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Discussion Starter · #1 · (Edited)
Some of you may recall my discussions about Rochester 4GC stumble issues in http://www.trifive.com/forums/showthread.php?t=104566 . As it turns out, Carter WCFBs also have a problem with stumbling, but it’s well documented as to the cause and the fix.

Here’s the technical explanation of what happens:

Hot air chokes use manifold vacuum to draw hot air up from the exhaust manifold, past the thermostatic spring coil, and eventually dumping it into the intake manifold. This vacuum port goes through the air horn, bowl, and throttle body (obviously). It’s located on the passenger side of the carburetor near the choke housing. In the WCFB, this port is in the primary fuel bowl wall.

When you make a quick left turn – let’s say because you’re stopped, and you have a narrow spot in traffic to make a left turn - the gas sloshes to the passenger side of the carburetor. If the seal on the vacuum port isn’t perfect, gas gets sucked into the vacuum port. At minimal throttle opening, this is enough to flood the engine and cause a “rich stumble”, and possibly stalling.

I have 2 WCFBs. One has a damaged air horn and is solid grime. The other is almost spotless inside and out, but missing the choke linkage. Hmm – looks like someone rebuilt it, used it for a couple thousand miles, then pulled it off and used it for scavenging parts. I decided to pull this one apart.

Upon disassembly, I discovered the bowl's choke vacuum port had a high spot right next to a low spot on the gasket surface – facing the fuel bowl. This could cause a vacuum leak into the fuel bowl. I also found the primary float had been set deliberately low, and an extra-strong accelerator pump apply spring installed.

Okay – looks like we’ve got a “stumbler” here someone tried to fix with a rebuild and a little tweaking, but eventually gave up and used it for parts.

On April 18, 1956, Carter issued service bulletin #497. It describes a fix for a stumble/stall when making a left turn. It instructs you to install a brass bushing #145-142 in the choke vacuum hole in the carburetor bowl.

Great! Now, where do I find a NOS 145-142? I’m sure my local Autozone has a stack of them on the shelf (NOT!!!).

Googling around, I discovered a discussion on another web site where this was being discussed. A guy there had a few NOS 145-142s that he was not about to sell, but was happy to measure with a micrometer. They were made from 5/32” OD brass tube, 5/8” long, with a gentle taper on both ends. It’s a “press fit”, so you have to carefully drive it about halfway in (5/16”).

Okay, I can do this.

I bought some K&S #5128 5/32” brass tube off eBay. I put a gentle taper about 3/16” long on one end by dragging and rolling it against a file. Then, I cut it with a tubing cutter and filed the other end. Note that the exposed end may fit into the air horn just fine if you remove the burrs only, but may not fit well once you’ve tapped it into place. Then you’ll have to file it with it already installed. (ask me how I know!)

Here are photos of the WCFB with my home made brass bushing installed:

100_5644.jpg 100_5645.jpg 100_5646.jpg
 

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Discussion Starter · #5 ·
You're welcome!

Note that 2x4 setups will need the change made to both carburetors. It doesn't matter if the carburetor has a choke - if the bowl has a choke vacuum passage, it needs the bushing.
 

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that should help someone, thanks for info. :tu
 

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You're welcome!

Note that 2x4 setups will need the change made to both carburetors. It doesn't matter if the carburetor has a choke - if the bowl has a choke vacuum passage, it needs the bushing.
Do all WCFB have a choke vacum passage? (Fitted on Trifives)

Do you know if the Rochester had any issues like that.
 

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Discussion Starter · #8 · (Edited)
Do all WCFB have a choke vacum passage? (Fitted on Trifives)

Do you know if the Rochester had any issues like that.
Yes - all tri-five WCFBs have this choke design, so all are susceptible to this problem. That doesn't mean a particular carburetor will have the problem - only that it could.

Rochester car carburetors have a similar choke design (the trucks usually had a manual choke). I would have to take apart a 2GC and 4GC to determine the likelihood of seeing this problem on them.

The Rochester 1 bbl has an important difference in its choke design. The vacuum passage uses an external tube (see http://chevy.oldcarmanualproject.com/1929_57chevyparts/57cmpc0296.htm ), so it wouldn't have this problem.

Note that this problem is probably unique to carburetors with the "high mounted" choke housing. Later designs had the "low mounted" choke housing, and probably don't have this same issue. (I would have to take one apart to see, but my guess is "probably not".)

In 1959, the choke housing was moved from up by the choke shaft to down near the throttle body (see http://chevy.oldcarmanualproject.com/shop/1959_60/59sms1009.htm and http://chevy.oldcarmanualproject.com/shop/1959_60/59sms1004.htm ). The usual reason given for this change was decreased hood clearance in 1959 required a shorter air cleaner assembly (they also went to a paper air filter in 1959), and the choke housing had to move. But, I suspect they were also seeing problems with clogged vacuum passages by then, and people were damaging choke shafts when removing the choke plate screws in order to clean the passages.
 

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Anyone know where to get some original brass bushings? Or repro's?
 

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Discussion Starter · #14 ·
One more thought on WCFBs I'd like to pass along.

All things considered, I like the '57 WCFB better than the 4GC. But, I had one problem on the WCFB on my car I'd like to share with you.

I've found that getting the secondaries to kick in smoothly on a 4GC can be a little tricky. On my '57, I found that I'd get a surging at about 45 mph somewhere between 1/4 and 1/2 throttle. It turns out it was caused by the secondaries coming in a little too soon, and the counterweight on the upper throttle plate wasn't working quite as well as it should to prevent it.

To diagnose the problem, I removed the secondary throttle linkage labeled "link" in http://chevy.oldcarmanualproject.com/booklets/55top26/5526top21.htm .
http://chevy.oldcarmanualproject.com/booklets/55top26/5526top25.htm shows the link in place with its retaining clips and washers. Once the linkage was removed, the problem disappeared.

The way to fix the problem is to carefully bend the link to make it a little longer. This is a trial and error process - bend it a little bit, reinstall, and road test.

Note that this link can be removed and installed with the carburetor on the engine. You have to work patiently, and it helps to remove the accelerator return spring. But, it can be done (I speak from experience).
 
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