Zenith Thermo Starter Valve, is it required?

waynesie

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My CS came to me with twin weber down drafts. They have worked fine, and are very simple, but I like to fiddle and want to see if Zeniths are better.

I aquired a set of Zeniths, but the thermo starter valves have been removed on both. The electric idle shutoff valves are there and both work.

Information about the thermo start valve say it further increase the richness at start up.... Not sure what exactly it does and when.

Is it necessary? And if so, when / what part of the start cycle does it help?
If not, do I need to cover up the holes?

Thanks for any help,
 

Stevehose

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This valve has a bimetallic spring that cuts some airflow to the carb under 57 degrees or so. Electricity will then warm the spring and open the valve, letting air in and leaning out the mixture.

You can try using without, depending on your climate they may not be needed. For example, my triple webers have a cold start jet that I blanked off because the extra enrichment is not needed on a well tuned engine where I live. A couple pumps of the accelerator pedal is enough. I suspect the same for you, especially with a choke plate system.

I think the hole on the side of the carb where it mounts must be blocked off or you will have an air leak (these valves mount with a crush washer for this purpose) where at best unfiltered air gets in or worse a nasty vacuum leak. I will check my old ones later and confirm.

Have these carbs been rebuilt? If not I would thoroughly go through them, and make sure the pieces of the carb bodies are not warped, they can be wet sanded true which is critical for consistent operation. Also proper accel pumps, jets, needle valves and gaskets etc.
 

HB Chris

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I never had those thermo valves, just a bolt where they would have mounted. I don't believe they are necessary unless you are in a very cold climate, and even then who knows. I also think most were removed by previous owners.
 

deQuincey

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you said they were removed,...hummm ?
well they were not always present, early euro coupes do not have those valves, i found them in carbs comming from the us that i used for spares,
i have checked virtually dozens of zeniths and i found that when those valves are present the upper body of the carb is different,
considering the operation correctly described by steve the valve is normally closed when outside temperature lays below 15 deg, and then opens by a bimetallic spring that is heated with an electric resistence, so the usual operation for warm climate is an open valve,
then IMHO, the best thing you can do is remove the tiny metallic cone that is forced to close the valve by the bimetallic spring,
you leave the valve there, no need to connect electricity, and the correct air ammount will travel through the right places to get the job done
 

Ohmess

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I took a look at two triple weber setups at the Vintage last month and neither had these operational. Amy's NC coupe had the mechanisms connected to one another, but no connector to actuate them. Gary's GA coupe had no connections.

These cars, and yours, operate in places where it doesn't get really cold. I recall seeing a post from someone in CT who connected his, using a cable through the firewall, who claimed his car ran a little smoother when first started on cold days.

I don't plan to mess with these initially (when I finally get around to installing my carbs.)

This valve has a bimetallic Aspring that cuts some airflow to the carb under 57 degrees or so. Electricity will then warm the spring and open the valve, letting air in and leaning out the mixture.

You can try using without, depending on your climate they may not be needed. For example, my triple webers have a cold start jet that I blanked off because the extra enrichment is not needed on a well tuned engine where I live. A couple pumps of the accelerator pedal is enough. I suspect the same for you, especially with a choke plate system.

I think the hole on the side of the carb where it mounts must be blocked off or you will have an air leak (these valves mount with a crush washer for this purpose) where at best unfiltered air gets in or worse a nasty vacuum leak. I will check my old ones later and confirm.

Have these carbs been rebuilt? If not I would thoroughly go through them, and make sure the pieces of the carb bodies are not warped, they can be wet sanded true which is critical for consistent operation. Also proper accel pumps, jets, needle valves and gaskets etc.
 

Honolulu

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I took them off and plugged the bolt hole... with a bolt. It'll be a cold day in Honolulu (Hell too) that I see 57 degrees. Outside just now is sunny, tradewinds and mid/upper 80s. Another cr*ppy day in paradise, me at work for another 25 minutes.
 
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