10 replies to this topic

[alpder]

I'm getting 0.2mm (8 thou) side-to-side movement in all directions on the throttle-lever end of my HIF38 carb shaft. Is this OK? A bit too much but bearable? Or crazy-too-much?

 

Also puzzled by the seals (one just visible below the dial-gauge tip in the pic). Although they are close-fit around the shaft, they also seem to be free-floating: nothing pushes them against the faces of the carb body. End-movement of the shaft therefore just pushes the seals outwards along the shaft, and they don't move back of their own accord, so leaving an air-gap. Am I misunderstanding how they're supposed to work?

 

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Edited by alpder, 22 March 2025 - 05:30 PM.

[alpder]

Even more puzzled by the seals, now I've started to remove them. I've never seen this kind of seal before, but... surely these are upside down?

 

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[Lplus]

Yes they are inside out.  When in place the vacuum in the manifold holds them against the seal face in the spindle shaft hole.

[alpder]

Thanks, that sounds right.

 

Trouble is I've got maybe 1.5mm end-float on the shaft. If I push the seals tight, then shift the shaft to furthest extreme, then centre it (as it will do when the butterfly is closed)... even then I can still blow into the outside of the seal and hear air going through into the venturi. No matter how hard I blow, the seal doesn't re-seat. Maybe the vac has a hell of a lot more puff than I do? Otherwise I worry that the vac won't re-seat it either - the gap between seal and face is just too big to overcome.

[Lplus]

Thanks, that sounds right.

 

Trouble is I've got maybe 1.5mm end-float on the shaft. If I push the seals tight, then shift the shaft to furthest extreme, then centre it (as it will do when the butterfly is closed)... even then I can still blow into the outside of the seal and hear air going through into the venturi. No matter how hard I blow, the seal doesn't re-seat. Maybe the vac has a hell of a lot more puff than I do? Otherwise I worry that the vac won't re-seat it either - the gap between seal and face is just too big to overcome.

Yep the engine can suck way harder than you can blow.  That's the way they are made so I wouldn't worry too much.  You will always have some end float on the shaft when the throttle is open, the only time it has no end float at all is when the butterfly is tightly closed with no gap at all around it.

[sonscar]

Why would you have end float?the butterfly is round and the throat is a circle.Steve..

[alpder]

Why would you have end float?the butterfly is round and the throat is a circle.Steve..

Once you open the throttle, the butterfly becomes a side-on disc (so almost a line) that's slightly shorter than the dia of the throat. Also, the throat has two 7mm-ish drillings where the shaft bushes fit, so the throat profile goes bigger than the circle at those points.

[timmy850]

You can get full size seals that fill the gap between the body and spindle

JZX1994
https://sucarb.co.uk...dle-seal-1.html

From memory the “end float” is taken up once you put the return springs and throttle quadrant on.

[alpder]

You can get full size seals that fill the gap between the body and spindle

JZX1994
https://sucarb.co.uk...dle-seal-1.html

From memory the “end float” is taken up once you put the return springs and throttle quadrant on.

Now, I really like those seals. For two reasons: (1) I can stop messing about with 3D printed parts to achieve the same thing and (2) I can take the rest of the night off because I can't reassemble the carb now until they arrive sometime next week. So... beer-time it is, then. Cheers

[Lplus]

You can get full size seals that fill the gap between the body and spindle

JZX1994
https://sucarb.co.uk...dle-seal-1.html

From memory the “end float” is taken up once you put the return springs and throttle quadrant on.

From my parts book there was a change in 1988.  I don't know if the bore of the seal housing was increased to take the seal though.  The HIF carbs I have are all pre 88. :shrug:

 

I don't think the throttle quadrant should be tight against the carb body when the butterrfly is fully closed.  I think there should be a small gap to ensure the butterfly doesn't fail to close to the idle stop.  ymmv

[timmy850]

The larger seals fit my HIF44 fine, and it’s date stamped 1982. I don’t know if this applies to all of them

I meant it as the return springs themselves that keep it centred, the quadrant shouldn’t touch, but the spring on each side keeps it aligned