4.3 pops and backfires thru carb, wont run

dirtdad

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Sep 10, 2017
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Its a 2001 mercruiser 4.3 v6, carburated. Had not run in maybe 1 1/2 years. I changed the oil and filter, and fuel filter/water separator, used earmuffs and a hose for cooling, and after some cranking and pumping the throttle, it started, ran for 5 minutes or so, and then it died. I could not get it to start again. Poured a little gas into the carb, and it would start, but not run. I tested the fuel pump, and oil pressure switch, had power to the pump, and when I direct wired the pump, it would not run. I ordered a pump, installed it, and verified that it would run (can hear it run while cranking, and then for a few seconds until the oil pressure drops on the gauge). Again, it would pop, run for a couple of seconds, backfire thru the carb, and die. I went ahead and replaced the plugs, distributor cap, and rotor. I am still having the same issue. I can hear the pump run, and I can pump the throttle, and it will start, but immediately dies, and it back fires thru the carb. Has me stumped. I can understand the fuel pump going bad, but what else? As I said, it was running prior to the pump going bad. I checked the fuel filter at the carb, and its clean. Could plugged jets from sitting with old gas cause this? The gas in the boat is probably 1 to 1 1/2 years old, but like I said, it did start and run for 5 minutes or so, until the fuel pump went bad. I did add some octane booster and a can of Seafoam to the tank. I am thinking of pulling the sending unit off the tank to look in it, in case its a mess in there. Any ideas or suggestions? I have owned the boat since new, and it has always been trouble free and reliable. Hour meter shows 419 hours.
 

GA_Boater

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Welcome aboard.

I would try using a portable tank or other way to avoid using the old gas in the tank. 5 minutes before the pump going bad isn't much and who knows how good that old gas is.
 

Scott Danforth

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The stuff in the tank is no longer fuel. Go thru your carb, clean your fuel system.

Your motor is ackfiring because its running extremely lean
 

dirtdad

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Sep 10, 2017
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So, drained the gas from the tank, it smells and looks O.K., I will put a few gallons of new gas in, and see how that goes. I used the electric pump and a clear hose, and I could see that its is not pumping freely, see some air bubbles. I took out the pick up tube, and blew thru it, and there is a restriction. I pulled it all apart, and there is a hose fitting with a one way valve in it. This valve was sticking, the fitting its in is made out of aluminum, and has that powdery, white corrosion. I think I can clean most of it with CLR, but I am wondering if I even need this valve. I think its there to keep gas from draining back, or if my motor was fuel injected, it would maintain pressure. But with a carburated motor, is it even needed?
 

dirtdad

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Sep 10, 2017
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one way valve in gas tank pick up

I have been trying to figure out why my motor wont stay running. I have another post under 4.3 V6 wont stay running. I took out the gas tank pick up tube, and blew thru it, and there is a restriction. I pulled it all apart, and there is a hose fitting with a one way valve in it. This valve was sticking, the fitting its in, is made out of aluminum, and has that powdery, white corrosion. I think I can clean most of it with CLR, but I am wondering if I even need this valve. I think its there to keep gas from draining back, or if my motor was fuel injected, it would maintain pressure. But with a carburated motor, is it even needed? If I replace it with a regular fitting, is there any reason I cant use brass? It would screw into another fitting that is stainless steel.
 

Scott Danforth

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Yes, you need the anti-siphon valve. Replace if needed.
 

Scott Danforth

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Its the anti-siphon valve. Yes you need it.
 

GA_Boater

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Dirt - Merged your two threads. It becomes confusing when we reply and you are getting offsetting advice when members don't see all the responses.

That's why Scott has the same answer two times.

On your question about the anti-siphon valve - Yes you need it. And it has nothing to do with carbed or FI engines. The valve is keep gas from siphoning out of the tank in the event of a fuel line failure, not to maintain pressure in the line.
 

djrowe007

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Sep 16, 2017
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Looks like I'm behind. Anti siphon valves are a safety item to prevent you from exploding when there is a fuel leak after the fuel tank. Required by the United States Coast Guard! They get stuck and fail, or may not open completely. It could also be the screen below your fuel pump preventing flow.
 

dirtdad

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Sep 10, 2017
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when I reassemble the pick up tube, can I use teflon tape on the threads, or do I need to use a different thread dope?
 

HT32BSX115

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when I reassemble the pick up tube, can I use teflon tape on the threads, or do I need to use a different thread dope?

Many of us don't like T-tape for gasoline NPTF pipe threads.... It doesn't always seal perfectly.............. Use any good gasoline rated pipe thread sealant like Permatex or equivalent......

Just click on the first link in the following google search ....How to properly seal fuel fittings

Regards,

Rick
 

dirtdad

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Sep 10, 2017
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I cleaned the antisiphon valve, and drained the gas tank of all old fuel, put in 3 gallons of fresh fuel, took apart the carburetor, cleaned it (it did have the fuel bowl full of gummy looking stuff) replaced needle and seat, power valve, very carefully removed the jets, blew air thru all orifices. Got it running, but after a couple of minutes, it dies out, I think the electric fuel pump (new, quicksilver brand), is sporadically cutting out, and the all of a sudden starts working again. I dont always hear it running, after the motor dies, although the oil pressure is still 40 lbs or so. Does this indicate a problem with the oil pressure switch? Funny thing, it has acted up in this manner when I was trying to start the motor before cleaning the carb, but whenever I used a test light to test if there was power to the switch, I have always found power to the red terminal, but I did ground the other end of the light to a ground source. Does the oil pressure switch control power, or ground?
 

Bt Doctur

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usually power if factory wired, the pressure switch is in series with the fuel pump
 

HT32BSX115

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If you're not sure, I would plumb a 0-15 psi gage into the fuel line and observe the pressure.
 

QBhoy

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Mar 10, 2016
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Check the distributor and leads. Check leads are in right order.
As mentioned check all fuel lines are clear and the wee gauze filter where main fuel line connects to carb.
Need fresh fuel and the suggested from an outboard tank is a great one.
If it's backfiring through carb...I'd be thinking electrical though, I must admit. Diz, leads and timing.
 
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