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Results 1 to 8 of 8
  1. #1

    Default Yamaha 5BS 661 problem

    Hi, I need help!
    I have a Yamaha 5BS 661 5hp 2 stroke outboard which appears to show all the signs of a broken reed valve. I have stripped it down though and can't find the valve! Am I going mad?I have removed the carb from the manifold - where is the valve and what should it look like?

  2. #2
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,627

    Default Re: Yamaha 5BS 661 problem

    If you are looking for a reed valve , it is a thin strip of metal that open inward due to crankcase suction.---It should be right behind the carburetor.----Reed valves are very reliable!---What are the " signs " you are seeing on this motor ?
    Last edited by racerone; 01-28-2013 at 09:13 AM.

  3. #3

    Default Re: Yamaha 5BS 661 problem

    Quote Originally Posted by racerone View Post
    If you are looking for a reed valve , it is a thin strip of metal that open inward due to crankcase suction.---It should be right behind the carburetor.----Reed valves are very reliable!---What are the " signs " you are seeing on this motor ?
    Hi, thanks. Well I certainly can't see anything between the two butterfly's and the piston! It was a non runner so I took it to get serviced and was told that it will run but it throws out all the exhaust through the carb so it won't tick over. They told me it was the reed valve. Is it possible there is no reed valve on my model or is that just impossible?

  4. #4
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,627

    Default Re: Yamaha 5BS 661 problem

    Take the time to look on U-tube on how 2 strokes operate.--------When you crank it over with the sparkplug out you should hear a " pooff, pooff " which is the crankcase compression.---------Without crankcase compression a 2 stroke will not run.-------------Induction into the crankcase can be done with #1 reed valves , most common today.---#2 special porting , known as a 3 port system of induction ---#3 a rotary valve set-up.-------Without knowing the details / age of your motor I can not say how it works on your motor.-----Pictures , pictures are required.----------A reed valve equipped engine can not run if the reed valves were missing !!-----If it throws air back out the carb then there is certainly something wrong with crankcase induction / compression !-----Looked up the parts manual ( on line ) for a 5BS and it does not appear to show a reed valve system.--------It appears to be the 3 port design.--Seeing the motor would make for instant identification on this set-up.
    Last edited by racerone; 01-28-2013 at 12:03 PM.

  5. #5

    Default Re: Yamaha 5BS 661 problem

    Many thanks, in which case I guess the piston rings are gone. I might strip it down further! Thanks for all the help.

  6. #6

    Default Re: Yamaha 5BS 661 problem

    Hi,
    I have now managed to get my 5hp Yamaha 5BS (air cooled) engine running but I still have a problem. So far I have replaced the piston rings, changed the spark plug and stripped down, cleaned and rebuilt the carb. (My text is too long so I will have to post as a few entries).
    The engine should be run at 100:1 oil mix, but I have been recommended a 50:1 oil mix, so thats what its running on.
    The problem is that when you try to accelerate, the engine starts to speed up then it just chokes itself and stalls. I have checked that there is a good jet of fuel coming from the carb and I can't find any air leaks (spraying all around the carb with carb cleaner the engine continues to run, but get any in the intake and it dies)
    There also appears to be a lot of oil coming from the exhaust, but being two stroke this may be normal I don't know. Putting newspaper under the exhaust will have it black and splattered fairly quickly, and the exhaust pipe will drip oil. Run it in water and the water turns milky with globules of oil on it almost immediately.
    Adjusting the carb will have some affect as to the speed of the engine, and as expected screw in the valve and it will stall.
    I found a parts manual and see that there are meant to be two o rings (or a washer and an o ring?) - parts 7,8 and 9 on the diagram below, on the needle (8) that runs through the centre of the float chambre. I know mine only has one o ring at the lower end (9) of the needle as I have just replaced it. I have no idea about carbs, so i dont know what this needles purpose is - I am guessing it is the only needle in this carb so I cant see that any o ring at the top end (7) will make any difference, although someone may tell me it does!
    Parts 2 and 4 are air jets whilst part 3 is a pilot jet. Could these be the problem? I have sprayed them with carb cleaner and have taken out one of them (not sure you can remove all of them) and they look clean.
    I am also slighlty confused by the purpose of the needle valve which closes off when the float rises - what is the purpose of that?
    If anyone is adept with carbs, I would really appreciate an answer to this as I have spent days trying to get this engine to run properly!

    Thanks in anticipation.

  7. #7
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    9,627

    Default Re: Yamaha 5BS 661 problem

    The needle that rises with the float controls the amount / level of fuel in the carburetor bowl.

  8. #8

    Default Re: Yamaha 5BS 661 problem

    Photo of carb parts has been addedClick image for larger version. 

Name:	5BS Carb.jpg 
Views:	893 
Size:	33.3 KB 
ID:	6342
    Last edited by Ajstars; 02-24-2013 at 04:42 PM.

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