# 25 hp Yamaha Outboard 4-stroke



## jmacrv8

This engine has about 200 hours on it. It's now stranded me a couple of times. It starts and runs great for awhile, then will only run at idle speed. If you try to throttle up, it dies. Pumping the bulb has no effect. The first time I'd been running about 45 minutes and it just quit. After cooling off for a few hours, it started up and I trailered the boat with no problem. On the next trip I got about 20 minutes out of it before it happened. Since I played the idiot and didn't use a fuel stablilizer over the past few winters, I removed the carb and cleaned with carb cleaner and compressed air. Didn't replace any parts. The fuel filter was also replaced. After this I ran it in the driveway with muffs at high idle for about 30 minutes. Then we headed to the lake and ran about 1.5 hours off and on with no problems. Yesterday we got about 45 minutes out and slowed to make a turn when it conked. We were able to idle back to a launch, but we were a few miles away. I'm generally hesitant about taking it to a dealer, as my experience is they don't deal well with intermittent problems, and I usually get charged for a lot of shop time (I'm speaking generally, although when I had a Hobbs meter installed at the dealer where I purchased it, they left the key on overnight, so I don't get a warm and fuzzy about them). Any advice on troubleshooting this problem will be much appreciated. Any common problems with these? I thought when I bought it that based on the Yamaha reputation I'd just change the oil and run it forever.

Thanks in advance.


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## octaneman

Hi jmacrv8


Outboard motors are very different from your average car engine, the diagnostic procedures are also different. To figure it out you have to determine if it's an electrical problem or a fuel problem. If its a fuel problem you will have to start the engine ( on the water) and disconnect the spark plug wire to see if the cylinder is firing. By moving the spark plug wire using the in/out method ( without connecting it to the plug) you will see the engine either stabilize or remain the same. If there is spark and no change in the engine then its a fuel problem or it has no compression. Electrical problems are allot harder to narrow down without the proper equipment. The ignition system on outboards consist of stators, triggers, and power packs.The most important thing on outboards is to check the timing advance, if the timing is off even slightly it cause a a myriad of problems, it could not start, cut out, run good to some RPM and then die out. To do more in depth testing you will require a DVOM and the manual for your engine to check specific tolerances of voltages,and resistances in the ignition system. Electrical components are very expensive on outboards, so before you decide to scrap a part be absolutely sure that it is the component at fault.


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## DCBD

Hi jmacrv8

I know your post is a few months old now but I'm wondering if you were able to solve your problem. I also have a 25 HP Yamaha outboard with I think the same problems. Mine is year 2000 and came to me second hand. At fisrt it would not start unless I held my hand over the air intake to choke it, then after a long warm-up it would idle. Once I put it in gear it would run OK until I started to increase the throttle, then it would begin to backfire and cough. I talked to a dealer about it and he told me it was probably the carb plugged up due to no use during the previous 3 years before I bought it. So rather than deal with the shop, I went straight to the parts department and ordered a new carb. Since instaling the new carb, it starts and idle just fine, but I cannot get full throttle for some reason. So mine is still a work in progress. I plan on rebuilding the old carb to keep as a spare if I can get the new one working properly. If not, I'm going sell it and get a Honda.


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## bozodog

Didn't the new carb caome with tuning instructions? What about the rest of the fuel system? Dirty tank, fuel filter...etc?


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## DCBD

The new carb didn't come with any tuning intructions. Yesterday I took the old one apart and I can't see any way to adjust or tune anything. The manual hints to everthing being factory set, but is not clear about this point. What I did find is the bottom of the bowl has a light coating of rust like material. When I changed the fuel filter I didn't see anything like this in the filter or the fuel. I also changed the fuel tank (plastic) but it seemed clean as well. It could be some type of bacterial growth such as you find in diesel fuel, but I've never heard of that happening in gasoline before. Everything I've checked on the engine is white metal or SS, so it either formed in the fuel pump (which I have not removed or pulled apart) or it got in through the fuel tank and was possibly cleaned before I bought it. I found some other forums yesterday, and there is a lot of talk about Yamaha carb problems. So it seems this is maybe a weak point with these engines.


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## Snoopdogie187

Depending on what they bowl is made of, it might have been water/corrosion found at the bottom of it. Just a thought anyway. The water wouldn't get pulled into the engine so easily and if it is just a little bit, it would collect in the bowl.


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## SABL

Is lack of full throttle due to fuel supply or linkage adjustment??


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## DCBD

The throttle adjustment seems to be OK. It fully opens the butterfly valve and mechanism that controls the piston which seems to control the fuel delivery also seems too fully actuate. I’m also convinced that the fuel delivery to the carburetor is sufficient. I suspect something in the carburetor needs to be adjusted but so far am not able to find any kind of adjustment short of changing parts. I think the problem is related to the piston but still investigating.


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## projectgator

I just joined...but I've got both Honda and Yamaha 4-strokes..... I wonder if you don't have one or the other problem. First, is the engine overheating? Do you have a good stream of water exiting from the engine? If not...the engine has "protection" to keep from damaging the engine. This could be a faulty impeller. Secondly, the engine brain may be confused and again goes to the self-protect to minimize engine damage. That would require a technician to determine this. Good luck.


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## DCBD

Actually I’ve solved the problem, it turned out to be a simple float adjustment. Under full throttle the bowl was being emptied faster than it could be replenished, so I adjusted the float to allow the needle valve to open a little bit more. Thanks guys for the imput.


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