1991 25HP Yamaha Outboard runs and then dies

My 25HP MLHP Yamaha is giving me fits. It starts and runs well but suddenly it will just die, as if it runs out of gas. This happens particularly if it is running at a certain RPM and I try to drop the throttle, even the slightest. The fuel pump is pretty new but I am going to try to get a gauge and run a vacuum test. I suspect an air leak or the diaphragm is bad, although it looks good.

I will run at higher RPM's well and dies when at idle or even when dropping down from higher speeds, before even getting to idle.

what are the top two or three items to check/fix/inspect first in this case.

Re: 1991 25HP Yamaha Outboard runs and then dies

fuel hose connections. clean out carbs good. test fuel pump.

Re: 1991 25HP Yamaha Outboard runs and then dies

Sparkie Boat - I gave the carbs a real good cleaning. cleaned the Pilot Jet and got the orifice to open up by cleaning. It runs well but still will just die while idling all of a sudden. Fuel pump looks good, diaphragm looks sound and not stretched out. Verified that the bowl had fuel in it when it was dying so I do not think it is a fuel loss situation, although thats what it seem like when it happens.

Are you familiar with the plunger Diaphragm unit that provides resistance on the arm that controls the timing advance/magneto? I was told that needs to be replaced. Its $100 so I am trying to make sure its bad before replacing.

Should the plunger make contact with the timing arm constantly?

It seems like the timing advance could have something to do with my issue.

My next step is to replace the fuel pump parts, although they look good, as well as possible new fuel hose and bulb and fittings.

Where else should I look to next?

Re: 1991 25HP Yamaha Outboard runs and then dies

I am not sure of the idle timing on your motor, a lot of motors are around 4 degress ATDC, you need to order a manual for your motor that will tell you how to check/set your timing. It should also tell you how to test parts like the plunger diaphram. I have not ever had to mess with the timing on a yamaha. But a simple check of what the timing is should tell you what is up. I do believe the plunger should make contact immediately, maybe you need to adjust your linkage. also when you cleaned your carbs did you clean the idle passages as well as the main jets? these sometimes get over looked.

Re: 1991 25HP Yamaha Outboard runs and then dies

I just had a similar problem on my C40 and she would not idle. I could get her running at higher rpm's but as soon as I attempted to drop throttle, she died. I cleaned out the main jets of each carb and still had the same problem. I cleaned the part of the carb that has the float and did not do the other side of the carbs. I took them back out and cleaned the whole carb on all carbs and now she idles really good. I know this is frustrating and hope you find an answer. I sounds like your motor is acting exactly how mine was though. Good luck!

Re: 1991 25HP Yamaha Outboard runs and then dies

I cleaned the carbs, including all passages as well as possible. I cleaned the jets and I was able to see a difference in the opening in the Pilot jet (looking through it with a white background). Once I re-assembled the carbs and mounted them back on, the motor ran "better" but would still die after a time while in idle.

I dont think the timing is bad because when the motor is running, it runs well, too well to have a timing issue. That being said, the timing advance arm does seem to have an effect.

I am going to replace the fuel lines/tubing inside the motor with some clear motorcycle tubing to try to visually verify a am getting good clean fuel flow through the system. particularly looking for bubbles, air pockets, poor flow, etc. \

Have you ever used clear fuel lines in a motor. I think I understand why exterior fuel lines need to be gray, non see-thru, but I do not see why fuel rated tuning inside the motor would be a problem.

Does anyone see an issue?

Re: 1991 25HP Yamaha Outboard runs and then dies

After cleaning carbs, any trash in the fuel lines and pumps that hit the carbs will plug things up again.
Timing is too simple to check to not do it, just to eliminate that instead of just thinking it is OK.

Did you do a link and sync after reinstalling the carbs? Needs to be done each and every time they are reinstalled.

Keep a very good fire extinguisher handy at all times in case that tubing gives way or pops off. No where to run out on the water to get away from a gas fire on a boat

Re: 1991 25HP Yamaha Outboard runs and then dies

I don't think that is a bad idea. But like 99yam40 said if you cleaned everything out, there may be more debris in the line. After I did my carbs I flushed the lines especially the one from the tank because I had water in my tank. I did check the fuel filter and it had some crap trapped in it and I cleaned that as well.

I don't know that you could rule out timing because I know that on Dodge Vehicles (I was a Dodge Mechanic for 13 years) after installation of a timing belt you could be off a half tooth and the thing would run great at cruise or higer rpms but as soon as she came down to idle, the car would die and the check engine light would come on. I think it is simple to check timing and this is probably a good idea. If you cleaned the carbs, you know that is done so there is not much more other than timing or fuel delivery problem other than carbs. Good luck!