1963 Cold Start

[Jeff S.]

Is there a measurement for the carb butterfly opening on a 340hp for choke and cold start?

[Duke W.]

Re: 1963 Cold Start

Everything you need to dial in the AFB or perform any other service/overhaul operation you ever attempt is in the 1963 Corvette Shop Manual.

Don't leave home without one.

Duke

[John D.]

Re: 1963 Cold Start

Jeff, This info may or may not apply to your 340 HP carb. But let me tell you something about a 63 fuel injection choke butterfly.
I used to think it had to close all the way to function well. I used to remove a roll pin that kept it from closing all the way. Now I am not completely at fault as the choke modification kit told one to remove this.
Anyhow I finally found out I was mistaken.
On an FI unit there is no need for the butterfly to close all the way for starting. If one has this then he may have a rich running unit until warm up.
I am going to guess then that your carb butterfly doesn't need to close all the way. Now to wait for guys that work on these carbs. I have not looked in the manual. John

[Thomas S.]

Re: 1963 Cold Start

The service manual is definitely a “must have”. Link to a general article about WCFB & AFB’s is below. Setting the cold postion is easy as long as you have a temperature around 60-65 degrees - the choke should be closed at that point. If there actual choke linkage has been damaged or incorrectly adjusted (bent) then you’ll have to go back to baseline measurements which are contained in the service manual.

There is small vacuum activated cylinder/pistion inside the choke housing that uses stuck or bind and without that mechnism working properly the choke can be very flacky. There are also two different types of springs. The originals were a slightly smaller diameter than the replacements currently available. Either one will work correctly.

There is a metal disk (zinc) that holds the spring inside of the Baklite plastic housing. The orignals had a small hole through the center to allow vacuum through the metal part of the choke housing to draw air through the plastic cover from the exhaust manifold tube. The replacements don’t have that hole.

Link (chike info about half way through the article.)

https://itstillruns.com/adjust-carter-carburetor-7902949.html

[Timothy B.]

Re: 1963 Cold Start

Is there a measurement for the carb butterfly opening on a 340hp for choke and cold start?

Jeff,

It's important for the choke blade to completely shut during cold cranking when first starting the engine. The cranking vacuum promotes fuel flow through the carburetor, if the blade were open slightly the cranking vacuum will pull air instead.

After start up, manifold vacuum acts on the hot air choke piston to pull the blade open approx 1/4" to allow air. As the spring inside the hot air choke warms it expands to allow the choke blade to further open.

If the engine is run then shut off and cools some, the heat in the spring controls how far the blade closes on restart and the warm up repeats.

As mentioned, the 1963 shop manual has a detailed chapter on the AFB Carter and it's adjustments.

[Thomas S.]

Re: 1963 Cold Start

Here are the two pages from the 63 service manual / AFB Carb. The only adjustment with a dimension relates to the of the choke valve gap when the choke coils is fully closed. I know this might sound a bit dumb, but in my experience, as long as the choke closes when cold, and is fully open when hot everything should work fine. The issue would be the fast idle speed which is easy to set.

[Duke W.]

Re: 1963 Cold Start

Another important adjustment is the choke vacuum break, but that's on another page.

The AFB is a very reliable carburetor. The fact that the bowls are in the main body casting with no "wet" gaskets means no leaks like Holleys are subject to.

The weak link in the design is the choke vacuum break. Over time the piston/bore get dirty/sticky, which may prevent the choke from fully closing resulting in hard cold starts. At room temperature the choke valve should fully close.

The choke housing can be removed from the carb to clean, but be aware that there is a tiny o-ring that seals the vacuum passage from the choke housing to the main body and this should be replaced. The o-ring should be included in zip kits, but I think it is often overlooked, and if not installed properly will cause a small vacuum leak that will cause improper operation of the vacuum break.

The piston/bore is less likely to get dirty with a paper air filter. The OE-type oil wetted urethane filter likely allows an oil film and dirt to pass through the piston/bore making is sticky. Use those for judging, only.

Duke

[Frank D.]

Re: 1963 Cold Start

Jeff,

It's important for the choke blade to completely shut during cold cranking when first starting the engine. The cranking vacuum promotes fuel flow through the carburetor, if the blade were open slightly the cranking vacuum will pull air instead.

After start up, manifold vacuum acts on the hot air choke piston to pull the blade open approx 1/4" to allow air. As the spring inside the hot air choke warms it expands to allow the choke blade to further open.

If the engine is run then shut off and cools some, the heat in the spring controls how far the blade closes on restart and the warm up repeats.

As mentioned, the 1963 shop manual has a detailed chapter on the AFB Carter and it's adjustments.

I adjust them so, that on a cold day, with the throttle linkage held wide open, the choke blade closes completely but lightly.
The piston chamber usually fails from being carbonized by exhaust gases -- many times because the channel in the exhaust manifold for the hot air tube has lost integrity... I've seen some so bad I took the welch plug out of the choke housing and scrubbed the cylinder with Hoppes #9 solvent and a brass bore-cleaning brush (just like a rifle)...NEVER sand them or use emery cloth (nor the piston)...

[Thomas S.]

Re: 1963 Cold Start

Frank, do you use a punch or small drill to remove the welsh plug and where would you find a replacement?

[Frank D.]

Re: 1963 Cold Start

Frank, do you use a punch or small drill to remove the welsh plug and where would you find a replacement?

If they're not super tight you can sometimes poke them out from inside the piston cylinder with some coat hanger wire or the like...else a super small jeweler's screwdriver to carefully pry up an edge from the outside works.... All the ones I've removed were able to be reused...

Many small engine repair shops will have welch plugs or you can get them online...