Yet another Yamaha 2.5 outboard question

[rab356]

I have a yamaha 2.5 MSH, 4 stroke. I have read posts with similar issues however, I don't see any actual resolution on any of the threads.
The issue:
Runs but spits and sputters when throttle applied and RPM won't increase.

I have checked the carb, taken it apart, replace gaskets etc.....replaced the spark plug....

Odd things, if I close the fuel cutoff switch, it continues to run poorly until it begins to run out of gas, then it appears to run fine a few seconds before running out of fuel, running rich I think.
Also, there are large carbon deposits on the plug.
Sometimes I can get it to run ok with the choke on or partially on, but that doesn't last long and doesn't seem right.

Do I need to adjust the float? There is a fuel air screw protected by a brass plug with a small hole in it, but I can't figure out how to get the plug off to adjust the screw, I downloaded a service manual and there wasn't any acknowledgement that it even existed.

Also, the gap on the plug says it should be between .6 and .7, but that seems quite a big gap when you look at it.

I am new to these kinds of engines and not sure if I am working on something that I shouldn't or if I should be looking somewhere else.

 

[alahol2]

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[VicS]

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You have replaced the plug so thats one possibility dealt with. 0.6 to 0.7 mm is a perfectly normal plug gap and correct for this engine.
I am worried that you think it looks a big gap. Are you sure you have set it correctly?

You have the correct plug type I take it?

Check the HT lead and the condition of the coil unit.

Check that you are getting a good spark, A good fat blue one that will jump a good few millimetres.

If all aspects of the ignition appear good i would look again at the carb and the fuel flow to it

Some of the things you mention seem to suggest a weak mixture others a rich mixture.

Soft tipped float valve I believe so check that the little clip between the needle and the float arm is in place

 

[Habebty]

I have a Yam 2.5, and had the same problem. Don't touch the needle, but make sure you have fresh fuel with no water in the fuel system. Then adjust the idle screw correctly. I had been using old petrol and filled up while there was water in the ribbed plastic can spout. Also, take off the fuel tank and rinse out (with a little petrol) all the crud that will be in there.

 

[Fillerymike]

My outboard behaved in a similar fashion, running on choke and half choke but lacking power. It had been not used in the previous season when the carburettor probably dried out. I cured the problem by stripping the carb and cleaning out the main jet, using a fine cooper wire. In future when the outboard is not going to be used for a while I will drain the carb.

 

[rab356]

Ok, you should just ban me from the board now.....stupid mistakes so far.

I was right, the gap was incorrect, the gap tool I have, has both inches and mm, of course I was using the wrong side, so I adjusted the gap correctly, and no change in the issue, it does run at idle very nicely though, but as soon as I increase the throttle, still no luck.

Took apart the carb again, blew it all out, looked for any crud, made sure it looked ok. Managed to get the brass plug out of the Air/Fuel mixture screw, again, neither of these affected the performance. I can screw the air/fuel mixture screw all the way in, and all the way out, nothing, still runs at Idle, no impact and still unable to increase the throttle.

Habebty, funny you should say check the fuel, I am going to try one more thing today, I am going to get a gallon of premium fuel and add the octane boost, the fuel I am using now is just the regular and has the sta-bil in it to get rid of the ethanol, I am hoping that this will be the issue as I am not sure where to go from here.

Very frustrating....

 

[rab356]

Ok, update to this post. I replaced the fuel and oddly enough, didn't make much of a difference if any, but I think I have a better Idea. When the fuel didn't work, I continued to take things apart and ended up taking off the top where the thermostat and anode is. This area had a bunch of crud in it and the thermostat doesn't look in good condition, I have ordered a new one of each. Once I cleaned it out a little bit it seems to run a little better, I can put some throttle to it and get a little higher RPM, not much, but more than before and the air/fuel screw seems to have a little bit of influence now. In addition to the thermostat, I also ordered a new Silencer Assembly Intake thinking that if this is plugged up or not functioning correctly that it might be creating an issue with the vacuum and air flow. I also ordered the water repair kit, just in case I need to replace the impeller and other parts to ensure consistent water flow in the event the cooling system is getting backed up and causing undo pressure. Since I am new to this used engine, I am wondering about the cooling, when running I get a good flow out of the water hole, water comes out and it is cold. When I shut down the engine, about 5 to 10 seconds later I think the thermostat releases as there is some additional water that exits the water hole and it is very hot. Can anyone tell me if this is normal?

Also, I am going to do these things one at a time until the issue is resolved so that I can post what actually resolved the issue for future reference.

 

[VicS]

Ok, update to this post. I replaced the fuel and oddly enough, didn't make much of a difference if any, but I think I have a better Idea. When the fuel didn't work, I continued to take things apart and ended up taking off the top where the thermostat and anode is. This area had a bunch of crud in it and the thermostat doesn't look in good condition, I have ordered a new one of each. Once I cleaned it out a little bit it seems to run a little better, I can put some throttle to it and get a little higher RPM, not much, but more than before and the air/fuel screw seems to have a little bit of influence now. In addition to the thermostat, I also ordered a new Silencer Assembly Intake thinking that if this is plugged up or not functioning correctly that it might be creating an issue with the vacuum and air flow. I also ordered the water repair kit, just in case I need to replace the impeller and other parts to ensure consistent water flow in the event the cooling system is getting backed up and causing undo pressure. Since I am new to this used engine, I am wondering about the cooling, when running I get a good flow out of the water hole, water comes out and it is cold. When I shut down the engine, about 5 to 10 seconds later I think the thermostat releases as there is some additional water that exits the water hole and it is very hot. Can anyone tell me if this is normal?

Also, I am going to do these things one at a time until the issue is resolved so that I can post what actually resolved the issue for future reference.

Id expect the water from the telltale to be cool perhaps becoming warm when the engine is running properly under load but I don't know the details of the cooling water circuit. The important thing is that water must be discharged from the telltale all the time it is running. This confirms that the water pump is functioning.

You say the thermostat did not look in good condition. It should obviously be closed when cold but if warmed up slowly in a pan of water should just begin to open at the specified temperature which may be marked on it. It should of course go on to open fully as the temperature is increased. It's possible that the fully open temperature is also marked on it.

Problems with the thermostat might lead to overheating but I dont think you will find have any bearing on your non running problems.

If the air intake silencer is clogged it is most likely because the engine has been laid down or carried incorrectly and engine oil has found its way in there. Is there a filter or gauze in there? Nothing is shown on the parts diagram.


Was wondering about valves but if it starts and idles Ok not likely to be a problem there.


My money is still on a carb problem

 

[ianat182]

Went for a trip in a friend's launch with his 2.5 Yamaha with much the same symptoms of no revs from the throttle. In our case it was the throttle cable and its linkage that was the problem and when a replacement cable fitted got it sorted.
In retrospect the real cause were the bunches of algae and weed we had collected on our journey from Hamble to Ashlett ,the water intake became blocked with weed and overheated the engine. Too much twisting of the handle to get it to rev above idle probably was the likely cause of the throttle linkage cable breaking/stripping.
20 mins in the Yamaha dealer shop got it sorted I understand.

ianat182

 

[rab356]

No, the intake is a completely sealed unit so I don't know if anything is in there or not. Going back to my other statement, since I have not ever heard one that was running properly and haven't owned it since new, I don't know what sounds it should be making and to me it sounds like there is a vacuum leak or something similar as you can hear that in the top of the engine around the carb. I made sure everything was in the correct place as far as gaskets and fully seated.

What about timing? I noticed on the Service Manual that you can set the timing, could this be the issue? Seems really strange to me that it runs and idles fine but as soon as you put any kind of throttle to it, it just lugs, won't pick up RPM and puts out some pretty good smoke. It also continues to put a good deal of black carbon deposits on the plug as well. I have changed this between .6mm and .7mm without any noticeable improvements.

 

[steve66]

If it runs better after the fuel has been turned off its an overfuelling issue.Take off the bowl and remove the float . Give the needle valve and needle valve seat a good clean out. Assemble it all back together and blow through the fuel union. It should blow freely. Then turn the carb upside down and blow again. It should now be sealed. If you can still blow freely you have a problem with your needle valve or seat.

 

[rab356]

Steve, thanks for the tip, I will try that, I have already replaced the valve and float and have blown it out a number of times already but I will try the above to see what happens.

 

[steve66]

Sorry if im teaching my granny to suck eggs but is the inlet to carb gasket good. Does it make any difference when its running if you spray wd40 or any other aerosol in the vicinity of the inlet to carb gasket whilst its running?

 

[rab356]

No worries, I stated up front that while I am mechanically inclined, I am very new to outboards and have not the first clue about what I am actually doing. It also doesn't help that to this point, I have never actually had a running motor , so I have nothing to compare it to.

So, there are a number of gaskets to the carb, are you asking about the one on the front that is between the carb and the Intake assembly? Or are you asking about the three others that are between the carb and the engine housing?

I replaced the ones between the carb and the engine housing, those were the first parts I replaced in addition to the Vacuum hose that goes from the top of the intake assembly to the top of the engine since it had a split in it and wasn't attached. I did order a replacement gasket for the one between the intake assembly and the carb however, I don't think it was the right one, seemed to be to small and when I tried to put it on it broke, so I put the old one back on which seemed to be in good condition.

With that said, I have not attempted spraying anything in the vicinity of the Carb, I will also try that. What should my expectations be with this? I am assuming that if there is a leak or incorrectly seated gasket it should almost kill the engine when running?

Again, thanks for the help, I am learning and all the information that I can get helps me understand how these things work.

 

[steve66]

Yes its the carb to intake you need to worry about , outside of the carb are not too critical. It depends on what your spraying a leak will either show up by increasing the revs or decreasing/ stalling the engine. I know from working with 2 Honda 4 strokes they respond massively to a new gasket and a s a rule i never ever use used gaskets any more. I have had thousands laser cut onto gasket paper and its saved me hours of messing

 

[rab356]

Ok, as promised when I got this boat anchor running, I would post the actual fix, I am happy to say, it is fixed finally! There was a missing part in the carb which I know I didn't take out , on the diagram, it is number 6, 69M-14966-00-00 BUSH. I am certain that when I took apart the carb it wasn't there. It is a black plastic plug that is under the pilot jet. Once we put that in, adjusted the float, it fired up and runs like a champ, and now the air fuel mixture screw actually works. I am certain I didn't take this out, I would have remembered since I would have had to figure out how to get it out to get to the pilot jet to take it out and clean it.