Webers to replace Zeniths?

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Brian R Adams
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Re: Webers to replace Zeniths?

#76 Post by Brian R Adams »

Al Zim wrote: Tue Dec 05, 2023 11:43 am Remember the newest 912 engine is 60 years old. al
But the newest 912 is 47 years old. :o
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Re: Webers to replace Zeniths?

#77 Post by Greg Bryan »

Martin Benade wrote: Tue Dec 05, 2023 2:23 am So no full throttle at 2500 rpm
Are you referring to Weber's propensity to have a flat spot at 2500 rpm?
I do not detect any flat spots with my present setup. Most of the time I drive like an old man, but not always, and it seems be very civilized in most cases.
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Re: Webers to replace Zeniths?

#78 Post by Martin Benade »

I was referring to how with a SC engine, nothing uncivilized happens ( not much happens at all) if you floor it at 2500 rpm, but half throttle gives more power. I’m sure bigger carbs would do this more so. It’s why 4bbl carbs usually have vacuum secondaries.

Why do Webers need long spacers under the balls? Wouldn’t the geometry be the same as the factory carburetor? Also if one downlink leaned back and the other one leaned forward by the same angle the geometry would be fine although I’m not sure if that could line up that way.
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Re: Webers to replace Zeniths?

#79 Post by C J Murray »

Martin Benade wrote: Tue Dec 05, 2023 4:56 pm Why do Webers need long spacers under the balls?
Two reasons, one because the throttle shaft sticks out more on one side of a Weber to make room for the Weber return spring, and two because the offset of most of the various levers is not designed for the offset cylinders of a 356, assuming there is any offset at all. It all makes sense when you are installing them.
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Re: Webers to replace Zeniths?

#80 Post by Brian R Adams »

Greg Bryan wrote: Tue Dec 05, 2023 2:19 pm
Martin Benade wrote: Tue Dec 05, 2023 2:23 am So no full throttle at 2500 rpm
Are you referring to Weber's propensity to have a flat spot at 2500 rpm?
I do not detect any flat spots with my present setup. Most of the time I drive like an old man, but not always, and it seems be very civilized in most cases.
My Weber flat spot never appeared if I stomped on it, it snapped ahead straight through the transition. It was when easing in the throttle that it reared its ugly head. As I recall, anyway, the Webers have been on the shelf for twenty years.
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Re: Webers to replace Zeniths?

#81 Post by Brian R Adams »

Martin Benade wrote: Tue Dec 05, 2023 4:56 pm Also if one downlink leaned back and the other one leaned forward by the same angle the geometry would be fine although I’m not sure if that could line up that way.
One forward and one backward is not likely to be possible, I think I pondered that at the time. Even if they formed the same angle off the end of the fan housing levers, the upper ball travels in an arc, so I don't think the carb openings would stay perfectly synced through the range of motion. Best to have both downlink rods in the same plane (i.e. both tilted away from the fan housing at the bottom, which is what comes to pass in the circumstances.)

FWIW Harry Pellow didn't bother with it at all, he had the rod on the passenger side reaching ~1.5 inches farther aft to reach its carb lever. (At least the engine on the cover of "Secrets ..." is that way, and that's his showcase engine. He never mentioned Weber linkage equalization that I recall.)
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Re: Webers to replace Zeniths?

#82 Post by Martin Benade »

“Are you referring to Weber's propensity to have a flat spot at 2500 rpm?”

I was just referring to overcarburetion, not a flat spot.
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Re: Webers to replace Zeniths?

#83 Post by Craig Geiger »

Adding an option to the party (that I used on my 63B with 1720 and s cam)... https://www.csp-shop.com/en/engine/dual ... 7029a.html I had close to zero experience dealing with carbs, zenith, weber or lawn mowers for that matter, and with the help of this forum, YouTube university, a buddy who lives close with a 356, and quite a bit of trial and error, it's running (and been running) great. Dodged the flat spot plague somehow too. It's always better to be lucky than good!
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Re: Webers to replace Zeniths?

#84 Post by Martin Benade »

So you put large-port manifolds on small-port B heads? At one time a step going the other way was said to be desirable
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Re: Webers to replace Zeniths?

#85 Post by Greg Bryan »

My retired machinist who had a flow bench said going from a smaller bore, such as a 40mm carb throat to a larger bore, such as a manifold that also accommodates a 44mm carb, did not have an effect on flow.
We didn’t talk about the other way, such as a C or 912 manifold on a B head, but intuition says it has to be deleterious.
There was a rep for a company that does cnc machining at the lit meet a few years ago that had B heads that had been machined to C head dimensions - that can’t be bad
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Re: Webers to replace Zeniths?

#86 Post by Martin Benade »

I didn’t exactly believe it but my good friend put his Solexes on a N and said it ran just like his S90. I do believe it ran acceptably though.
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Re: Webers to replace Zeniths?

#87 Post by Greg Bryan »

Martin Benade wrote: Tue Dec 05, 2023 8:40 pm “Are you referring to Weber's propensity to have a flat spot at 2500 rpm?”

I was just referring to overcarburetion, not a flat spot.
A 40mm Weber is not dissimilar to a 40mm Solex, so I don’t think one is over-carbureting an SC motor with converting to 40 IDAs. On a stock C, I don’t know because I’ve never personally done anything with them.
I like the Webers because they are so much easier to work on and parts are readily available, and it doesn’t have the unfortunate feature of dumping a bowl full of gas onto a hot engine when you pull a main jet.
I have also had mixed results with having Solex throttles repaired where the idle is still spotty - I can’t discount my ineptitude as a contributing factor, however.
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Re: Webers to replace Zeniths?

#88 Post by Martin Benade »

I’d compared it to an SC which is equally overcarbureted under those conditions, meaning don’t floor it until about 3000 rpm.
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Re: Webers to replace Zeniths?

#89 Post by David Nicholls »

Dear Readers
What about the Fuel Lines from the Pump to the Carburettors ?
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Re: Webers to replace Zeniths?

#90 Post by C J Murray »

Greg Bryan wrote: Tue Dec 05, 2023 10:29 pm My retired machinist who had a flow bench said going from a smaller bore, such as a 40mm carb throat to a larger bore, such as a manifold that also accommodates a 44mm carb, did not have an effect on flow.
We didn’t talk about the other way, such as a C or 912 manifold on a B head, but intuition says it has to be deleterious.
There was a rep for a company that does cnc machining at the lit meet a few years ago that had B heads that had been machined to C head dimensions - that can’t be bad
Complicated subject. CFM flow, velocity, signal strength, turbulence, tumble? What are you after? Any abrupt change of diameter causes unwanted disruption. The best combination of carburetor, manifold, port, and valve size is the one that provides a velocity of the speed of sound at the valve at an rpm that maximum power is designed to be for the intended use of the engine. A too large manifold slows the velocity of the intake charge which can cause the fuel to drop out of the mixture and coat the walls of the manifold which causes obvious problems. Same with too large ports. Too large intake valves reduces the charge speed at the valve which slows the combustion process in the cylinder, reducing power. Envision your thumb over the end of the garden hose and how that makes the water move faster. High velocity is good to stir the mixture in the cylinder for better combustion. Too large carburetors means that the fuel circuits don't respond in phase with engine demands.

It is pretty easy to get the combination wrong. The funny thing about making changes like a larger carb is that your mind can play games with you. You want the large carb to make more power. That is why you go to the trouble of changing it. Once installed you feel a point during acceleration where the car surges forward. Success! However the reality is that power is reduced in the area before the surge and the surge doesn't last long before going flat. Since most street driving happens at low rpm the car is actually slower now but your brain can't accept that so you tell everybody you are happy.
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