3.0L Merc carb questions.

[Chris Hays]

Finally got around to fitting and starting my fully rebuilt engine in the boat, also rebuilt the carb, (new gasket kit, cleaned out all passages ports etc), engine will attempt to fire but will not run, still just guessing with the ignition timing, (set it static), as need it running to set it correctly. Once i finished turning over the engine after it would not fire, i looked down the barrells into the carb and gas was still trickling out of the venturi assemblies, flooding onto the top of the throttle plates??? I left the float adjustment alone during the re-assembly as it was set up correctly, why would it be overfueling?? My main question is with the fuel pump. How does it supply the exact amount of fuel to the carb without having a return to tank?? Is it set-up to supply the correct amount as the engine speed increases?? It seems as though its over fueling the carb and there is no where for the excess to go but down the throats. I purchased this pump brand new from a boat supply outfit on Ebay, the P.N. was correct and it looks identical to my old one. Any thoughts guys???
P.S. I also set the mixture needle out a few turns as it says in the book, but i also fiddled with this and it has no effect. Once i cut the fuel supply to the pump and tryed to run the engine to remove the rest of the fuel from the lines , it ran for about 3 seconds at a time, (which it would not do with the pump sucking from tank), although it would only run if revved, which is making me think its the pump even more. Ran out of time to play with it so thought id get some thoughts on this before i pull it out again.
Thanks in advance guys.

 

[chiefalen]

Re: 3.0L Merc carb questions.

It's the float ether you didn't set it correctly or the needle is not sealing, or the seat is messed up for the needle, you put the new seat in and needle.

You put the float in backwards or it hanging up or the float got gas in it and it ain't floating?

 

[supercab78]

Re: 3.0L Merc carb questions.

A trick that sometimes worked on these type of carbs when they flooded was to slose off the rubber fuel line in front of the fuel pump with needle nose vise grips. Start the engine, it will run for a few minutes on the gas in the carb. After it stalls out, release the vice grips and restart. This will some times flush debri from the needle and seat. The float and seat kind of works like a toilet tank. Float drops more fuel comes in. A little debri in the needle/seat and it just keeps coming in even if not needed. The two screws in front are idle mixture screws 1 1/2-2 turns should be very close. Sucks having that problem when trying to break in a fresh engine.

 

[chiefalen]

Re: 3.0L Merc carb questions.

Well he should have a metal tube connected to the carb so no vice grips are gonna crimp it temp like.

What it will do is crimp it and then the tube will be garbage

Want to fix it right or you want to mess around with this all summer.

Take back off the carb first see if the float is floating. Second make sure the seat is still good, And that the needle is installed correctly when you put the float back on the right way and it ain't hanging up on something.

The tip of the adjustment needle still good or did you bend it when you screwed it in for the adjustment? Gently with the adjustment needle that round tip is all important. Gently in and then out 2 turns to start and adjust from there.

 

[CharlieB]

Re: 3.0L Merc carb questions.

Everyone is right, your problem is still within the carb, and NOT with the fuel pump.

The fuel pump output pressure is fixed by the spring pressure on the pump diaphragm, the needle/seat is plenty capable of withstanding fuel pump pressure as long as the surfaces are in good shape, clean, the float is free to swing up/down, and the float level is set correctly.

 

[Bondo]

Re: 3.0L Merc carb questions.

P.S. I also set the mixture needle out a few turns as it says in the book, but i also fiddled with this and it has no effect.

Ayuh,... That's because the carb is Leaking.....

Rebuilding Old Carbs is a 50/ 50 proposition,... At Best...

Your's just might be Junk....

Either way,... Your's isn't Right,+ needs another Rebuild, or another Carb.....

 

[supercab78]

Re: 3.0L Merc carb questions.

Steal line !? I know that's why I said the rubber line in front of the fuel pump. It's between the pump and fuel tank.

 

[Chris Hays]

Re: 3.0L Merc carb questions.

Thanks for all the responses guys, ill be pulling the carb apart this weekend to see whats going on with it, probably stuck or sunken float as ya'll said, or bad needle/seat.

 

[Chris Hays]

Re: 3.0L Merc carb questions.

Well, had a bit of time to spare today so tried again, took air horn assembly off carb, float is not sticking, not full of gas and i double checked the measurements, all good there. The needle and seat look fine, if i blow into the fuel inlet and gently push the float up, there is no way i can blow through it anymore so this has got to be sealing correctly. Re-assembled it all and now it seems the fuel trickling out may have stopped but it still wont even think about running, All i get is about a 1 second rev of the engine with the throttle held half open and then it dies. I also looked at the mixture needle and the end is worn, looks like it was overtightened into the housing at some point, would this make it not run at all? Really disheartening when trying to get a newly rebuilt engine running and this crap happens!! I guess thats the reason im a Diesel specialist by trade!! LOL.

 

[Don S]

Re: 3.0L Merc carb questions.

What year is this 3.0L?
If a points system type distributor, you may have a bad something from the switch to the coil. When it starts, the power to the coil if fed by the starter. When you let off the key, the power comes from the ignition switch, sometimes thru a fuse and then thru a kill switch, and then the resistor wire and to then to the coil. Also includes a couple of connecters.
Turn the ignition switch to run and see if the is any power on the + terminal of the coil. There should be.

 

[chiefalen]

Re: 3.0L Merc carb questions.

Got a spare spark plug, pull a plug wire off one plug, and insert the spare plug into it and hold it to the block and see if you got spark.

Maybe do it in the dark to see if there are any other stray sparks flying around.

 

[Don S]

Re: 3.0L Merc carb questions.

It's a lot easier and eliminates the cap, rotor, wire, and spark plug if you just pull the center wire (aka coil wire) out of the distributor cap and hold it about 1/2" from a good clean ground to check spark. You should NOT have to do it in the dark either. You want a bright blue spark. Yellow is about the same as no spark.

 

[JustJason]

Re: 3.0L Merc carb questions.

Got a spare spark plug, pull a plug wire off one plug, and insert the spare plug into it and hold it to the block and see if you got spark.

That's not a proper way to check for spark. Even if the plug fires it doesn't mean the engine has good spark.

Once i finished turning over the engine after it would not fire, i looked down the barrells into the carb and gas was still trickling out of the venturi assemblies

That usually means(but there can be more than 1 reason for it also) that there is pressure on top of the fuel inside of the carb.
Fuel is not "sucked" out of a carb, it is "pushed" by atmospheric pressure.
If your dribbling fuel out of the discharge nozzles, then it is being pushed out by something. More than likely have a clog/restriction in the bowl vent, or a clog/restriction is 1 of the many air bleeds that feed the main nozzles.

If this boat won't start with half a dixie cups worth of fuel poured straight down the carb then you have an ignition problem. Double check your firing order first. Then with a good battery have someone crank the engine over and check your base timing... if your timing is to far out the engine is not going to do a thing.

 

[Chris Hays]

Re: 3.0L Merc carb questions.

OK!! Now im starting to get aggrevated with this thing, pulled top off carb, AGAIN, set float limits to the precise measurements given, pulled venturi assembly just for a look, all was well, put it back together, Pulled distributor, checked points gap, adjusted static timing again, (just to be sure), grounded a plug and cranked engine, nice bright spark, put plug back in connected everything up, cranked it and there was the fuel running out of the venturis again for about 10 seconds after i stopped cranking it. OK, so im happy with the condition of the carb, needle, float level, ignition system etc, time to check fuel pressure to it, rigged up a pressure gauge with a piece of hose to the fuel line from pump, cranked engine and oops!! 15 psi, WHAT THE HELL!!!, I know it should only be around 6 or 7 for my engine, the clear hose i was using from the line to the gauge was so hard i could'nt even squeeze it!! i guess buying things off Ebay your really at the merci of the seller as to the description, although the pump is identical to the one i took off the engine. Is there a pump that looks like the one for the 3.0, that puts out 15 psi??? I guess thats the problem, wrong pump.

 

[chiefalen]

Re: 3.0L Merc carb questions.

That fuel pump should not give more than 6 and less than 4 psi. Thats overpowering your carb.

Change that sucker. Will never run right with 15 psi or won't run ever.

I came back to edit, send a email to the guy who sold you that pump and tell him it's the wrong one, and you want to exchange it for the right one.

 

[Chris Hays]

Re: 3.0L Merc carb questions.

What would the part number of the pump ive got on there now be?? Which engine would use this high pressure pump?? Just thought i may need this info to back up my argument.

 

[chiefalen]

Re: 3.0L Merc carb questions.

You got me some 4 barrel 7.4, 8.1 monster motor. I still don't think it needs that pressure maybe yes.

That ain't the point you need one that pumps between 4-6 psi no more no less.

That one you go ain't it. Who's the guy on ebay, he sells a lot of pumps, it a new pump what?

You want part numbers go to the top of the forum look up the pump #.

When i first got my boat i hooked up a cheap electric dc pump that i used to pump my tank dry, the pump was no good that came on the motor.

It was a 6 psi one and i used it just to get the motor running, while the one i ordered was getting delivered. I hooked a switch to it temp like so when i went to start the motor i first flipped the switch.

But it was only TEMP. not marine and not safe to go on the water with.

 

[Don S]

Re: 3.0L Merc carb questions.

There isn't a mechanical fuel pump out there for a carbed engine that puts out 15psi.
It's a bad pump. Get a new pump, and not some cheap used pump off ebay that some seller says is good because he wants to clean out his garage.

 

[chiefalen]

Re: 3.0L Merc carb questions.

I spent a little time last night looking a for a carb that needs a 15psi pump none need that much. I saw 7 psi and finally quit.

Fuel injection needs a hell of a lot more.

 

[JustJason]

Re: 3.0L Merc carb questions.

if your handy you could dissasemble your pump and clean out the valves and valve chamber. It's probably full of sediment and causing such hi pressures.