[stoveboltgeek]

I'm slowly starting to resume work on a long-dormant restoration ('55 Bel Air 2 door sedan, 6 cylinder, 3 speed, low mileage original car). When I bought the car, it ran okay except for a really rough idle. I rebuilt the carburetor, which mostly solved the problem, but it would still occasionally run rough momentarily at idle.

Having recently pulled out all kinds of goodies as display items for a "Carburetors 101" session presented by a club member who spent his career designing Rochester carbs and fuel systems, I got motivated to dig into this carb and see if I could solve the rest of the problem. In the process, I discovered what appears to be a manufacturing defect and an important part missing from the carb rebuild kits of the era. But, I digress...

Now, being a reasonably intelligent individual, years ago, I ran the engine out of gas before pulling it. But, as I just found out the hard way: a) running out of gas doesn't mean there's no gas left in the carb; and b) said remaining gas may not evaporate.

What gas had remained in the carb had turned into that rock-hard varnish mess. I now had a gummed up carb where the only thing that still moved was the choke plate. The gaskets were stuck. The float was stuck to the bottom of the bowl. The accelerator pump, power valve piston, check balls, and throttle shaft were all stuck solid.

Oh crap.

But, I was able to get everything freed up and cleaned up without damaging a single part. So, I thought I'd share how I approached it.

 

[stoveboltgeek]

To understand the situation, here's the exploded diagram of the carb from the Master Parts Catalog. Note the names and locations of the various parts.

http://chevy.oldcarmanualproject.com/1929_57chevyparts/images/57cmpc0296.jpg

 

[stoveboltgeek]

When I first tried to get the air horn off, the gasket wouldn't let go. Okay, no big deal. I'd had good luck getting them loose with a shot of PB Blaster. Shot it around the outside edge, came back in an hour, and still wouldn't budge. Uh oh. So, I soaked the outside and the carburetor throat. Then I shot a bunch down the bowl vents (which turned out to be a wise move).

A couple hours later, I was able to get the air horn loose. That's when I discovered the float had been stuck to the bottom of the bowl. Fortunately, the PB Blaster I'd shot in the bowl vents had loosened the gunk to where the float was almost free.

Once the air horn was off, I realized everything inside was gunked up solid. I decided to call it a night.

The next day I hosed everything down with PB Blaster. I got the accelerator pump loose, but the power piston was stuck solid. Now, the power piston has two passages - a vacuum and a carb throat passage. I tried putting PB Blaster in the holes, but I mostly got foamy spray, and not much in the passages. The piston was still stuck solid. Okay, this is a problem.

 

[stoveboltgeek]

That evening, I got a bright idea. I need an old-style oil can with spring bottom. Next day, I bought the only one available locally: Goldenrod® Spring Bottom Oiler 6 Oz. Capacity 4 1/2
and filled it with PB Blaster. I figured - if the PB Blaster didn't work, I could try lacquer thinner. (Unlike a plastic syringe, lacquer thinner can be used in a metal oil can.)

Now, PB Blaster is a whole lot thinner than oil, so you don't need to squeeze the bottom - it'll run out the hole much faster than you need for these purposes just by tipping it over. I filled the vacuum and carb throat passageways with PB using the oil can. After several minutes of messing with it, the power piston came loose. Using the same technique, I was also able to get the power piston check ball and spring loose.

The accelerator pump check ball proved to be a bit trickier. I filled the passageway wih PB, but the check ball is way down in the hole and wasn't inclined to fall out on its own. I considered compressed air, but was able to get it loose with a wooden toothpick.

With the carburetor completely disassembled - without damaging any parts not found in a rebuild kit - it was time to get rid of the PB Blaster. I hosed everything down with a can of carburetor cleaner.

 

[stoveboltgeek]

At the beginning of this thread, I mentioned an apparent factory defect and a part missing from the carburetor kits of the era. BTW - I bought a spare kit around the same time from the same manufacturer, so I was able to confirm said missing part.

When I rebuilt the carburetor years ago, I found the power valve had been done incorrectly. The check ball was jammed inside the main well support so badly, I had to knock it out with a very small punch.

The ball was clearly too big, and had caused all kinds of problems. The PO had put the bigger accelerator check ball in the power valve, and put no check ball at all in the accelerator pump outlet. When the idle was much better but still not perfect, I figured the main well support had been damaged by using the too-big ball.

I had since bought a parts carb at a swap meet for all of $10, so I was going to swap the well out of that one into this one. But, when I tore everything apart, I was in for another surprise.

The power valve has 2 concentric holes. There is a small one for the check ball, and a larger one for the spring and retaining nut. The parts carb check ball fit nicely in its support. My "good" carb's check ball didn't go down into the smaller hole. As I experimented, I discovered both power valve check balls fit properly in the parts carb's hole, but neither check ball fit my "good" carb.

I put each check ball in each support (without the spring) and then shone a flashlight up the power piston hole. I could see light around each check ball when put in my "good" carb's support, but no light when using the parts carb's support.

Because there wasn't a good seal, my "good" carb was essentially running with a partially open power valve all the time.

Given that both main support wells had the same casting number, I can only assume my "good" carb was incorrectly manufactured. The only thing I can figure is someone used a smaller-than-spec check ball as a workaround when it was assembled at the factory, but that check ball was discarded when the carb was previously rebuilt.

The weird things we find...

I also discovered the choke suction tube packing was missing from my "good" carb. Looking at the carb kit I bought and tucked away years ago, I discovered it didn't have said packing. If the packing wasn't there when I rebuilt the carb (on account of not being in the first rebuild kit), and there wasn't a packing in the kit I used, I wouldn't have thought anything about it when I put it back together.

The problem, of course, is the choke suction tube exits below the throttle plate. The absence of the packing presents the opportunity for a vacuum leak.

A leaking power valve and a vacuum leak - this might explain the idle problems I'd been unable to correct...

 

[stoveboltgeek]

As a continuing part of my research into getting everything right, I was able to find a NOS carburetor kit made by Rochester. Now, I have no plans to use this kit for the rebuild; rather, I plan to keep it as a reference for comparing the parts found in current-day aftermarket kits.

The Rochester kit does include the choke suction tube packing. I'm checking with both Daytona Parts and Quadrajet Parts to see if their kit contains said packing. The photo of the current Standard Motor Products kit found on Rock Auto appears to have the packing, but the unopened one I had from years ago does not.

Anyway - once I buy a new kit, I can compare the parts to those found in the NOS kit.

 

[SeeTheUSA]

Enjoying the thread, keep us updated on your progress.
(PB Blaster is a must have tool when restoring these old cars.)

 

[angs1957]

Was the stumbling being caused by the incorrect accelerator check ball?

 

[stoveboltgeek]

Enjoying the thread, keep us updated on your progress.
(PB Blaster is a must have tool when restoring these old cars.)

Will do. I won't be able to do anything on it the next few days, but will follow up. I thought of a couple things that might be worthwhile to show. And, if anyone has questions or wants to see something in particular, just say so.

Was the stumbling being caused by the incorrect accelerator check ball?

The consistently bad idle was corrected by installing the missing accelerator check ball. There was still an intermittent momentary bad idle afterwards. It would run fine, cough for a moment, then run fine again. It literally sounded like the engine had a cough! So, there's clearly other things amiss - two of which I've already found.

The missing accelerator check ball should not be confused with the incorrect check ball in the power valve.

Other than the idle, the car ran fine. And, it never stalled on account of the idle problem.

I'm definitely on a hunt for clues. Finding the culprits in this case is truly "the Devil is in the details" as described here: troubleshooting - the Devil is in the details

 

[Churchkey]

Nice write up.
Here's my shop go to = a gallon can for soaking it comes with a wire basket for small parts.

Carburetor & Choke Cleaner 1 GAL MAC 6402 | Buy Online - NAPA Auto Parts

Buy Carburetor & Choke Cleaner 1 GAL - MAC 6402 online from NAPA Auto Parts Stores. Get deals on automotive parts, truck parts and more.

www.napaonline.com

 

[Granpa49!!]

My carb had sat for 42 years but was in no way gummed up like yours. Must have something to do with gas. Ethanol had not been incorporated into our fuel supply when I last drove it. However there was a little orange residue in the bottom. I put water and Simple Green in my ultrasonic cleaner and ran it with the hot water feature for several minutes. Parts looked like new. None of the dull finish etching done by carb cleaners. I squirted aerosol carb cleaner thru all of the passages just to be sure.

 

[stoveboltgeek]

Nice write up.
Here's my shop go to = a gallon can for soaking it comes with a wire basket for small parts.

Carburetor & Choke Cleaner 1 GAL MAC 6402 | Buy Online - NAPA Auto Parts

Buy Carburetor & Choke Cleaner 1 GAL - MAC 6402 online from NAPA Auto Parts Stores. Get deals on automotive parts, truck parts and more.

www.napaonline.com

Years ago, I read some of the carb cleaners were powerful enough to actually damage the metal carb parts, so I've always stayed away from these dunk-type cleaners (using spray cans instead). If this doesn't harm the metal, it's worth looking into.

My carb had sat for 42 years but was in no way gummed up like yours. Must have something to do with gas. Ethanol had not been incorporated into our fuel supply when I last drove it. However there was a little orange residue in the bottom. I put water and Simple Green in my ultrasonic cleaner and ran it with the hot water feature for several minutes. Parts looked like new. None of the dull finish etching done by carb cleaners. I squirted aerosol carb cleaner thru all of the passages just to be sure.

Hmm. I've heard of using ultrasonic cleaners, but never looked into it. Maybe I ought to...

My carb - I ran it till the engine quit, but didn't remove the carb and pour any residual out of it. It had a layer of what looked like caramelized sugar on the bottom. Or, picture maple syrup, but hard as a rock. I'll see if there's still enough left to get a good picture of it.

 

[Granpa49!!]

Harbor Freight - $85. It was just barely big enough to get both halves of the 6 cylinder single barrel carb in it but it worked great. I don't know if a quadrajet will fit in it. You can see youtube videos on using simple green in ultrasonic cleaners for carburetors. That is where I got the idea.

 

[Dragsix]

Ted, dont mess around here, Get in touch with Jon at the Carb shop and get one of his kits. He makes his own kits and they are the right kit with all the right parts. He rebuilt carbs professionally for years, happy to give advice, posts ofter over at stovebolt. He is also a wiz with carter carbs, has probably the most extensive library of carter factor literature and production materials, and owns quite a few of the Carter production tools and machinery. A good guy to deal with. I think you will be happy getting a kit from him.

THE CARBURETOR SHOP / Carburetor rebuilding kits

 

[stoveboltgeek]

Ted, dont mess around here, Get in touch with Jon at the Carb shop and get one of his kits. He makes his own kits and they are the right kit with all the right parts. He rebuilt carbs professionally for years, happy to give advice, posts ofter over at stovebolt. He is also a wiz with carter carbs, has probably the most extensive library of carter factor literature and production materials, and owns quite a few of the Carter production tools and machinery. A good guy to deal with. I think you will be happy getting a kit from him.

THE CARBURETOR SHOP / Carburetor rebuilding kits

That's quite a recommendation! I'll contact him next week.

 

[Dragsix]

Jon and I chat quite a bit on the Stovebolt website. He really is one knowledgeable guy.

 

[stoveboltgeek]

From the "you can't make this stuff up" department:

Same casting number (7005926), same diameter check ball - one goes down in the well, and one doesn't.

Now, you may be thinking, "if you're such a smart guy, why didn't you notice that when you rebuilt the carb?".

Good question.

It turns out - the edge that the ball on the right is sitting on is beveled; it looks just like a properly machined seat for a check ball. Having never rebuilt a Rochester BC before, I just assumed that was where the ball was supposed to go. It wasn't until I pulled apart my parts carburetor and compared the two that I realized the problem.

So, why is that area beveled? My best guess is you get better flow (less turbulence) than if it had a sharp edge.

 

[stoveboltgeek]

So, the next dilemma is - jet size.

Being a correct '55 manual transmission carb, it's currently equipped with the 7002957 jet (stamped "57"). So is my parts carb.

But, there's an interesting discrepancy in the factory documentation.

The Master Parts Catalog lists 7002957 for '55 1 barrel: 1929 - 1957 Chevrolet Master Parts & Accessories Catalog

But the Rochester carb data sheet lists 70002958 for '54-56, including '55 (the '58' being one step richer than the '57'): RCB-9C-306

I checked my paper copies of the Master Parts catalogs from 1957 and 1962 versus my Rochester catalog from 1967. The inconsistency is consistent.

I started poking around last night, and found a discussion on stovebolt.com about 7002958 jets. The first reply is from none other than Dragsix, who points out that both '54 and '56 used the '58' jet: Jet size

Thinking about this some more: when I was first looking for a '55, I wasn't satisfied with the performance of '55 6 cylinder Powerglide cars. I found '56 and '57 6 cylinder PG cars to be better.

Could it be that the difference in performance can be traced to the bigger jet?

The '55 in question is my second '55. My first '55 was a beat-up 4 door, with a somewhat worn 265 V8, 2 barrel carb, and PG. I found my 235 6 with a 3 speed stick performed about the same as that 4 door, so I was happy enough with it.

As I mentioned - even after the rebuild I did years ago, my carb's power valve had (unbeknownst to me) a slight leak 100% of the time. Could it be that a slight leak plus a '57' jet was effectively the same as a '58' jet?

So, I'm seriously considering going with the '58' jet when I put it back together.

 

[Granpa49!!]

Interesting stuff. I'm building a '61 truck 235 for my '55 and I plan to use the '55 manual choke carb so I can use the '55 car air cleaner. Do you think there is a difference in the jets from '55 to '61?

 

[stoveboltgeek]

Interesting stuff. I'm building a '61 truck 235 for my '55 and I plan to use the '55 manual choke carb so I can use the '55 car air cleaner. Do you think there is a difference in the jets from '55 to '61?

My Rochester catalog says "yes".

1953-58 truck 235 uses 7002958. 1959-62 uses 7002956, 7002943 or 7002655, depending on which carb you have.

It's worth noting that Rochester was playing around a lot with jet size in that era. The 1959-61 car BC carbs varied from 7002954, 56, and 57 jets, as well as 7002642. There were undoubtedly tweaks in the fuel bowl, air horn, and throttle body at the same time, so what goes with what requires some careful research while assembling parts.

Maybe Dragsix knows the difference between these later jets (I don't).