1976 mercury 115 possible fuel pump issue?

[1976mercury115]

So I’ve had this boat for a while now and the motor has been giving me a lot of trouble it’s a 76 mercury 115. So far I’ve had to replace the spark plugs, coil, distributor, and clean all the carbs. So far it’s only ran like crap and I’ve taken it out 2 time to end up stranded each time because of the motor. The problem I’m having now is I cannot get it to start at all now one of the wires on my distributor corroded so I got a new used distributor to replace it. Before replacing it it would start and run alright but after I replaced it I started it and let it run for a bit then shut it off. Now the next time I went to start it it wouldn’t start at all just pop and act like it’s going to start but not actually start. I removed one of my fuel lines and put it in a jar to see that it’s not pumping much fuel. So I figured I just needed to rebuild my fuel pump well I got the rebuild kit and rebuild the pump, but now it’s pumping better but idk if it’s enough. When I crank it still doesn’t wanna start and when I run a fuel line to a bottle it’s will barely fill the bottom of a water bottle after thirty seconds of cranking. Is that about how much it should pump cause I know it only makes like 3 psi of pressure but I figured there would be more flow than just a little drizzle. Did I not do the rebuild right or is there something else that can cause it to barely pump fuel?

 

[racerone]

Serial # of your motor?-----You start the trouble shooting with a compression test.----Regardless of what you think about your motor.----Post the numbers here.----The fuel pump is driven by crankcase PRESSURE pulses.

 

[1976mercury115]

Serial # of your motor?-----You start the trouble shooting with a compression test.----Regardless of what you think about your motor.----Post the numbers here.----The fuel pump is driven by crankcase PRESSURE pulses.

Sorry forgot to add the compression is an even 120 on all cylinders and I believe the serial number is 4238855

 

[racerone]

With 120 PSI compression , good crankcase compression in each of the 6 crankcases.-----Good spark that jumps a gap of 3/8" or more , your motor should run and purr.----These motors are just wonderful if in good condition with clean carburetors.----You have installed a new impeller since you bought this motor ?

 

[1976mercury115]

With 120 PSI compression , good crankcase compression in each of the 6 crankcases.-----Good spark that jumps a gap of 3/8" or more , your motor should run and purr.----These motors are just wonderful if in good condition with clean carburetors.----You have installed a new impeller since you bought this motor ?

I have not but water does come out in a good stream how can I tell if I have good crankcase pressure when I was testing the fuel pump I could see each time it’s pulse and it’d push out a few drops of fuel so I’m thinking fuel delivery is why it will not run currently.

 

[racerone]

Your motor may not have an overheat warning horn.-----Check fuel hose from motor to the fuel tank and pickup tube inside the tank.

 

[1976mercury115]

Your motor may not have an overheat warning horn.-----Check fuel hose from motor to the fuel tank and pickup tube inside the tank.

I’ll definitely get a new impeller before taking it out again but my pickup tube in the tank did go bad and I put a new one in could my crankcase seal be bad and not let it make enough pressure? I found a video of someone testing the fuel pump and they pump a lot of fuel very quick.

 

[Chris1956]

The fuel pump condition does not affect the starting of the motor, as the primer bulb fills the carbs with fuel. The fuel pump will only affect the running of the engine at high speed or after she idles for a couple of minutes to use up the fuel in the carbs.

On the backdrag carbs, there is a fuel filter screen under the brass fitting, on the top of each carb cover. These can clog, but it is rare. The non back drag carbs have a banjo filter, which should be checked/replaced.

Make sure the choke plates are closing and adjust the shutters to help them seal the carb throats, without binding. Make sure the starter is spinning the engine at 300RPM so proper choking can occur. If she is cranking slow, check the starter cables for corrosion, which can occur under the insulation.

How is the wiring on the engine? The red and adjacent white wire on the switchbox must have +12VDC at all times for you to have spark on that engine. The white wire runs to the ign key, and red wire is hot at all times. At this point the original wiring is pretty well shot, as the insulation has fallen off. Replacement wiring harnesses are available. Try surplusunlimited.

Those old distributors often suffer from intermittent trigger function. The good news is that they usually completely fail after a while, so you can diagnose that easier. I presume the carbon electrode under the distributor cap is good? If in doubt, replace it. You should put a spark checker into the spark plug wiring to monitor your spark. Initially check all 6 plug wires, and they can corrode after all these years. They are replaceable, if you look on the web. Search for "yellow Tiger Tails".

 

[1976mercury115]

The fuel pump condition does not affect the starting of the motor, as the primer bulb fills the carbs with fuel. The fuel pump will only affect the running of the engine at high speed or after she idles for a couple of minutes to use up the fuel in the carbs.

On the backdrag carbs, there is a fuel filter screen under the brass fitting, on the top of each carb cover. These can clog, but it is rare. The non back drag carbs have a banjo filter, which should be checked/replaced.

Make sure the choke plates are closing and adjust the shutters to help them seal the carb throats, without binding. Make sure the starter is spinning the engine at 300RPM so proper choking can occur. If she is cranking slow, check the starter cables for corrosion, which can occur under the insulation.

How is the wiring on the engine? The red and adjacent white wire on the switchbox must have +12VDC at all times for you to have spark on that engine. The white wire runs to the ign key, and red wire is hot at all times. At this point the original wiring is pretty well shot, as the insulation has fallen off. Replacement wiring harnesses are available. Try surplusunlimited.

Those old distributors often suffer from intermittent trigger function. The good news is that they usually completely fail after a while, so you can diagnose that easier. I presume the carbon electrode under the distributor cap is good? If in doubt, replace it. You should put a spark checker into the spark plug wiring to monitor your spark. Initially check all 6 plug wires, and they can corrode after all these years. They are replaceable, if you look on the web. Search for "yellow Tiger Tails".

The motor has been rewired and that carbon electrode is that the little cylinder thing that sits in the distributor cap? I’ve also rebuilt the starter and checked with a timing light the rpm’s the starter is spinning over the motor. I’ll check the screens or banjo fitting on the top of the carbs cause the screen in the fuel pump was a little clogged when I got the boat. Thanks for your help I’ll try everything you suggested.

 

[racerone]

Install an ELECTRIC PRIMER valve off a 1980 to 2005 Johnson / Evinrude on this Mercury.----Hard starting is then not in your vocabulary anymore.

 

[1976mercury115]

Install an ELECTRIC PRIMER valve off a 1980 to 2005 Johnson / Evinrude on this Mercury.----Hard starting is then not in your vocabulary anymore.

I’ll defiantly look into that it’s been pretty hard to start even when it was running ok.

 

[Chris1956]

Remember, when starting, you cannot normally flood them. So, pump the primer up until hard, cold start lever all the way up, hold choke button and crank her until she fires. After that, feather the choke until she is warmed up, and lower the cold start lever.

 

[1960 Starflite]

Remember, when starting, you cannot normally flood them. So, pump the primer up until hard, cold start lever all the way up, hold choke button and crank her until she fires. After that, feather the choke until she is warmed up, and lower the cold start lever.

That's exactly the way I start my 850. Once warmed up, it starts with just a quick hit of starter.