needle jet questions

[xsgeoff]

I took my carbs apart to clean and check the floats, and now I have a few questions regarding the needle jet:

1) How much "play" should there be between the needle and piston? My Haynes manual says: "The jet needle is retained by a nylon plate and is a push fit." The needle is fairly loose though, able to "wag" around a few degrees when installed in the piston. And the nylon plate just sits on top of the needle fairly free-moving, so that the needle can push the plate up. Is this correct?

2) How should the needle be seated in the jet tube? Does it need to be fully in contact with the tube? Right now, my needle doesn't fully seat on the tube, it sits at its lowest point in the piston, and leaves a very small amount of play giving a very thin gap between needle and jet. I guess I need to adjust the needle position clip, or even replace the needle and jet?

Cheers!

 

[DogBunny]

You definitely do NOT use the needle position clip to adjust play.
Pictures would help a lot. Need to know what you have, and in what order it is assembled.

 

[xsgeoff]

It is a standard Mikuni 38mm from a 447. I will try to take some photos and get back.

 

[xsgeoff]

http://imgur.com/a/sfdgC#0

First image shows assembly. Needle goes into piston with retaining clip on the middle of 5 clip grooves. Nylon plate goes into piston on top. The plate has a semi-loose fit, the piston barrel guides it straight, but it can slide up and down freely (meaning the needle can also slide up). Is just the weight of the nylon plate and gravity what keeps the needle down in the piston?

Second and third images show how the needle diameter is smaller than the piston hole diameter, so it is not guided perfectly straight, it can angle in all directions.

Last image shows the seating of the needle in the jet. The needle never drops far enough axially to completely close off the valve.

 

[DogBunny]

What you show is correct, but there is supposed to be a big spring in side the slide pushing on the nylon plate.
http://cdn.boats.net/diagram/yamaha/MCY/1973/XS1_XSB_XS2_TX650/IMAGES/CARBURETOR_XS2_TX650.png
part #20. I have that part if you need it.
The needle is not supposed to ever completely close off the opening, and the opening is not a valve, it is a jet.

 

[gggGary]

The diaphragm spring keeps pressure on the plastic plate.

Otherwise all correct, they tried a rigid needle early on and had problems, changed to the spring pressure on the button keeping it more or less centered with some wobble as part of the design. It is correct that at idle the jet needle does not "hard seat" to the needle jet there is a gap. Low air velocity keeps it from pulling up much fuel till the throttle shaft is opened.

 

[gggGary]

The diaphragm spring keeps pressure on the plastic plate.

Otherwise all correct, they tried a rigid needle early on and had problems, changed to the spring pressure on the button keeping it more or less centered with some wobble as part of the design. It is correct that at idle the jet needle does not "hard seat" to the needle jet there is a gap. Low air velocity keeps it from pulling up much fuel till the throttle shaft is opened.

 

[xsgeoff]

Ah, of course. I have the spring, just didn't make the connection about it pushing on the nylon plate Thanks though, DogBunny!

Thanks Gary. One more question: How does one tell when the needle is worn enough to warrant changing? The o-rings that hold the jet tubes were dickered, so I need to order them. May as well change some other parts if they're at all worn and I'm getting stuff in the mail anyways. Mine looks pretty good I guess, but that is my zero-experience opinion. Here's a slightly blurry photo:

 

[gggGary]

That looks pretty good if the last bit by the tip looks like the rest does in the blurry pic you should be good.
Needle wiggle must be a big deal, in the later BS34's the needle has it's own tension spring held in place by a screwed down plate.

 

[5twins]

Yes, it looks OK, as long as that's not a groove worn into it I'm seeing where the color changes about 1/3 down from the top. Yes, the needle is a loose fit in the slide. It has near a half MM of up and down play, not real good for exact mixture regulation. You can add a flat washer above or below the e-clip to eliminate that up-down slop. The '78-'79 carb set changed the needle mount in the slide. They spring loaded the needle to eliminate the play. That carb set operates noticeably smoother through the low and mid RPMs. I have to think it's because the needle isn't bouncing up and down through half a clip setting position.

 

[gggGary]

Edit; Even a less than perfect genuine mikuni needle will probably be better than any "wilfitz" needle

 

[xsgeoff]

Thanks, guys!