Topic: Emulsion Tube Fit

[minimo]

1975 400F
- Air pods
- Aftermarket exhaust/muffler (Outex, Japan)
- Aftermarket carb rebuild kit (via 4into1, likely Emgo brand).
- Upsized Main Jet: 92 (stock: 75)
- Aftermarket Pilot Jet: 40 (stock: 40)
- Aftermarket Needle, Clip 4th groove from top (stock: 3rd from top)

I’ve had good working results with these aftermarket carb parts on a stock setup, but now on this other 400F with air pods and aftermarket exhaust (bigger muffler), I was curious how critical the gaps around these aftermarket emulsion tubes are as they seem to drop in quite easily down that part of the Main Jet stem.

See the gap around the emulsion tube head here. A friend suggested getting an O-ring to fit on there but curious to know what y’all thought.



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[dave500]

are the factory ones any tighter?are the factory ones damaged?

[minimo]

The factory ones are a tighter fit, yes. However, there is still a noticeable gap between the OD of the emulsion tube and the inner wall of the stem, as though it has gone oval.

Is the fit around the emulsion tube critical?

Following the flow of fuel from the bowl, the main jet’s o-ring should be the seal to the passage of fuel, correct? So, as the needle lifts, it opens up a passage of air through the tiny holes in the emulsion tube walls to mix with the fuel. If that’s the case, then a gap would be necessary, would it not? The o-ring on the main jet is important to help create this suction occur, maybe?


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[Deltarider]

... as they seem to drop in quite easily down...

That's not abnormal.

Is the fit around the emulsion tube critical?

Not at all. Needle jets don't need to be tight. Right under them is the main jet which has a little O-ring. That must seal tight.

[minimo]

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[dave500]

the top of the tube must seal tight where it fits into the body,all the discharged fuel must exit the top of the tube,if its bypassing at the top of the tube itll be unmetered by the needle,it should also really fit tighter at the base area shown in your photo,sure only metered fuel via the main jet enters this area how ever it is designed for vacuum from above to draw air via the by pass port which enters those tiny holes along the tube,if the bottom isnt sealed well enough that draw will be disturbed somewhat,the design and action of emulsification is over looked by most guys,if its even known about,the idle or slow jet also has a built in emulsion tube.

[minimo]

Gotcha.

So then I’m looking for a good seal on the main jets.

Thanks to this handy page on carb o-rings, I was able to determine for the 400F Main Jet: 1.2mm cross-section x 3.5 mm ID
http://www.sohc4.net/carb-o-ring

I ordered some vía McMaster-Carr:
Oil-Resistant Buna-N O-Ring, 1.2 mm Wide, 3.5 mm ID
https://www.mcmaster.com/9262K248

But I’m finding that these o-rings don’t quite give my mains that “snappy” fit. These main jets easily slip in, so I fear that these o-rings aren’t creating a good seal. It is also possible there was some attempt of polishing in and around the stem of where the main jets get pressed in, so that may have opened up the hole some.

The next size up on cross section that I have access to is 1.30mm but @ 2.50mm ID, but that’s a whole mm narrower on the ID.

O-Rings, 1.3 mm Wide, 2.5 mm ID
https://www.mcmaster.com/1302N123

Being a tighter fit around the main jet, would there be concern these smaller diameter o-rings would break (faster)?


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[dave500]

yeah maybe it was corroded and some one tidied it up,the bottom of the tube should still be almost a press/slide in fit,it should really seal better in that spot for the vacuum to have a good effect on the tube.

[minimo]

Thank you, Dave and Deltarider


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