Probably a silly question - about current idle mix screws

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Mervyn Hyde
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Probably a silly question - about current idle mix screws

#1 Post by Mervyn Hyde »

While recently checking and then re-tuning my Zeniths, I noticed that the new idle mixture screws that I received from an Italian source appeared shorter than the originals. That is, the taper was sharper on the new ones. It may also be that the newer ones have a shorter threaded shaft. This seemed to reduce the amount of adjustment that I had when tuning. It became a very fine process and the normal 1.0 to 1.5 turns out as a starting point was reduced. The older ones had a length from the base of the shaft where the spring rests to the tip of about 21.7mm.

Has anyone else noticed this and is it even relevant?

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1963 356B T6
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John Jenkins
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Re: Probably a silly question - about current idle mix screw

#2 Post by John Jenkins »

Correct idle mixture screws for the Zenith with the longer taper are made with Swiss screw machines. Same applies to the Solex. I discovered this when I was looking for a machine shop to make our idle mixture screws as the ones in the kits had too sharp of a taper which made it tough to get the mixture correct. A small movement made a lot of difference. The Swiss screw machine cuts the material as it comes into the cutting area thus allowing the cutting to be done on the thickest part which is supported and not hanging in the air. The support is better and allows the longer taper to be made. You'd have to see it to really understand it. The cheaper way is with a lathe but it can't make the longer taper without bending the part. The shop I used cut a body out where the idle mixture screwed in. I could then see why the longer taper works better. More of the mixture screw is close to the walls of the straight tubular passageway. For the Solex this is 1.35 mm in diameter and 7 mm long. The longer taper goes about 3 mm into it vs. about 2 mm for the short one. This passageway is not tapered. This is another example of "you get what you pay for."
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Vic Skirmants
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Re: Probably a silly question - about current idle mix screw

#3 Post by Vic Skirmants »

Hint on Zenith idle screws: the carb body is cast iron, the screw is brass. It is very easy to deform the screw if tightening too much, which is easy when the spring is in place.
Therefore: turn the screw in without the spring. You can feel the point when it contacts the fully-closed position easier than when the spring is in place. Now count the revolutions until the screw comes out. Put the spring on, and turn the screw in 1.5 turns short of full disengagement. Adjust from there.

Dick Weiss
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Re: Probably a silly question - about current idle mix screw

#4 Post by Dick Weiss »

I had a set of mixture screws which were in an aftermarket kit many years ago and their threads were not correct and wouldn't enter the holes! Also, some other Solex sets had blunt tips, not sharp-pointed, and they were not the extended tip version into
the throats used on the '68/'69 split-shaft Solexs.

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Mervyn Hyde
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Re: Probably a silly question - about current idle mix screw

#5 Post by Mervyn Hyde »

Interesting. Most of the available mixture screws for the Zenith (at least) seem to come from the same Italian origin, with whom I have had some differences of opinion before. Thanks John, that confirms my suspicions about the taper length. Vic's method is a good one and one I used with this current adjustment. The short taper on the new screws however, seems to mean the starting point is further in and the minimal difference adjustment even finer.
Merv
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1963 356B T6
1968 911 SWB

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