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.