one back to the manufacturer for refund. The old carb had the same problem of continuing to dump gas after the engine stalled. (But not as bad as the new carb). Opening up the carb, inside seems very clean no varnish or other obvious problems. The needle valve looks fine no outward sign of problem. Checked the float height and it was around -½ inch (vs 9/16 recommended for spring type). So that was off but not sure if it would cause the gas running and flooding the engine. Next, I weighed the float and it weighed 13 grams. Specs say the float should be no more than 9 grams. Could the combination of poorly adjusted float that is also 4 grams heavy be my problem??
Hope to be receiving a rebuild kit containing a new float tomorrow. Should be able to rebuild and test tomorrow. Hope I have found the problem. Anyone have experience with a heavy float?

 

[BillK2632]

Yes - the float can absorb gasoline over time and throw everything off. Really it works just like a toilet tank. The float weight + drop height being off - I think you found the problem - go find Mikes Carburetor videos on YouTube - very detailed step by step on rebuilding a Mercarb.

 

[mrrickb]

Well I replaced the float and needle valve with seat. Still having the same problem with gas continuing to flow causing the engine to stall and even flows after engine stalls.

Not sure what to do next, I guess I will have to go to the mechanic at my marina

 

[Bt Doctur]

until you check the fuel pressure, your chasing a ghost

 

[Shaps]

until you check the fuel pressure, your chasing a ghost

+1.... check the fuel pressure

 

[mrrickb]

It is a new fuel pump, but I guess I will check to eliminate that as a problem (I hope)

 

[BillK2632]

Does the fuel pump continue to run after the engine stalls? It should not if it does it was not installed correctly with an oil pressure switch. Also the throttle cable should be adjusted so the throttle plate is completely shut when in neutral. If not it allows fuel to continue to flow - common cause of run on after engine is shut off. Agree with others - time to check fuel pressure and installation of the new pump.