Mercruiser carb backfire and fuel pump issues

Mulligan18

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Jun 1, 2016
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Friends, I am new to this forum and new to forums in general. I have read much about my problems in this and other forums and believe I have made good progress but I need help bringing these problems to conclusion. My boat is an 1994 StarCraft 1810ss with the 4.3LX Mercruiser 4bbl weber carb. I purchase it and used it a little last fall. It is in the water at a dock now.

Allow me to start with the fuel pump. My pump does not run when the key in in the on (not start) position. I hear conflicting advice on this. Should the fuel pump operate with the key in the on position without cranking or running the motor? There is no power to the pump at all when the engine is not running. The pump will run if I jump the connections at the solenoid (with the engine off).

Last year and again this year, I have sputtering and backfiring through the carb. I have replaced the plugs, battery, installed a rebuilt carb (old one was black as coal) and installed a new check valve at the gas tank. The old valve had been defeated so I assume the prior owner was chasing this same problem. (A little cold blooded, he said). I have cleaned all connections at the solenoid and the main ground at the block. I will be ordering a new cap and wires.

I plan to purchase a new fuel pump. Not sure if this is my problem for the sputtering and backfiring but the current pump looks original and probably should be changed out at this time. It does not have a clean whirr sound when running. I hear a rattle like sound so I am assuming it is worn out not putting out enough to keep up with the demand when accelerating. Searching on-line and chatting with west marine (on-line) I cannot come up with a fuel pump match for a 1994 merc 4.3 lx. The closest I can find is a Sierra 18-7326 (8224065). Most places indicate this fits the 92, 93 and some say 96 as well but I can find no listing indicating the correct pump for a 1994. Any suggestions? I will be going out to the boat toning to see if I might be able to read a part number from the current pump.

Thank you,
JBB
 

alldodge

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Howdy

Your fuel pump should not run in the ON position "until" the key is turned to START. After the motor starts and the oil pressure increases enough to close the oil pressure switch, then the pump will run in the ON position.

To check your pump you need to remove the fuel line feeding the carb from the pump, and install a Tee fitting between carb and fuel line, and attach a vacuum fuel pump pressure gauge to the third connection. Fuel pressure should be between 4 to 7 psi while at idle or full WOT.

Give us your motor serial number so we can figure this out

Merc Elect fuel pump wiring.jpg
 
Last edited:

stonyloam

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Have you checked your timing? Before potentially wasting money on a new pump, go to, Autozone and borrow (free) a fuel system test kit and see if that is really your problem.
 
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Mulligan18

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Thank you both for your prompt replies and information. I just went out to the boat and looked for a part number on the current pump. A label on the side of the pump says.. "SAE J1171 Marine". Stopped at West Marine. The guy at the counter said his records show a Sierra Pump 18-8867. But, that pump does not look like mine,. I believe the motor number is 0F132498. If that number does not look correct, let me know where to find such and I will get it tomorrow. I have not yet checked the timing. I don't realty mind spending a little on a new pump for the peace of mind. It's and older boat with a very much unknown history. I would feel better getting as much brought up to date as possible.
 

alldodge

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Mulligan18

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New fuel pump installed last night. The engine runs much better but I still have some sputtering and backfiring when accelerating from low RPM's. The condition improves when the engine is warm. It stalls out when I reduced to idle after running at a cruising speed. I think the distributor cap, rotor and wires may be the next necessary step. Took it out for a ride on the bay. WOT gave me only 25 mph at 3000 RPM. I think the rebuilt carb is not functioning correctly. It seems to me that the secondary barrels of the 4bbl carb are not opening. I can't understand why. Worse, though, I am now hearing a knock of some sort in the engine. This is completely new. Don't know what to think about this.
 

Bondo

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I think the rebuilt carb is not functioning correctly.

Ayuh,..... Welcome Aboard,..... I'd be lookin' at the carb again too,.....
 

Mulligan18

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Jun 1, 2016
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I think I might reinstall the original carb and see how that goes. I will get to that task tomorrow morning. I am highly troubled at the thought that something might have dropped down through the intake manifold. Major surgery may be on the horizon.
 

sjm32157

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Jul 29, 2009
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Mulligan,

I have been fighting the same problem on my 1995 Maxum 4.3LX for the last couple of seasons. Gas tank and pickup tube are always clear. No signs of contamination in the sep filter. I also had a tired looking Carter-Weber carb so I replaced with an Edelbrock 1409. Wife and I had the boat in the water yesterday for the first splash of the season. Ended up limping back to the ramp with fuel starvation symptoms. I ordered a new fuel pump last night. I put 12 volts on the old pump and it sounded like an electric razor.
 

Mulligan18

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Jun 1, 2016
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Seaman, I stopped at a Mercruiser dealer just today and picked up a rotor and cap kit. Raining today. Hope t install tomorrow. I was telling the parts guy about my problems and he had a couple of suggestions. He said to pull the fuel pick up from the tank and check to see if there is a screen that might be blocked. He said also to check the fuel line from the tank. If it is a Sierra line it might be degrading from within and causing problems. In any event, the fuel pump made a big difference for me but the problem is not entirely gone. Hopefully the cap and rotor will be the ticket.
 

Mulligan18

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Gentlemen,

Rain stopped so I installed a new distributor cap and rotor and also reinstalled the old original carb. It starts and runs at the dock just fine with no hint of hesitation, sputter or backfire even with aggressive throttling from low RPMs. The knocking sound seems to have subsided as well. Oddly, the idle speed was about 1,200. I brought it down to about 900 with the idle screw adjustment. I'll probably bring it down a little more after I have a chance to run the boat a little on the bay. So, it appears I probably had three problems working together. 1. Corroded connections hear and there reducing voltage. 2. A bad fuel pump 3. A bad cap and rotor. The carb replacement was a waste of time and money. I'll probably just return the rebuilt one as the core. Lesson learned.

Quick last question. What motor oil should I be running? The guy at West Marine recommended 25w40 Quicksilver. Should it be strait 40 instead?

I recall a story my Dad told me a long time ago. Boat builders often times kept a chair in the shop called a "moaning chair" for such times that something got cut to short instead of too long or some other crisis. Thank you all for your advice and information so that I could avoid the moaning chair.

Mulligan18
 

alldodge

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Oh no, another oil thread :D

I think of it this way, my boat doesn't start if I have to put on a jacket. So running straight 40W is what I run. Now my rebuild done by a performance builder says to run 20W50 Castrol, and since I paid him to build the motor I'll do as he ask. If you run in colder weather you want to use a multi grade oil
 

Mulligan18

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Jun 1, 2016
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Thank you Mr. All dodge. I am on Lake Ontario in western NY. Could be 50 one day and 95 the next during the boating season. I guess I am ok with what I have.
 

NHGuy

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Great, it's often little things that mess up boat engines.
 
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