Nissan Carb issue?

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
I have a 70 hp 1985 70 hp Nissan outboard. Took out this weekend to see how she ran and it did fine until full throttle. After about 15-30 seconds full throttle it falls on its face/bogs down and will die unless I back off throttle and give it a few seconds. Seems like possibly carb isn't keeping up with fuel demand for some reason. Recently took carbs apart and cleaned them, as there was some old residue clogging the lower carb. Floats appeared to be in good order. No carb rebuild kits available for this motor but some of the carb parts are available individually. Not even sure it is carbs but that is just my guess. Any thoughts on this one guys?
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
Try squeezing the primer bulb when it acts up, sounds like an air leak or failing fuel pump. It could be a plugged filter of fuel line too.
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
Thanks for the response. New fuel pump, new filter, new fuel lines have been installed already. Runs great at 3/4 throttle. Bogs down at WOT
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
New doesn't mean there's no air leak, and frequently when a carb has been dirty for a while, even after cleaning it there's still gunk in it that plugs things up quickly again.

​Like you said, there's a fuel flow issue at high RPMs, when you back it off the float bowl fills up again and it runs OK. Squeezing the primer bulb will let you know if it's an air leak or fuel pump issue.
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
I will sure give it the squeeze test when I get it back together. Good idea. Will let you know what I figure out. Many thanks
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If bulb press test provides no solution, could be a head gasket issue, completely dried out, water passes inside combustion chamber at full throttle and OB misses, bogs down, will assure you that head has never been retorqued to speces after break in period finished. Was OB used in salt or fresh water ?

Other issue could be an electrical related problem. Spark plugs or electrical component heating up when on max demand at full throttle, misses, bogs down, cools down al less throttle to miss again when at full throttle again.

You have a 31 year oldie, expect problems, parts are not that eternal. If it's not a fuel related issue, personally would start going for a new head gasket, take advantage to decarbon head and pistons heads as well, clean all water paths.

Happy Boating
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
Many thanks for the replies. I will repost after checking the suggested culprits
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
Ok I was able to take the boat out last night and determined that if I squeezed the bulb when the engine started to fall down at full throttle, that it would supply the needed fuel to the carbs temporarily until it started to starve again. So the squeeze test was in fact successful. So I will be looking for air leaks in the fuel lines. Maybe putting some better clamps on the fuel lines perhaps? So should I be checking for leaks specifically "before" the fuel pump? (the fuel pump is brand new).

I have fuel line from primer bulb to filter, another from the filter to the pump, and from the pump to the carbs. The guy that worked on the motor before, replaced the fuel lines but I want to make sure he used the right size replacements. Is the fuel line to the carbs supposed to be smaller than rest of the fuel lines? I am not sure but the schematic seems to show they are the same size diameter, but I am not sure I am reading it right. The fuel line from the fuel pump to the carbs connects to a "T" and then splits to separate lines to each of the two carbs. Then it is a smaller hose to the two carbs from the "T". I wish I could post a pic but dropped my phone in Davy Jones Locker (the ocean) last night in about 13 feet of water when I took the boat for a test run, and so I am phone-less. So lines to both carbs are smaller lines than the rest of the lines. I don't know if he just replaced them with the same size that was on there or he reduced the size. At any rate I was wondering if the fuel starvation at high throttle could possibly be the restriction of these smaller lines.
Also is there any way to test the pump for fuel volume? BTW... I love this forum and all the phenomenal help offered!
 
Last edited:

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
So to continue my thoughts, if I am able to get more fuel to the starved carbs by pumping the primer bulb does that mean the fuel pump is not getting enough fuel supply to it? Or does me squeezing the bulb fill all lines with more pressure to the "out" side of the fuel pump line as well? Since I am not sure how the marine fuel supply system works, I am a little ignorant of how to pin point exactly where the problem might be and where to concentrate my efforts. Before the pump or after? Or possibly smaller lines to carbs restricting flow?
 

ondarvr

Supreme Mariner
Joined
Apr 6, 2005
Messages
11,527
Squeezing the bulb will pressurize the entire system all the way to the carbs, even if you had no fuel pump it would run correctly as long you kept squeezing when needed.

​The air leak would be on the suction side of the pump, or in the pump itself (that's if it's an air leak).

​The fuel pump may not be working correctly, hard to tell at this point though.

Try putting a clear fuel line between the pump and carb, look for air bubbles.

 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
My bulb and line from tank is pretty old but I do have a newer one I can put on the tank and try. Good idea. I think I will try replacing the clamps too as some of them looked sprung/stretched out and not really apply much pressure to the hose fitting. And I will pay special attention to the suction side of pump and see if those things yield some results. will post what I find.
Anybody have a Tohatsu outboard that can look at their fuel lines and see if the carb has smaller hoses than the rest of the system?
Many thanks
 

pvanv

Admiral
Joined
Apr 20, 2008
Messages
6,500
All fuel lines are 5mm ID, 10mm OD. Odds are you have a leak before the pump. Could be in the tank pipe, at the tank connector, at the motor connector, or between the motor connector and the pump. The pump cover itself could also be leaking soemwhat. If you place the tank above the level of the motor, that should reduce the vacuum in the inlet fuel line. O-rings for the primer hose do go bad with time, and are not available separately from the manufacturer, but you may be able to match them up at a decent hardware store.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Testing with new fuel line should get rid of that bogging situation if happens to be a fuel related issue. Both female fuel line connectors can go bad with time, becomes dried, looses external diam, does not seals tight against male correctors, surrounding air is let in fuel line and OB misses, worse at full throttle..

Happy Boating
 
Last edited:

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Oops, should be..

Both female fuel line connectors o'rings can go bad with time, becomes dried, or lose internal diam, does not seals tight on top male connectors for a perfect snug fit...

Happy Boating
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
All fuel lines are 5mm ID, 10mm OD. Odds are you have a leak before the pump. Could be in the tank pipe, at the tank connector, at the motor connector, or between the motor connector and the pump. The pump cover itself could also be leaking soemwhat. If you place the tank above the level of the motor, that should reduce the vacuum in the inlet fuel line. O-rings for the primer hose do go bad with time, and are not available separately from the manufacturer, but you may be able to match them up at a decent hardware store.

Thanks for that Paul. I could see in the parts book 5-10-? but wasn't sure if that was ID and OD and length or what. There was no mm designation but I wondered if they were mm designations. I did check the screws on the new pump and the screws were not really as snug as they could be so perhaps I was pulling some air at the pump.
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
Oops, should be..

Both female fuel line connectors o'rings can go bad with time, becomes dried, or lose internal diam, does not seals tight on top male connectors for a perfect snug fit...

Happy Boating

I am going to try and track down new O'ring for the female fuel connector, nothing in my O'ring supply would fit, all were too thick. I also noticed the lubrication Cap was loose on both cylinders, so I snugged them back down. I did replace fuel lines and put new clamps on them. Unfortunately I overtightened the fuel line clamp to the fuel pump and broke off the hose nipple. I endeavored to repair ir with some JB weld. Hopefully it will hold and I won't have to discard my new fuel pump. So we will see if all this produces some results as soon as I can get her down to the bay and test her again.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Can get fuel o'rings at auto parts stores, remove current o'rings with a thin needle to have both measured to their exact size. Seems that for 70 HP OB's there's no available fuel pump body spare part nor fuel connector o'rings for any other HP fuel line as well. Ain't that a real double crime..

Happy Boating
 

mikkydee

Seaman Apprentice
Joined
Feb 20, 2015
Messages
39
Went to Napa auto parts today trying to track down the fuel connector O'ring. Big zilcho! Any O'ring close to proper size was too thick :(
 
Top