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Mercury 7.5 1981 fuel issues

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sobr0716

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Hi all, just bought a small aluminum boat with a Mercury 7.5. I have down the research and it's a 1981 model. This is the issue. Starts right up after priming with bulb (brand new) runs great for about 3 mins then stalls. Lack of fuel.. . If I continue to pump while running it does fine.

This is what I have done. Replaced all the fuel pump gaskets, ensured float was at 1/4 inch, replaced plugs. Blew out all jets with compressed air carb looks good except for some corrosion in bottom of bowl.

any help is appreciated,

mark
 
Possibly an issue with what drives the fuel pump.--------------Crankcase compression that is.-----Possibly a bad seal on the bottom of the crankshaft?-----And check the gasket between the carburetor and block as it must have a hole in the right place to drive the pump.
 
kimcrwbr1 said:
http://www.ebay.com/itm/Mercury-New...142-/400699822271?hash=item5d4b9210bf&vxp=mtr
You need to confirm the right part number for your motor and give the carb a thorough once over. Pull the jets and soft plugs, lightly surface the flange, Soak it good in carb cleaner and blast all the ports with high pressure water and air. The ears on the carb flange can get bent and create a vacuum/pulse leak, On a glass table with 220 grit pull the carb in one direction only until it all shines the same. Do not overtighten the carb down nice and snug is all that is necessary.
Click to expand...

So there is a small vacuum line that runs from what appears to be the crankcase to the front of the carb. It connects to a brass nipple that just opens up to the main throat of the carb. Is that what drives the pump?

i did get the correct rebuild kit. Same as the one mentioned from eBay. However I did not install the brown looking gasket as there was not one when I took it apart. This is the gasket in the middle top on the picture from the eBay post. It is identical (except in color) to the one in the lower left that is black. I have looked at parts schematics and I don't see where that on would go.

i appreciate all the help!
 
kimcrwbr1 said:
What is the serial number? Compression numbers? Does it jump a open air gap of at least 7/16 in on both plug wires? If it runs good as you pump the primer the pulse port is likely plugged. That has the integrated fuel pump carb if you dont know when the carb has been rebuilt get a complete kit and clean all the ports with high pressure water and air. Are both plugs burning the same color?
Click to expand...

Not sure about compression numbers but when running full throttle in the water it planes the boat and runs great. Both plugs are identical in color.

Where is the pulse port? Is that the small nipple connecting to the crankcase with a vacuum line to the carb?

serial number 5700307
Carb tillotson/mercarb bb7a

thanks, mark
 
Some of those motors used a fuel pump that wasn't up to the job when the motor aged (lost crankcase compression). Merc put out a bulletin on how to replace it with an aftermarket pump. (I've done a dozen at least of them myself.)

Can you post a photo of the fuel pump?

Jeff
 
The small hose just returns oil to the intake for re-use.------Pulse port is most likely a small hole on the carburetor flange.
 
fastjeff said:
Some of those motors used a fuel pump that wasn't up to the job when the motor aged (lost crankcase compression). Merc put out a bulletin on how to replace it with an aftermarket pump. (I've done a dozen at least of them myself.)

Can you post a photo of the fuel pump?

Jeff
Click to expand...
Sure

Here are a couple of the pump
 

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All bets are off; that's the later carb with a good set up.

The vacuum to run that pump comes in right from the crankcase via a small hole in the intake/ carb gasket. See if that hole is open and the gasket is sealing it properly.

Jeff
 
fastjeff said:
All bets are off; that's the later carb with a good set up.

The vacuum to run that pump comes in right from the crankcase via a small hole in the intake/ carb gasket. See if that hole is open and the gasket is sealing it properly.

Jeff
Click to expand...


Thanks Jeff, When I pulled the carb I found the issue. In the computer biz we refer to the problem as "an issue between the "seat" and the "keyboard". Basically user error!!!!

I installed the flange gasket upside down blocking that little hole :mad:. flipped gasket around and installed starts right up and runs forever, no pumping of the bulb. appears to be pumping gas like a champ!!!! I am sooooo grateful for you guys making me re-check my work.

On to easier tasks. how do I make sure I have the idle needle in the correct spot?

Thanks again!!!!
 
AAahhhh! Did that myself once. Good man!

Start her up, warm her up good, put her in gear (on the trailer or tied to a dock), then SLOWLY turn the idle screw in until the motor picks then tries to stall. Open the needle about a 1/2 turn and you'll be close.

Jeff
 
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