Vino poor running solved

[deaninmilwaukee]

I bought a well worn 3000 mile 02 yamaha vino which ran ok for 50 miles and then suddenly would do no more than idle. Adding throttle caused it to die as fast as revs came up. This coincided with the first morning of cold weather.

After pulling the fuel system apart, getting a little dirt out from the petcock but finding a clean carb, I noticed that the intake manifold was heavily cracked, so I ordered up a new one and got it installed.

The problem is solved, with the engine running very smoothly. What I surmise happened is that at idle, engine vibes were low enough that the cracks stayed closed, but as it revved, vibes caused the cracks to open and let air in, so much so that it would lean out and flame out.
I believe that the cold weather made the rubber stiffer and allowed this to happen more easily than when it was warm and the rubber was more pliable.

While it was apart, I added 72.5 main jet to compensate for the exhaust being derestricted ( by past owner) and derestricted the variator myself.
I now have a scoot that will take my 200lbs up fairly steep hills near my house at 33 mph, and will top out at around 37 mph.

This post is a thank you to scootdawg community. I lurked here for a while finding other posts that helped me with my project and I hope my post is helpfull as well.

Dean

[Fox]

What was really happening is that at idle the vacuum level was low enough so the crack didn't allow too much air in so the fuel/air mixture was still within the range where it would run but when you opened the throttle the vacuum increased opening the crack allowing a bunch of air in throwing the whole fuel/air mix out of whack.

[jlee]

Nov 16, 2012 16:34:13 GMT -5 Fox said:

What was really happening is that at idle the vacuum level was low enough so the crack didn't allow too much air in so the fuel/air mixture was still within the range where it would run but when you opened the throttle the vacuum increased opening the crack allowing a bunch of air in throwing the whole fuel/air mix out of whack.

I think you've got that backwards (right idea, though).

At idle, the vacuum is high, sucking the cracks closed. As RPMs increase, the vacuum decreases, allowing the cracks to open up and lean out the mixture enough to kill the engine.

[Fox]

Whatever...

[jazzman]

Remember guys correcting someones bad info sometimes causes bad feeling and then the sh!t starts. So you have to be very diplomatic about the words you use when correcting bad info ;D

[deaninmilwaukee]

Well it seems there's some dispute about why the manifold caused the symptoms it did, but it definitely was the cause of the problem. The bike runs smooth as silk now.

Dean

[Fox]

I love that feeling when you finally get the damn thing to do what it's supposed to do and you can wash your hands and ride it.

[jazzman]

What causes the manifolds to crack is that the Carb is not supported on the other end and it bounces up and down and over time it causes the manifold to crack. If you can support the carb on the Air Filter side so it does not move, your manifold will last for years.

[deaninmilwaukee]

Its funny you should mention that as the previous owner clipped off the the two tabs that are supposed to support the air cleaner to the transmission assembly, apparently to be able to open the air cleaner up. I guess they didn't have a impact driver to get the screws loose. Now I am confronted with either fabbing up some other way to support the assembly or bite the bullet and buy a whole new air cleaner cover. Right now the air cleaner assembly is solely supported by its connection to the carb.

Dean

[Fox]

Post a pic of what you got. I think I most likely have an exact replacement for you cheap.

[deaninmilwaukee]

That would be great! I will try to get a good photo and I'll post it here.

Dean