Carburator with electric pump question

[i_timmers]

Can somebody describe at what point will the fuel pump turn on and run? I have a Mercruiser 1996 4.3L LX V-6 4BBL that is not getting fuel to the carb. Will it stay running, or will it just become a lift pump from the tank on startup?

Thanks,

Tim

BTW, the carb is fine, it's been checked by a rebuilder with a great reputation

 

[thrasher]

Re: Carburator with electric pump question

The fuel pump is usually supplied power from 2 seperate locations. The first is an oil pressure switch. This switch will not operate until the engine has started and oil pressure has built up. As long as you have oil pressure you should have a positive supply to the fuel pump. The second supply is from the starter motor, when the starter motor engages, it also supplies positive to the fuel pump to allow the engine to have a fuel supply when starting the engine. If the starter motor is not operating and there is no oil pressure, the fuel pump will not operate.

Hope that helps.

Gary

 

[i_timmers]

Re: Carburator with electric pump question

When we first started the boat, we cycled the throttle 2 times. The boat started and then died out. Pumped it 2 more times, it started and ran. So, I'm thinking the original problem might be the fuel pump is not energized or it is burned out. My original post was my boat is starved of fuel under load(like it was running lean). I hold down the choke plate, it richens out the mixture, and the boat planes out and it practically threw me into the engine compartment!

The added vacuum the carb is developing is possibly pulling the additional fuel from the tank and not utilizing the fuel pump?

Possible?

 

[Don S]

Re: Carburator with electric pump question

Have you tried giving more throttle when it starts? The choke may also be too tight and causing it to die the first time.
When first starting, the 2 pumps, used fuel. When you start cranking the starter, only then does the fuel pump come on. If when starting, you only crank for 1 second before it start, and it dies in 2 seconds because the rpms are too low, and it takes 3 or 4 seconds for the oil pressure to build up for the pump to start again. You are in a loosing battle.
Pump it twice and give it about 1/4 throttle , allow it to get to 2000 to 2500 rpm and back it off slowly and see what happens.
It's a carb, not EFI. Cold, carbed. engines do not like minimum idle.

 

[180shabah]

Re: Carburator with electric pump question

Between this and your other thread, I am not convinced that you have a fuel pump problem. If you wnat to double check it though, unplug it from the harness, and feed it a constant 12v. I doubt that it will start any easier though.

 

[i_timmers]

Re: Carburator with electric pump question

Now, my main reason for asking this question was to figure out the main job of this fuel pump and when it starts and stops working. I figured it wouldn"t be the same as an EFI but is it giving me the same problem I have seen with weak pumps. Being the boat starts, would this be the work of my manually pumping of the throttle? If so, can the carb, by itself, pull fuel into itself when the fuel pump is shot/inop? Now, it isn't pulling enough fuel to run WOT, but just enough to maybe run 2400rpm(under load) and then bog down at any more throttle given to it?


BTW, To everyone!

I want to thank you all for the help everyone is putting forth in this. I believe in applying all my efforts and suggestions in the same way you all have, If I can!!!

Tim

 

[i_timmers]

Re: Carburator with electric pump question

Between this and your other thread, I am not convinced that you have a fuel pump problem. If you wnat to double check it though, unplug it from the harness, and feed it a constant 12v. I doubt that it will start any easier though.

Not so much the starting issue that bothers me, more about not being able to plane or run WOT that is irritating me. The carb mechanic tells me I need to check my fuel supply and I want to check all avenues including the power to the fuel pump. If it ends up to be the fuel pump, that would be great! At least I will have my problem fixed. I might be able to get a good nights sleep! LOL!

If it's feesable that a carb can run(within reason) without a fuel pump, then I am more convinced this would be the problem.

Thx,

Tim

 

[Bondo]

Re: Carburator with electric pump question

If it's feesable that a carb can run(within reason) without a fuel pump,

Nope,.......

A Carb will Not suck fuel,........

It needs to be supplied with about 5psi, by the Fuel Pump..........

Btw,.......... Pumping the Throttle will Not pump fuel either..........
Only the Fuel Pump pumps fuel.......

 

[thrasher]

Re: Carburator with electric pump question

Bondo is correct, a carb cannot suck fuel from the tank. Either your pump is not working (or so worn its not pumping enough fuel) or you have an issue with the fuel delivery system. Have you checked all the inline filters and cleaned them? There is usually a main filter near the tank, a small filter on the inlet to the carb and sometimes another in the fuel pump. Then there is also a screen on the fuel tank pickup.. I was going to suggest a blocked fuel tank breather, but as you have this problem from the minute you start the engine, it's unlikely to be the breather. If it won't rev up, have you checked the timing and spark plugs/ht leads?

Gary

 

[180shabah]

Re: Carburator with electric pump question

We're missing the key point here. when he was running with the flame arrestor of he was able to manually close the choke down, and it ran fine. The fuel system is working the problem is in the carb.

 

[Bondo]

Re: Carburator with electric pump question

We're missing the key point here. when he was running with the flame arrestor off he was able to manually close the choke down, and it ran fine. The fuel system is working the problem is in the carb.

Ayuh,......

Actually the Key Point is that this is a 2nd thread about the Same Issue, by the Same Poster...............

BTW, the carb is fine, it's been checked by a rebuilder with a great reputation

Ayuh,..........

I'm having a Hard Time buying That...............

I think your Rebuilder with the Great Reputaion is 1 one the Thieves,+ Scammers you were talking about in your Other thread.........

A Reputable Carb Rebuilder,..... Rebuilds Carbs,+ Charges you for it..........

 

[i_timmers]

Re: Carburator with electric pump question

Actually Bond-O, He DID NOT charge to open it up, inspect, and do a mild cleaning. He checked the float bowls and the jets were clear. The filter inside the carb(Weber thing) was clean! "There was no need to rebuild it", he said. I don't know of a rebuilder that would pass up the opportunity to not make money.

The reason I reposted this is because I was curious about the pump issue, If I added it to the last post, it would have been overlooked.

So if I am understanding this, without the pump, I have no flow whatsoever? Ok, that answers that question. I do realize there will be cold start issues with a carb, that's ok with me also. Since I have been trying to locate the proper tools to check the fuel pressure, I have been collecting the parts needed to replace possible failure points(Fuel/water filter, anti-siphon, new section of fuel hose and a transfer pump to empty the fuel tank out). Once this is empty, I will have to inspect the pickup tube and in-tank filter for clogs and debris. Then I will reassemble and fill with fresh fuel.

Basically, I will then be left with a weak electric fuel pump as the last resort. Unless there is something else I am missing?

 

[rodbolt]

Re: Carburator with electric pump question

wow
bond-o got red dots, DonS got red dots.
maybe I can get one
Dude your missing the whole thing, time and time again.
the fuel pump simply supplies the carb float bowl with fuel. model T's used gravity to do it and worked ok.
on your merc, the R terminal on the starter supplies the pump purple yellow wire with 12volts when cranking, once the key is released the 12v positive comes from the purple wire,to the oil pressure switch mounted just above the oil filter, through the switch and onto the pump.
a working switch,with clean contacts and connectors along with a good ground path is imperative.
as its a positive displacement flow through type pump they dont get weak. they normally work or not.
check all the wiring with a voltmeter to insure you have the correct voltage at the proper time to actually run the pump. most all the pumps have a screen at the inlet that can clog.
from there a fuel pressure/vacum gauge,availible at wally-world or napa, will be your best friend and can quickly narrow down the problem.

 

[i_timmers]

Re: Carburator with electric pump question

Please make no mistake, any discussion about mechanics is based on local experience. If I had the referral for an excellent mechanic, the boat would be there. The great thing about this site is this is just about a bunch of people getting together and talking about boats and helping each other out... If I can lend a hand, I am the first one to offer up experience or the time!

 

[Daddy5295]

Re: Carburator with electric pump question

I'm with ya timmer's, The best advice I give my buddies is don't buy a boat unless you have alot of know how, or ALOT of money.

 

[180shabah]

Re: Carburator with electric pump question

wow
bond-o got red dots, DonS got red dots.
maybe I can get one

RED DOTS???????

 

[180shabah]

Re: Carburator with electric pump question

Basically, I will then be left with a weak electric fuel pump as the last resort. Unless there is something else I am missing?

Seriously - you manually closed the choke and it took off. means there was already fuel in the carb bowl, means the pump already put it there, means it made it from the tank, through the pickup, through the A-S valve and through the filter. Ther may be issues with the filter, pump, AS valve etc, but they are NOT causing THIS problem.

THIS problem is in the carb. Or you have a pretty severe Vacuum leak.
What jets/rods are in the carb?
What step up springs are in the carb?
what is the measurement for float height and drop?
what is the measurement for the choke pulloff?
What is your vacuum reading at idle?

The carb can be clean and free of debris, that doesn't mean it is setup/tuned for YOUR application.

 

[i_timmers]

Re: Carburator with electric pump question

Well, the only thing I can say is that: this carb, that came off this motor, on this boat, is from Phoenix, where I also live. This boat is originally from here, no altitude differences to speak of. Why would an owner change the carb settings if it is a local boat? Registered here, purchased here, lived here.

Originally thought there was residue in the float bowls, or jets, or the carb internal filter. All is clean and was when it was opened up yesterday by the mechanic who was gonna do the rebuild.

I asked about the carb pulling fuel into itself because I was not sure of any vacuum that was generated in the carb besides the choke.

 

[180shabah]

Re: Carburator with electric pump question

Still need to check the following:

what is the measurement for the choke pulloff?
What is your vacuum reading at idle?

 

[i_timmers]

Re: Carburator with electric pump question

The boat is back together. I siphoned the fuel out last night, that was fun. The pickup tube was clear of any blockages, and the tank wasn't too dirty either. I installed a new anti-siphon, fuel hose from the tank, fuel/water filter and then the carb. I did pull the fuel pump to check for blockages in the inlet screen, it was clear. Also, I cleaned the flame arrester for good measure.

Heading to fuel up shortly and we'll see how she runs. I also will get a vacuum gauge on it and then I can get the vacuum idle readings. The choke pulloff is 1/8th inch(3.5mm), just checked it!