Unusual case, motor bogging

[nelsonag]

Hello All.
So I was given a Merc 18xd as a courtesy and it ran great to the best of my knowledge. Since everything was working great, I decided to mess with it to get more hp. I read on some forums that model merc is the same as the 20 and 25xd models with exception of the carb and adapter plate. SO... I did some research, found it was identical, then bought a WMC15a carb (same as 25 model) and put a new adapter plate on it. I put the new carb on and noticed that it will idle fine, will travel under low rmp fine, but when i gun it it will run fine for about 5 seconds then will cut out.

Also, I switched it from tiller to remote throttle, and had to mess with the timing arm and idle arms in the process. The motor runs fine with the 18hp carb after I was trying to diagnose the problem so I dont know if timing would be the case but I am wondering if it is maybe a sticky needle valve? or is there maybe a timing issue? I have read its hard to mess with the timing on a 2 stroke.

I cleaned the WMC carb and rebuilt with new gaskets and still have this issue.

 

[Weldorthemagnificent]

Maybe not getting enough fuel? Is the jet size in the new carb correct for that motor?

Sent from my CLT-L04 using Tapatalk

 

[Shaugh]

Does that motor have a vacuum cutout switch ?

 

[nelsonag]

Yeah so the only differences between the diff hp models is the adapter plate and the carb, so Im assuming the jet is the correct, the parts manual for the motor shows it as compatible.

As far as the vacuum cut off valve I dont believe so, I couldnt find anything labeled as that in the parts diagram.

 

[user 21074]

On my Johnson 20hp to 35hps the timing is only critical at full throttle.Sound like your starving for fuel.Do you need bigger fuel lines or fuel pump different?Check float level?

 

[maintenanceguy]

If it ran 30 seconds or a minute at full throttle, I'd agree that it's starved for fuel. But there's at least 30 seconds of fuel in the carb bowl. If the carb wasn't getting enough fuel, it would run for 30 seconds on what's in the bowl before shutting down. Since it's only running 5 seconds, I think there's something else wrong.

The only thing that was changed was the carb. Something's not right with that. Assuming the carb is new from the factory, it's unlikely that it has the wrong components or that it's bad. I would find the service manual and follow the procedure for doing all of the carb adjustments before I did anything else.

 

[CedarRiverScooter]

Could be something wrong with carb or gaskets.

Not a boat motor, but this year I had heck of a time with a Honda mower carb. it had only about 10 hours since new (brand new mower) & was whistle clean. Would leak like float was stuck up. Had it apart 10 times (warranty out). Finally had to buy new carb now it works.

Moral of story - just cause it's new doesn't mean it's OK.

 

[RaisedByWolves]

If it ran 30 seconds or a minute at full throttle, I'd agree that it's starved for fuel. But there's at least 30 seconds of fuel in the carb bowl. If the carb wasn't getting enough fuel, it would run for 30 seconds on what's in the bowl before shutting down. Since it's only running 5 seconds, I think there's something else wrong.

The only thing that was changed was the carb. Something's not right with that. Assuming the carb is new from the factory, it's unlikely that it has the wrong components or that it's bad. I would find the service manual and follow the procedure for doing all of the carb adjustments before I did anything else.

Starving ie jetting, not fuel delivery from the pump.

Bogging after 5sec is bad jetting most likely, could be float height too.

Op, you can try running it with the choke partially on and see if the situation improves.

If it does, your jetting is off regardless of cross reference saying the components are a match.