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1976 Johnson 15hp help

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Rteague

New member
Ok, just got the motor to run at a high idle, carb needed cleaning. When I let it idle for about 30sec to a min it starts to sputter and die. If I give more throttle it will pick back up. After it dies it won't start again until I reprime the bulb. I've tried venting the tank, and closing the vent on the tank. Still pulls the bulb half empty then dies.

Checked every line and bulb, even direct plumbed the fuel line into the fuel pump ( to rule out loose quick connect).

Anyone got any ideas?

Pulling my hair out on this one. Need it to stay running to fine tune the carb and get a smooth idle.

thanks,
russell
 
It sounds like an aftermarket plastic tank? Will it run properly when the fuel tank is up on the seat, rather than the floor? How old is the plastic tank and what brand is it? Has the tank spent lots of time out in the sun and has it been exposed to any ethanol? Thanks, Tim
 
Brand new 6gal plastic tank. just bought yesterday. Not sure model of tank. Bought it at academy sports. Have had the tank on the bench beside the seat the whole time while working on it.
 
What happened to the original tank? Did it run the same way? How far below the fuel pump is the tank when you are testing it? Thanks, Tim

Also there are many who believe that the 9.9/15's have inherent fuel pump problems, especially when exposed to ethanol blended fuel.
 
Pump the bulb manually and see if it stays running if it does you have a fuel pump problem. Is the arrow on the primer bulb facing the correct way the arrow points towards the motor. The pumps on those motors are good, all pumps that age had problems with ethanol. The motor is supposed to empty the bulb when it starts running. Do you mean it actually sucks the pump flat.
 
Tim- when I bought the boat, the tank had a pinhole in it, so I replaced it. Sitting on the back bench it's almost level with fuel pump.

Scott- tried that, I believe it's the fuel pump. It would run as long as I kept pumping it. Gonna find me one and replace it. I was thinking that may have been the issue last night after thinking about it.
 
Rteague said:
Tim- when I bought the boat, the tank had a pinhole in it, so I replaced it. Sitting on the back bench it's almost level with fuel pump.

Scott- tried that, I believe it's the fuel pump. It would run as long as I kept pumping it. Gonna find me one and replace it. I was thinking that may have been the issue last night after thinking about it.
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I would not recommend the pump kit unless you are good with tedious tasks, but it can save you some cash.
 
Before buying a new fuel pump or rebuild kit I would recommend taking off the fuel pump and taking it apart to inspect. Get a clean rag or clean work surface and carefully take it apart being careful to not lose any small parts such as the little springs. Keep pressure by pinching the assembly together before removing the two screws and slowly let it apart. I have had the same problem as you are describing and the last person rebuilt it incorrectly. Just google how it should be rebuilt and with a little time and patients you could discover it being in there wrong.
 
had the same problem with my 6hp. this is how i checked the fuel pump. i crank the motor ,got it operating temp , shut it off , pumped the primer bulb to fill the carb bowl, crank the motor again(the motor will run for a little while with the bowl full) pulled the gas line from the carb and see if its pumping gas to the carb.if its weak then you either have a cylinder low on compression or the plastic piece on the fuel pump has a crack in it( the one that the fuel line connects to) those are to be finger tighten.
Rteague said:
Ok, just got the motor to run at a high idle, carb needed cleaning. When I let it idle for about 30sec to a min it starts to sputter and die. If I give more throttle it will pick back up. After it dies it won't start again until I reprime the bulb. I've tried venting the tank, and closing the vent on the tank. Still pulls the bulb half empty then dies.

Checked every line and bulb, even direct plumbed the fuel line into the fuel pump ( to rule out loose quick connect).

Anyone got any ideas?

Pulling my hair out on this one. Need it to stay running to fine tune the carb and get a smooth idle.

thanks,
russell
Click to expand...
 
Compression has nothing to do with the fuel pump. Those pumps are fairly easy to rebuild with a little patience. The rebuild kit is around $15 an afternarket pump is about 40 or so and a factory one is $90 plus.
 
flyingscott said:
Compression has nothing to do with the fuel pump. Those pumps are fairly easy to rebuild with a little patience. The rebuild kit is around $15 an afternarket pump is about 40 or so and a factory one is $90 plus.
Click to expand...
is this an electric fuel pump ? because the older ones worked on compression where as the fuel pump is mounted on the side of the block where a port allows compression off the cylinder to operate the fuel pump.
 
The fuel pump use a crankcase pulse to operate. One pressure pulse and one small vacuum pulse to work for each cycle. Cylinder compression is in the cylinder only if you tried to put cylinder compression into the fuel pump it would not last long.
 
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My idea is that crankcase pulse/pressure is directly proportionate to cylinder compression, however in this case I don't think that is the problem. I have a little gauge I made to monitor case pulse strength at the fuel pump pulse port. The pulse diaphragm in the pump is probably stretched and weak (ethanol) and cannot supply enough movement to effectively pump the fuel. If you put the tank up above the motor once it is running, the fuel will siphon in and then we will know that the fuel pump is tired if it continues to run. This, however, will not hold true if there is a blockage.
 
timguy said:
My idea is that crankcase pulse/pressure is directly proportionate to cylinder compression, however in this case I don't think that is the problem. I have a little gauge I made to monitor case pulse strength at the fuel pump pulse port. The pulse diaphragm in the pump is probably stretched and weak (ethanol) and cannot supply enough movement to effectively pump the fuel. If you put the tank up above the motor once it is running, the fuel will siphon in and then we will know that the fuel pump is tired if it continues to run. This, however, will not hold true if there is a blockage.
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Believe what you want cylinder compression has NOTHING to do with crankcase pressure.
 
Also note, if the original steel tank had a "pinhole" in it, then the previous owner likely had ethanol laced fuel stored in it. Although the tanks can rust as water is of course heavier and sits on the bottom, ethanol will "invite" water into the fuel and while it is stored for a reasonable length of time, it can promote faster rust formation. Remember, 10% ethanol can suspend its weight in water. That is all fine and dandy until you go to store the fuel. Humidity will actually find its way through the breather port on these steel tanks, (they are always vented automatically when the fuel line is connected to the tank, the left hand pin under the coupling is what opens the vent), that is why you should place the tank in a stable temperature/environment and unplug the coupling at the tank for longer term storage.....off season. So to put it in a nutshell, since this motor was stored with ethanol, and/or water in the fuel, I am almost certain that your pump is damaged. Thanks, Tim

Tim's Repair Duluth MN, since 1969.
 
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Fine, I stand on my knowledge and experience, ask Mr. Reeves or Racerone........anyone, for that matter. If you have low cylinder compression, the motor will provide less vacuum, thereby a reduced crankshaft pulse and that is what activates the fuel pump diaphragm. I respect you for your expertise and professionalism, but I will not stand corrected on this one.
 
Ok Answer me this my 70 hp has 140 psi compression and my 15 hp runs 90 psi and they use the same pump. How does that pump not blow up? Another question, I have a motor right now an 1989 70 hp with 0 compression on #3 cylinder and the motor still runs. The fuel pump runs off of the #3 cylinder. Blaming Ethanol on a 40 yr old metal tank rusting out is a stretch. And expecting a fuel pump diaphragm to last the life of the motor is another one they go bad. I am way past blaming ethanol for everything that goes wrong with a motor.
 
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PLugg..... If you're handy and mechanical inclined, and if a kit is available for that fuel pump, rebuild it..... If not, install a new fuel pump.

Scott, Tim..... I'm not getting into that squabble... however, I've never encountered a condition whereas a compression failure would have any effect on a fuel pump... BUT... (believe it or not)... I did one time encounter a older 6hp model (forget the year) that had a bad diaphragm in its fuel pump... and this affected the top cylinder's compression to a point where the service manager wanted me to replace the powerhead.

I instead installed a new fuel pump, the compression was restored, and a happy customer was born.

Now, in this case, my case..... If the situation was reversed... would a bad head gasket have affected the fuel pump? We would never know as when encountering poor compression, the first thing we check is the head gasket anyway. Interesting huh?

Now, if what I encountered this one time many years ago holds true with the 9.9hp or 15hp models, that could be found out simply by loosening the fuel pump, then checking the compression.

If the compression drops.... Hmm, sonofagun! :)
 
Thanks Scott and you too Joe. The compression is above the rings, on a 2 stroke pressure and vacuum is created below the rings. If the cause of low compression is the rings, a piston hole or lower cylinder scoring, then the crankcase will not do its work. If the cause is a head gasket, then we have a new ball game. In Mr. Reeves discovery, the vacuum loss caused by the substantial fuel pump problem , robbed the little 6 from developing adequate vacuum to effectively draw the air/fuel mixture past the reeds. The question that Mr. Reeves presents is quite important as it highlights the importance of crankcase integrity. To address Scott, no, I am not blaming ethanol for all todays problems, that is far stretching the concept I'm presenting here. As far as fuel pump diaphragms, I have done uncountable repairs on small engines and the ethanol horror stories are endless. I started guiding in 1967 with a new Evinrude Fisherman and it runs yearly, yes..........it has the original fuel pump. It has never, ever tasted ethanol and as long as I'm around, it never will. To address the 70 hp, the lack of compression has not effected the crankcase pressure/vacuum situation because the damage is upper cylinder or head gasket. You can take the head right off a healthy crankcase pulse activated outboard and the fuel pump will still work. As for pump blowing up, that is easily explained by the fact that crankcase pressure/vacuum is engineered into the operation to keep consistent numbers. The upper cylinder pressures are certainly different.
In effect we are both correct, because the CAUSE of compression loss will determine crankcase pressures. You guys are great. Thanks.
As for the 15 owner, have you gone to school with us?
 
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As far as the 70 hp goes the scoring was a so bad it has to be resleeved. The scoring started underneath the port and went to the top so no it was not a head gasket or upper cylinder damage.
 
It is obviously creating enough impulse to operate the pump. I would bet that both pump pressure and delivery have been compromised, especially if a higher lift is required. OMC has been quite generous over the years with fuel pump performance. Even the early pressure tank system would supply ample fuel with worn out cylinders. But should the crankcase leak pressure on the downstroke, then fuel supply would fall off.
 
For the typical fuel pump it takes 2 to 3 psi to adequately activate the pump. A 1958 pressure system will operate fine on 1.5 to 2 psi as long as the lines and tank are sealing. If you have the spark plugs out you have 0 psi compression. The pump will work. But then drill a hole in the top of the pistons and you put the plugs back in, you still have 0 psi compression. However, the pump will not work because the crankcase will produce 0 psi on the downstroke now. My crankcase pressure lesson is now over. We have to understand that generalizing the compression/crankcase pressure rule has exceptions. You cannot state that compression has NOTHING to do with fuel pump operation and I cannot positively state that it has EVERYTHING to do with fuel pump operation.
 
This is my last post on this. The 70 hp is a VRO with the oil side connected that still works because he drove it back to the dock like that.I did the test with the bad cylinder.
 
Okay thanks for the discussion, you are a top tech and I appreciate your input. I stand partially corrected. Keep up the good work, you are one of the forum's best. Take care, Tim
 
Interesting conversation.
So, Rteague.... did you replace that fuel pump? (BTW there is a $15 difference between the kit and a ready to mount after market one on Ebay... I did rebuild one once, never will again. The $15 are well worth the peace of mind of having a well assembled pump, and save you the time and aggravation from crawling on all 4's looking for the damn spring on the floor....)

I'd be interested to hear the outcome because I have the same situation with a 1974 (9.9) that I just got started after a couple of years of rest, and after cleaning the carb twice (last time was last week and I did a thorough cleaning, removing both welch plugs and making sure every passage was free).
The thing starts right up and revs up awesome, but will sputter (sounding like a backfiring) and die if I try to get it to idle.
 
Better start a new thread, we cannot get Scott to help us on this one. A spit or pop at low idle is a lean fuel/air ratio. This can also be caused by a failed crankshaft seal, otherwise running good at higher rpm's. Is there oil leaking from below the flywheel?
 
timguy said:
Better start a new thread, we cannot get Scott to help us on this one. A spit or pop at low idle is a lean fuel/air ratio. This can also be caused by a failed crankshaft seal, otherwise running good at higher rpm's. Is there oil leaking from below the flywheel?
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Thanks Tim.
I did not see any trace of leak or abnormal oil residue anywhere on this motor, including under the flywheel (that I removed to change points/condensers and set timing)...
Additionally, the compression readings are excellent on both cylinders (around 115)

I agree that it does sound like a fuel/air ratio adjustment issue, and I'm wondering if it could be this invisible gasket, or o'ring I read about on Leeroy's ramblings, that is supposed to seal the pointy end of the idle adjustment screw far inside the carb body.....

Guess I'm gonna have to pull it again. :(
 
Jupiter Dinghy said:
I'm wondering if it could be this invisible gasket, or o'ring I read about on Leeroy's ramblings, that is supposed to seal the pointy end of the idle adjustment screw far inside the carb body.
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Leeroy's ramblings? Invisible gasket?..... Apparently I missed that one!
 
I only meant that that was my last post on pump pulses. There is no o-ring on the low speed mix screw it's a plastic bushing. I have seen where people have rebuilt those carbs and did not remove the old bushing and put the new one on top of it. The 2 bushings can cause a lean condition if that happend. Another thing to check is the stator assembly and make sure it does not have excessive play in it. You have the low tension magneto system which had a lot of quality issues. If you replaced the points did you use factory or aftermarket that ignition is hard on points. Another thing to check is do you have the low mix screw set to lean. On those motors the best setting for the idle to be smooth will not be the best overall setting for running. Start your own thread don't hijack somebody elses.
 
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