SUCCESS !!! need outboard repair thoughts from the brain trust.

Scott Farris

Scott Farris

Well-known member
Hi guys, My Nissan 25 HP 2-stroke is giving me head aches. It is starting and idling great, but as soon as I start to run it up under load the boat will plane out then shortly after the motor stalls out. If I carefully feather in the choke I can get it to run at a higher load w/o stalling but only enough to get the boat up to 4-5 mph. This is the outboard I converted to a Jet pump.

To date I've tried two different tanks, new gas, two different hose assemblies, fuel filter has been cleaned, plugs inspected and gaps checked. I ran 2 gal of the new gas through her at the slow speed. Still didn't run well so I replaced all three small internal fuel hoses - connector to filter, filter to pump, & pump to carb, all with real hose clamps. I've taken the carb off, drained it, removed the needle, and sprayed everything down with carb cleaner making sure everything was free and all spring assemblies appear to be working correctly. Reassembled it. Took her back out and nothing changed operationally. Frustration reigns!

Any thoughts or suggestions? (please keep them physically possible and comfortable ;^)

Scott
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Have you tried running under load while continuously pumping the primer bulb on the gas line? Doing so could make up for a weak fuel pump.
 
I'm interested in what you find out is causing this.

Looks like you've tried different bulbs?

Have you checked the fuel pump for a pinhole in the diaphragm?
 
Carbs could be gummed up. I had a Suzuki DT25 that did the same thing. Had all 3 carbs clean and adjusted then it ran great
 
Gordon, yes to the constant pumping of the bulb. I've done this numerous times even thought the bulb stays fairly firm the whole time on its own.

I guess it's into the fuel pump next. Searching back I see I last rebuilt the pump in 2009. Thanks Pete & Carl. I will definitely follow up with results. Photos from last rebuild in 09.
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View attachment CarbInternals.JPG
Thanks all !!
 
Carl said:
Sounds like fuel pump
Click to expand...

I agree. If you haven't rebuilt the fuel pump I would start there.

Have you checked the reed valves?

Have you done a leak down test on each crankcase? (not a compression check on the cylinders) Poor crankcase pressure will cause the symptoms you describe. Poor crankcase pressure will also affect the performance of the fuel pump.
 
Dave, Reed Valves, I hadn't thought of them but sounds like they could be an issue. Trying to find something on them on You Tube I did come across a video showing a Nissan 25 doing almost the exact same thing as mine but his is actually running better than mine as I have to carefully feather the choke to get any real high speed out of her but the stall is very similar.

Thanks,
Scott
 
If its any consolation, I'm in the same boat (pun intended).
Similar symptoms with my 25 Merc, replaced the fuel line & cleaned out the filter but I havent had time to clean the carb yet but I'm betting I need to order a fuel pump kit.
 
Guys, also I mentioned crankcase pressure but failed to point out the culprit, worn crankshaft seals. Typically only shows up in motors with a lot of hours on them. The seals hold enough pressure that an engine will start and idle just fine. Sometimes will even run wide open seemingly fine (under light load, like in a test tank.

The seals can leak under vacuum, when the crankcase is drawing fuel/air mixture thru the carburetor and under pressure, when the crankcase is pushing that fresh fuel to the cylinder. This leakage can be confined to the internal seals between between the crankcase chambers. (each cylinder has it own chamber). In the case of an internal seal leak, the fuel air mixture will leak from the side building pressure to the side creating vacuum, which means all of the mixture is not reaching the combustion chamber. This internal leakage, if present, is hard to detect and often overlooked. (it is also not real common except in engines with a lot of hours)

On the other hand, leaking reed valves will some times revel themselves by having fuel spitting back out the throat of the carburetor.

Hope it is just a fuel pump for each of you, simple and relatively painless.
 
Scott,
I don't think it's your fuel pump because you said you tried running it while continuously pumping the in-line primer bulb. That bulb literally is a manually operated fuel pump. It's whole purpose is to pump gas to the carburetor when the regular fuel pump is not working (meaning when the motor is not running). When you squeeze the bulb, it simply forces gas through and past the fuel pump and then on to the carb. I suggested pumping the bulb continuously as a way of eliminating the fuel pump as a suspected issue. You could remove the fuel pump completely and the motor would still run if you continuously pumped the gas line bulb.
 
A quick related question then:
On my 25 Merc, when I tested this last time out, the bulb was firm and trying to further squeeze the bulb made no difference in my motor bogging down when trying to throttle up.
Should I assume a clogged high speed jet or other carb problem?
 
Carl said:
A quick related question then:
On my 25 Merc, when I tested this last time out, the bulb was firm and trying to further squeeze the bulb made no difference in my motor bogging down when trying to throttle up.
Should I assume a clogged high speed jet or other carb problem?
Click to expand...

When mine does that it is one of the jets clogged in the carb. I believe I have three jets per carb and my F40 has three carbs and they are apparently more complicated than most to meet fuel efficiency standards at the time. To have them rebuilt is really costly, but I've only had to have it done twice in 20 years. Only work I've paid to have done on the motor.
 
Again, probably not a fuel pump issue.

Under normal circumstances, pumping the fuel line bulb pushes fuel past the fuel pump into the float bowl on the carburetor. When the bowl is full, the float rises and the float needle cuts off the intake of more fuel, preventing the bowl from overflowing. Because the float valve is blocking the passage of any more fuel, the bulb becomes firm.

Once the motor is running, the regular fuel pump takes over and draws fuel from the tank, through the bulb, and pushes it into the carburetor float bowl, but only at the rate at which the float needle valve allows it to enter the bowl. Your bulb staying firm implies that the fuel pump is keeping the float bowl full. If it weren't, the bulb would become soft because as the fuel level in the bowl dropped, the float valve would open, allowing you to pump more gas into the bowl.

Of course, another, more common reason for a soft or collapsed bulb is a failure to open the vent on a vented tank or an obstructed fuel line on the intake side of the bulb. This has happened to me more than once when something heavy, such as an anchor or the fuel tank itself, was sitting on top of the fuel line, pinching off the flow of fuel. That has nothing to do with your problem, but I just thought I'd throw it out there.
 
Scott,
Here's one more long-shot troubleshooting trick you can try. (the nice thing about troubleshooting tricks is they don't cost money)

Try pulling the spark plug wires off one at a time. In other words, try running the motor on just the top cylinder, then just the bottom cylinder.

Here's why I'm suggesting this:

I have a garden tractor that started well and sounded great, but had no power, struggled to cut the grass, and sometimes stalled unless I stopped completely and let it "catch its breath". And, I had to use increasing amounts of choke to keep it running. In other words, the symptoms were rather similar to yours.

I tried all the usual fuel and carburetor-related stuff, and it didn't make any difference.

Then, one day, just out of desperation, I pulled a spark wire off while it was running. It died instantly.

I put it back on and re-started the motor. When I pulled off the other spark wire, the motor kept running exactly as it had been. No change whatsoever.

I'm a little embarrassed to admit this, but I had bought this tractor used, and had been using it for three or four years running on only one cylinder. It just started so well, and sounded so good, that it never even occurred to me that it was only running on one cylinder. Now, having replaced the bad ignition coil, the thing is a beast, cutting through even tall thick grass at full speed ahead, and uses way less gas than it used to.
 
Scott I saw you disassembled the carb and sprayed it down but did you remove the jet? Specifically remove it and blow out whatever could be in it then look through it to see that it was clear ? Also Dave is spot on with seals causing a lean condition in 2 stroke motors. If it starts and idles fine but won run high revs under load and more fuel helps this seems to be a lean condition. You can also pull the plugs and take a reading on each one seeing if they are over or under fuel the should be brown.
 
She's back running great! I really thought Dave's reed valve suggestion was going to be it, but they were in great shape. With the carb in hand again, I dissembled it, and verified all the ports were clean and open. Even scrubbed a bit of smut z off the bottom of the bowl, but that was mostly cosmetic. When I took the fuel filter off it's mount I noted some gas/oil? on the mount, so I replaced the whole 20+ year old filter assembly with a horizontally mounted inline see-through filter. This ended up being very educational and key to the whole repair. I elected to leave the fuel pump alone since the motor definitely spooled up before dying and I didn't have a replacement gasket or diaphragm. The photos below are for those like me who are unfamiliar with reed valves:

This photo was taken with the carb and air/fuel intake cover removed, leaving the reed valves in place from the intake side.

View attachment ReedValveMTD.JPG


Below is a close up of the reed valve assembly from the backside - The thick curved u-shaped pieces are the reed stops while the thin Mylar flat u-shaped flappers underneath are the reeds in their relaxed closed position.

View attachment ReedsCloseUp.JPG


And since the inline filter plays a role, I'm including a photo to make sure we're all on the same page.


View attachment InlineFilter.JPG

Once it was all buttoned up, I wiggled a garbage can up underneath and fired her up with the cover off. Now unlike prop motors, the jet pump doesn't have a true "neutral" it has a scoop that pivots up to direct half of the output forward, to balance the remaining half blowing to the rear. So the old bucket test is not a dry activity, even with a bit of a cover over the can. I quickly noted that even pumping the fuel line bulb, the horizontal filter never fully filled up with gas, leaving a bubble above the outlet. When I'd rev the motor, the bubble would grow. While my tank test never drew the gas level below the outlet level, it was close. I'm sure under full load it would have, interrupting the fuel flow like I was experiencing.

I also took the opportunity to pull the spark plug wires. The first pull was shocking and I decided doing this with the motor running wasn't the best idea. I was wearing a neoprene glove but I'm sure the water spraying around helped with the conduction. The engine did take a load/rpm hit with both (one at a time) wires, but the motor continued to run indicating both cylinders were getting spark and firing. Guess I need to see under the hood after dark some early morning to see if any arcing is taking pace indication to me that the wires may be deteriorating.

At this point it was shut down and back to work - fuel filter overhaul and fuel hose maintenance. Since I was very uncomfortable with the air bubble in the horizontal filter, I overhauled the original assembly, making sure the bowl was properly seated and hopefully air tight. This mounted filter has the outlet at the top making sure that no air bubble can develop in the filter and airlock the system. The air can and apparently has been drawn though and into the carb fast enough to stall the motor. The two fuel hoses were disassembled, ends trimmed and bulbs and end fittings reinstalled into the "new" condition hose ends.

With everything back together, it was off to the small local impoundment. Back to the ramp, will the fourth try be the charm? If not, I was probably going to cry and bring her to a pro (god forbid). Luckily she fired up and ran great! 3 plus miles up and another 3 back. What a flipping little shallow draft rocket. (I know it's not as fast as a Schellinger)

So I really want to thank you guys for the thoughts, discussion and for keeping my morale and motivation up as I worked though this.

Scott

ps: Carl, hope your motor issues are a simple to correct. Good luck!
 
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