Throttle Plates

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Mervyn Hyde
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Throttle Plates

#1 Post by Mervyn Hyde » Mon Sep 10, 2018 1:32 am

The throttle plates on my Zenith 32's are about the only part that I haven't replaced. Where is the best place to buy these? I can see them in kits in some places but neither Stoddards or SM seem to have them. I assume the 8 degrees ones.
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Martin Benade
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Re: Throttle Plates

#2 Post by Martin Benade » Mon Sep 10, 2018 7:25 am

Probably that guy on ebay, I believe his store or name contains "alfa 1750"

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Mervyn Hyde
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Re: Throttle Plates

#3 Post by Mervyn Hyde » Mon Sep 10, 2018 7:43 am

Hmmm. I have had some issues with his spares in the past.
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Vic Skirmants
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Re: Throttle Plates

#4 Post by Vic Skirmants » Mon Sep 10, 2018 8:15 am

Why in the world would you need new throttle plates? They don't wear out.

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Ron LaDow
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Re: Throttle Plates

#5 Post by Ron LaDow » Mon Sep 10, 2018 3:59 pm

Vic Skirmants wrote:Why in the world would you need new throttle plates? They don't wear out.
They're also a whole lot of "fun" to align correctly. You'd need some sort of a leak tester to make make sure the two in one throttle body 'seal' the same. Hint; your eye isn't nearly good enough.
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Dick Weiss
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Re: Throttle Plates

#6 Post by Dick Weiss » Mon Sep 10, 2018 4:48 pm

The screws should be loosened (if the plates aren't centralized when fully closed w/the adjusting screw backed off), shaft endplay be shimmed to .003"-.004" each end, and finally the plate screws re-tightened. Then adjust the stop screw to check the plate edges
to the 1st and second hole in the throats for both being equal as best as possible. Hope they line up to get an equal balance/ref.

Unfortunately, the thicker Zenith shaft(s) can't be twisted like the solid/1-piece Solex shafts except the adjustable split-shaft version.
Fortunately, I've had good luck w/not needing a relief added to the edge(s) shown in some tech sheet(s) and not seeing attempts to bending the plate(s) to suit the edge-to-hole refs.

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Re: Throttle Plates

#7 Post by Mervyn Hyde » Mon Sep 10, 2018 5:51 pm

Thanks Vic. That is useful to hear. Also Ron and Dick, great advice. I will do a little forensics this weekend. I had checked, shimmed and set the end play a while ago but the 'eyeball test' was all I applied to checking the sealing.
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Re: Throttle Plates

#8 Post by Ron LaDow » Mon Sep 10, 2018 9:53 pm

Merv,
(What FUN it is loading an image, too, since it can't be 'bigger' than what you can take with that camera made of rocks)
Once more:
003.JPG
This will do it, but you have to arrange a rubber-band on a lever or gravity to hold them closed and you'll need some clay to keep the water from sneaking back up the idle circuit.
Anyhow, what 'looked' equal turned out to be nowhere close measured with this, some water and a stop-watch.
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Re: Throttle Plates

#9 Post by Mervyn Hyde » Mon Sep 10, 2018 10:12 pm

Terrific thanks Ron. I will be making one of those this weekend.

BTW - I always use 'Postimage', free 'site for hosting images of any size.
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Re: Throttle Plates

#10 Post by Ron LaDow » Mon Sep 10, 2018 10:34 pm

Merv,
That got an O-ring on the sealing surface, since I had one and the thing got regular use. A decent fit and some petroleum jelly would do as well for a one-time (or similar) use.
Because I ended up testing them all, I also ended up with a milled plex block, 'glued' to the throttle body with P. J. (sealed the idle circuits at the same time) and a trough between the bores; fill it all the way and stop-watch the difference. 10% meant reject.
When Alan did that Beijing/Paris car say 15 years back, he dug out some old parts which leaked *zero* in the two minutes I was willing to watch. 1 minute leak between the indices on the part in the image meant you could kill the engine by backing off the idle screw. I considered that the working standard.
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Re: Throttle Plates

#11 Post by Mervyn Hyde » Mon Sep 10, 2018 10:45 pm

Ah! I was looking at the base and was originally thinking the tube was inserted in the opening. I will use some suitable silicone adhesive perhaps. Now to find some suitable clear pipe ...

In the meantime, I checked the shimming and end play on the shafts and all good. Up to the sunlight there was some unevenness in the location of the butterfly valves (small gap showing on one side of each) and following Dick's advice I loosened the screws holding the 'plates to the shaft and they re-centred at fully closed. Then retightened. I can see about 1 and 3/4 of the two small holes, evenly in each throat. Seems much better, but there is a slight glimmer of light around the plates when held to a very bright LED flashlight. I will check the sealing with Ron's device when I can make one up.
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Re: Throttle Plates

#12 Post by Mervyn Hyde » Tue Sep 11, 2018 9:30 pm

Ron, I had some time today and made up this to do some testing with, hopefully. The clear perspex is drilled and tapped to take the throttle plate screws and with some Vaseline or similar should seal to the plate and cover the idle aperture holes. I still need to mark it up for measurement.

Just checking was my mild concern above about the throttle plates fitment after centering a problem? "Up to the sunlight there was some unevenness in the location of the butterfly valves (small gap showing on one side of each) and following Dick's advice I loosened the screws holding the 'plates to the shaft and they re-centred at fully closed. Then retightened. I can see about 1 and 3/4 of the two small idle holes, evenly in each throat. Seems much better, but there is a very slight glimmer of light around the plates when held to a very bright LED flashlight."

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Re: Throttle Plates

#13 Post by Ron LaDow » Tue Sep 11, 2018 10:56 pm

Merv,
I'd say you got it. The Vasoline is a good water-seal, especially with a 1"-2" head. And the flat surface will block the leak up the idle circuit, too. The marks on the one above were totally arbitrary until the dyno tests which assigned rough values to them after the fact.
Maybe your eyes are better than mine; the leak tests told me I couldn't "see" similar fits for beans. Please let us know how it works out.
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Re: Throttle Plates

#14 Post by Mervyn Hyde » Wed Sep 12, 2018 1:00 am

Just tested the LHS carb with the throttle stop backed right off. On both sides I get, in 2 mins, just < 20ml water through. Or <10ml/min (just over 1/4floz). The difference between the throats is very minor and well within your 10%. Time wise with the tube full (around 60ml), as there is surely an initial column effect, it still plenty of fluid in there after 16 mins.

So, I guess that both throttle butterflies are satisfactory, centred and operating equally?

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Last edited by Mervyn Hyde on Wed Sep 12, 2018 4:41 pm, edited 1 time in total.
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Re: Throttle Plates

#15 Post by Olivier AUVRAY » Wed Sep 12, 2018 3:18 pm

Ron LaDow wrote:Merv,
(What FUN it is loading an image, too, since it can't be 'bigger' than what you can take with that camera made of rocks)
Once more:
The attachment 003.JPG is no longer available
This will do it, but you have to arrange a rubber-band on a lever or gravity to hold them closed and you'll need some clay to keep the water from sneaking back up the idle circuit.
Anyhow, what 'looked' equal turned out to be nowhere close measured with this, some water and a stop-watch.
Another way................. with my favorite Solex tool:
SNC15494.JPG
SNC15495.JPG
SNC15497.JPG
With the proper setting of this tool, you can:
-check the leak when the throttle plate is fully closed
-adjust the throttle plate exactly at idle position (In this case you need data from the carb. manufacturer)
-adjust the mixture screw (In this case too, you need data from the carb. manufacturer)

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