Nissan, (Mercury / Tohatsu) 2.5 - 2 Stroke - Poor WOT

[samuelh1987]

Hello once again, This time I'm having issues out of a 1998 Nissan 2.5hp 2 stroke. I installed a brand new carb and It starts and idles great. It runs wide open for a few seconds and begins hesitating then shuts down like it does when you run the carb dry after use. It will run perfectly up until half throttle then the issue starts. Once you back off it's perfect again. It also starts back up with no issues. I'm going to replace the fuel lines as one tore today when checking the carb for a blockage. Just curious if a faulty reed could cause this issue. It doesn't do it when ran in my test can, only when on my big canoe.

 

[nccatfisher]

It is starving for fuel/sucking air somewhere in the fuel system I would bet. I would check all fuel lines, filters, connectors(if applicable) I am betting they are suspect. If it has a primer check that. Sounds like it is in the fuel delivery system. If it was a reed it would do it in the tank.

 

[samuelh1987]

Thanks for the reply, I actually got looking and someone replaced the gasket under the petcock with a length of hose that was too long and didn't have the petcock going into the tank. Not sure if this could be an issue, but, I cut the hose to proper length and butted the connections like they came. Maybe that will help too. Going to pickup a new cap in the morning and bit of hose to replace the old stuff. I'll pick up a filter also.

 

[muddywaders]

Sam your motor is equipped with a variable venturi carb that utilizes a tapered needle in the main jet.The needle has multiple grooves that hold it in the throttle/slide and can be positioned higher or lower by means of an e-clip to adjust for altitude.I suspect you need to raise the needle(richer).
Tohatsu motors 2.0-3.3hp are the same and the horsepower ratings are achieved with a restrictor plate in the carb which can be easily removed.
I have bought and sold many of these single cylinder two stroke motors as they remain popular for dingies,Common issues are reed valves,fuel petcock leaking and also the impellers need to be fresh to prevent overheating at idle/trolling speed. m.w.

 

[samuelh1987]

Thanks a bunch for the tips. Turns out my carb was already missing the restrictor plate, but, I wasn't aware. How does one adjust the E-clip? Is it part number 7 in the exploded diagram?

 

[Sinkingfast]

Yes...but the needle controls mid range usually though. My bet is there are different main jet sizes with and without the restrictor plate. The slide cutout controls when the main jet starts to feed. I would check part numbers for the mains before anything else.

 

[jethro]

One thing that I found helpful in diagnosing an issue with my 6 horse Merc was to install clear fuel lines. Doing so helped me identify an air leak by seeing air bubbles building up in the tiny fuel line as I ran it.

 

[DaleH]

If that part #37 is called a 'restrictor plate' or such and appears to be limiting the entry into the throttle body, removing it will transform that motor into a 3.5hp OB! At least that is what is done on the Nissan versions. You might want to check some online schematics to confirm.

 

[samuelh1987]

Part #37 is a oring. I believe I got her going this afternoon. The idle adjust screw was backed out about 4 turns. In my reading I found out to start at 2 and adjust accordingly. It ended up needing to be set around 1 3/4. I now have even transition between idle and max. I also replaced the fuel lines, cleaned the carb (mine was missing the restrictor plate), and added a inline fuel filter. Seems to be running good now. My telltale went from being a steady stream to a steady drip while messing with it, so I got a new impaler on order just in case.

 

[muddywaders]

Sinkingfast is correct wot mixture is a function of main jet size,however only two sizes were offered #92 for the 2.5 and 3hp models #94 for the 3.3hp.Needle position was the only adjustment for elevation.The jetting chart I have does not cover 2002 Tohatsus so it is possible other sizes are available.
I think the restrictor plate was used in the 2hp and 2.2hp versions.Hold onto your old carb as many are becoming nla.Care must be taken when cleaning this carb as the emulsifier tube is prone to breaking when being removed(use heat).m.w.

 

[GYPSY400]

Hello once again, This time I'm having issues out of a 1998 Nissan 2.5hp 2 stroke. I installed a brand new carb and It starts and idles great. It runs wide open for a few seconds and begins hesitating then shuts down like it does when you run the carb dry after use. It will run perfectly up until half throttle then the issue starts. Once you back off it's perfect again. It also starts back up with no issues. I'm going to replace the fuel lines as one tore today when checking the carb for a blockage. Just curious if a faulty reed could cause this issue. It doesn't do it when ran in my test can, only when on my big canoe.

If you can verify that all the fuel lines are Good, my next step would be the float level.. it may have been "adjusted" during shipping..
You have a fuel delivery issue no doubt as the motor needs more than the fuel system can supply at WOT. And considering you said the motor will run briefly at WOT and then die tells me that the float bowl is filling up, but draining faster then it can be refilled.

Sent from my SM-G903W using Tapatalk

 

[samuelh1987]

Fairly certain it's lined out. I replaced the old lines, added a new fuel filter, cleaned the carb and adjusted the float level to make it level with the bottom of the carb when upside down. I used a tach and set the idle to 1100 rpms as the manual suggested. If I have any more issues I'll definitely be back lol.

 

[GYPSY400]

Fairly certain it's lined out. I replaced the old lines, added a new fuel filter, cleaned the carb and adjusted the float level to make it level with the bottom of the carb when upside down. I used a tach and set the idle to 1100 rpms as the manual suggested. If I have any more issues I'll definitely be back lol.

So was the float level out of adjustment?

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[samuelh1987]

It was sitting about .25" too high when the carb was inverted.

 

[samuelh1987]

https://youtu.be/i254IY5ayMA