How To Access the Air/Fuel Mixture Screw of the Carburetor? By carefulrider (Read 649 times)

[carefulrider]

In the process of trying to get my 2006 JMStar/Jonway VIVA YY250T-2, 244 cc (copy of Honda CN250, upright or vertically oriented) engine to run normally, it's getting close to the time to adjust the air/fuel mixture on the carburetor. This is the carb whose adjusting screw is not on the side, but underneath, facing down toward the ground.

Does anyone have a good way to access the adjusting screw, with the carburetor attached to the intake manifold & the engine?

Otherwise, would it be acceptable to lie the scooter down on its side on the grass, remove the bottom guard or shield & access the screw using a really long length screw driver? I don't even know if there is clear access to the screw head from underneath all the way from say, where the starter motor is. Of course, this would be undertaken with the engine off.

I look forward to your reply posts.

[ellpee]

When a guy did mine, he loosened the clamps at both ends of the carb (one for air intake, one for intake manifold) and just rotated the carb until he could get at the screw. Note also, however, that the F/A screw was "plugged," I'm told some kind of California emissions thing, and he had to get that plug out before he could put a screwdriver on the actual adjusting screw.

[alleyoop]

[carefulrider]

Thanks both for the reply posts:

Upon closer inspection, I found that I was unable to access the air/fuel adjusting screw from the underneath with a long reach screw driver.

Before I read your posts, I removed the carburetor & turned in the screw until it stopped & then backed it out 1-1/2 turns & replaced the carburetor. The motor accelerates without hesitation when the throttle is opened quickly. Is this the correct adjustment for the air/fuel screw? I'll await your replies before I put plastic panels back.

I have made a note of the idea of rotating the carburetor instead of removing it, to turn the the adjusting screw. Furthermore, I shall try the screwdriver set pictured in the post, if you suggest that additional adjustment is needed.

I think I shall end up riding this scoot next season, as there is still additional work to do on it, & the temperatures are beginning to dip into the 30s here in upstate NY.

Thanks for your instruction. Am learning, with your help.

[ellpee]

I've read 2 1/2 turns here, but that's all relative I'm sure. I run one scoot at about 2500' and another at 7500', and the correct answer is quite different. Size of carburetor jets also comes into play. There are threads/posts here about tweaking the mixture by watching the rpm, and that's probably the best way to go, but of course you need a good RPM gauge or a good ear or both.

[alleyoop]

To adjust the mixture correctly you need the motor to be running so you can hear the difference as your adjusting the mixture. Doing it while the motor is off and turning it a little this way and that way you will be lucky to get it right and may be to lean and or to rich.
Alleyoop

[carefulrider]

Well, I attached a RPM gauge & started the motor; waited till it reached operating temp. Adjusted the RPM to 1860. Then moved the air/fuel mixture screw in one revolution, & then out two revolutions. The RPM did not change.

However, when I disconnected the vacuum hose to the (rubber) intake manifold, the RPM went up by around 500. What does that mean?

I think I am going to be obliged to take off the carburetor & clean it. Don't know how to do it. So it's back to the class room.

[alleyoop]

Oct 22, 2014 14:14:43 GMT -6 carefulrider said:

Well, I attached a RPM gauge & started the motor; waited till it reached operating temp. Adjusted the RPM to 1860. Then moved the air/fuel mixture screw in one revolution, & then out two revolutions. The RPM did not change.

However, when I disconnected the vacuum hose to the (rubber) intake manifold, the RPM went up by around 500. What does that mean?

I think I am going to be obliged to take off the carburetor & clean it. Don't know how to do it. So it's back to the class room.

When you took the vacuum line off the intake that created a LEAN condition and is why your rpms went up. It was sucking in more air that made it LEAN..
Alleyoop