1985 7.5 hp 2 stroke Mercury won't pump gas into the bowl

AnthonySci

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I bought a used 7.5 hp, 2 stroke Merc a month ago. Before using it, I bought new, non-ethanol gas, added the Quicksilver 2 stroke oil, 50:1 and then ran it once for a couple hours fishing. It started right up with full choke, ran normal at idle and also with throttle open.

A week later when I went to use it, before starting, I noticed the fuel line squeeze primer bulb only would pump to "water-balloon semi-hardness." With full choke, the motor started on the 2nd pull of the rope. In less than 2 minutes, the motor coughed a few times, sputtered and then quit. (I'm fairly certain that I ran the motor out of gas---that it was running only on the gas that had remained in the bowl. Then, I was unable to start the motor.

Since then, I have replaced the entire fuel hose and the primer bulb and the bayonet shaped fuel connector.
I replaced all the fuel pump diaphragm gaskets. I removed and inspected the fuel inlet needle and the main jet, both are completely clean and clear. The short rubber vacuum hose that connects between the cylinder fitting and the carb throat is clear.

Gas is getting to the fuel pump diaphragm--- but NOT passing through it. The fuel filter visually appears unobstructed--I even removed it and then re-installed the carb and still no gas pumping into the bowl.

When I remove each spark plug and squirt a bit of fuel mix into each cylinder, then replace the plugs--with full choke, the motor starts right up, runs for about 2 seconds then dies (no fuel.) So I'm getting spark and compression.
Any ideas?
 

GA_Boater

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Welcome aboard.

What is the serial number of the motor so we can look at some parts diagrams?

How did you check that gas is not passing through the pump when squeezing the bulb?
 

AnthonySci

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Serial number A172695
Walbro WMB-4 carb
When I remove the fuel hose from the inlet on the outside of the fuel pump / diaphragm mechanism---and then squeeze the primer bulb--I see gas squirting out of the end of the hose. However, when I remove the fuel bowl--it's bone dry inside!
Today, online I ordered a new needle inlet w/ spring and a new main jet. Even though visually the existing parts look exceptionally clean, I see daylight through the holes / orifices
I have not verified if a vacuum is getting to the carb venturi through the small rubber hose attached to the fitting located on the block at the lower cylinder.
(Produced by pulling the rope start, moving the piston up & down, then the negative pressure drawing gas through the diaphragm into the bowl)...Perhaps, I could test by attaching a small plastic bag over the end of the black rubber tube, pull on the rope start, and see if the bag moves in or out. What do you think?
 

racerone

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No, no-----------That wee rubber hose just feeds crankcase fuel / oil from bottom of block back into the air intake.--------And it has nothing to do with operation of the carburetor !
 

ondarvr

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Nothing pulls fuel into the bowl, it's pushed in by the pump.

The jets have nothing to do with filling the bowl either.

Pull the fuel line off the carb side of the pump and see if fuel comes out when you squeeze the bulb.

Are you sure you installed the floats and/or needle correctly? They directly affect the fuel moving from the pump to the float bowl.
 

racerone

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???----I believe the fuel pump is built into the carburetor on this motor !
 

AnthonySci

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When I pull the fuel line from the side of the carb and squeeze the bulb, yes---it pumps gas out the end of the fuel line.
My fuel pump is built into the side of the carb.
I bought the Merc diaphragm gasket kit, replaced the entire set of fuel pump---and still not pumping gas into the bowl.
Update: The orifice / opening that the float opens & closes against I have NOT verified as clear & unobstructed; that's about the only fuel location remaining.
For a novice mechanic like myself, your help has been immeasurable, thank you.
 

AnthonySci

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With the carb removed from the motor, I dropped the float out of the way and then slid out the spring loaded-half inch long-"inlet valve." The needle was clean, chrome bright; the spring was OK . The opening allowed my breath to pass through it. I reassembled.

With my gas tank attached to the motor. After squeezing the primer bulb 6 or 7 times, enough for it to become hard, when I squeeze the bulb one additional time, raw gas escapedfrom around the blue plastic cover (part #29) cover/strainer/inlet that the fuel line connects onto.

After moping up the spilled gas, I sprayed starting fluid into the throat of the carb. On my 2nd pull of the rope, the motor started, ran for a second until the starting fluid was burned up.

I don't know what to do next.
 

ondarvr

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With the carb removed from the motor, I dropped the float out of the way and then slid out the spring loaded-half inch long-"inlet valve." The needle was clean, chrome bright; the spring was OK . The opening allowed my breath to pass through it. I reassembled.

Did you squeeze the bulb at this point to ensure gas was making it into the bowl?
 

AnthonySci

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With the carb removed, AND with the FUEL PUMP BODY removed from the carb---yet intact---when I squeeze the bulb, gas pumps out through the 2- side by side holes farly low on the FUEL PUMP BODY part #22. However, NO GAS is pumped out through the hole/opening that lines up with the top LH hole of gasket #20 that aligns with the top LH chamber of the carb that feeds by gravity into the bowl as long as the float is in the down position.

Further, when I reattach the entire FUEL PUMP BODY to the side of the carb and then pump the bulb---gas escapes out the overflow channel that's built into part #29, the blue plastic fuel inlet cover. Just to be sure, I lowered the bowl and it is BONE DRY.

This seems to pinpoint the blockage is somewhere inside either part #25 or inside part #22. (Perhaps, a fragment of the old fuel hose or the old deficient squeeze bulb broke loose, entered the gas stream and clogged one of the internal ports???) Any advice other than replacing parts #25 and #22 ?
(I already have bought and installed the entire new diaphragm gasket set.)
 

ondarvr

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I don't have the parts diagram, but your explanation is very good, sounds like you found out where to look.
 

AnthonySci

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Update:
I replaced the INLET NEEDLE & SPRING that allows gas into the bowl when the float is "down." Also, I replaced the SPARK PLUGS. (Previously I replaced the entire fuel pump gasket set including the diaphragm. On my Walbro WMB-4 carb., this entire fuel pump assembly is mounted to the side of the carb.
Non-ethanol gas I bought in October 2016.

I squeeze the (new) primer bulb until firm, Full Choke, after a few pulls on the rope, the motor starts, runs for maybe 30 seconds; then it dies. It runs long enough for the water cooling to begin exiting the motor. Then to restart, I need to Re-Squeeze the PRIMER BULB.

After a Merc. motor is started and running, what keeps "the gas flowing?" What keeps the diaphragm moving: inhaling , then releasing?

www.marineengine.com/parts/mercury_marine_parts/sn/0A172695/ 1007225/297_40
shows the WMB-4 carb. parts diagram.
 

racerone

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The pump diaphragm is operated by pressure pulses from the crankcase.-------A positve pressure pulse on the down stroke of the piston and a slight negative pulse on the up stroke.---------Sometimes the wrong gasket is used between carburetor and engine and the wee hole for those pulses is blocked.
 

AnthonySci

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Thanks for pointing toward investigating the series of gaskets that make up the diaphragm within the fuel pump.
Of the 5 gaskets, I switched the positions of the 3rd and 4th one---eureka, motor starts right up and continues running!
Again, thanks to all who responded, I learned much.
 

AnthonySci

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It's been said many times: There's no such thing as a stupid question.
On this website, where do I go to start a new thread???
For the life of me, I can't find anywhere to "click onto" to type in a new thread, a different motor, different question.
 

racerone

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Scour this page near the top.---------New topics------See if that works.
 

AnthonySci

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How to begin a new thread?
Log in.
In the SEARCH box, type in a very broad topic. Like "Mercury Outboard."
This type of search will then open a result page show a list of TOPICS.
At the very top of this list, on the left side---there's a SOLID ORANGE box: " + NEW TOPIC "
Click on this box.
Type in your question and you've begun a NEW THREAD.
 
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