The Stovebolt Site Forums The Parking Lot Big Bolts '55 GMC COE carb

Hey guys, I just aquired a '55 GMC 350 COE. it's the strangest thing I've ever seen how the carb is set up. anyhow, I need to get this thing running so I can find a new home for it, the carb hangs down below the manifold...I have no idea how this works. Any input would be nice to help me understand.

I'll get some pictures of it sometime today and put them in my webshots

Standard procedure on a COE. That's a Zenith 63AW updraft carb if it's original. If the carb has been sitting for an extended period, it's common for the accelerator pump to corrode. If you have full open and close movement on the carb butterfly, you're probably OK.

This is the carb that is on my Forward Control single barrel updraft carb (Carter BB-1)
http://www.tocmp.com/manuals/index.html
The Old Car Manual Project home

this is the one that is not on line I hope it will be soon
Carburetors - index
Carter
coming soon
Maxx

Keep it Travis, & join the ever exspanding COE
Fraternity. Looking forward, to seeing pictures of it.

OMG, this thing is a pain the arse to work on. I saw that it's a Zenith carb by the way and thanks to all who took the time to respond.

I can't get the cap off the master cylinder so i have to remove it to further struggle with the cap...there are bolts behind some tin, so i'll remove the tin to get to the MC nuts that are attatched to the bolts that hold the master cylinder on. Still searching for the digi camera.

I was told "it ran when I parked it"

it's been sitting at least 8 years
anybody in the Wenatchee area ???

I'll be back

Oh boy, I don't think I like this COE thing very much...I'm way too spoiled being able to reach everything under the hood of my others trucks.

I think I have it figured out how to get to the carb. I'll certainly learn a thing of two working on it/ cleaning it out after sitting for a few years.

I'm at my parents place this weekend and can't seem to get into my webshots account...forgot the password so I'll get some pictures up for whoever is interested in seeing it when I get home

Oh no doubt, they can be a pain to work on, but you forget all about that, once you have them running, & can go out on a drive with them. I recently installed NOS Updraft carb.'s, on my '53 Chevy COE, & my '52 GMC 450 COE. The Zenith Updraft, on the 450, is larger, & heavier, than the one on the 350 COE. Can't wait to see pictures of yours. Here are my Webshots, http://community.webshots.com/user/snubnose4754

Spanky, WOW...you have done well with yours huh. I took some pictures of this thing, the more I tinker with it the more I like it. it's just too darn cool IMO. I got through one of the sets of wheel cylinders, pretty easy job.

One thing I found just to f'in cool is the GMC letters just like the letters on the front above the grill are stamped right into the rear axle cap... I have not ever seen that before...so it's new to me. later.

http://community.webshots.com/user/squall_5053

Very Nice. All in all, one of the best looking COE's ever made IMO. Looks like it's in good shape too.

Nice look'in COE Travis. It definitely has potential.

I spent another day tinkering with the '55 COE, got the MC off and rebuilt and took the carb off and cracked it open. I really don't understand this excel pump on this. it doesn't move much...maybe an 1/8 and only the top part moves at all. Rusty said something about that. the carb looks very simple. Rusty, will you look at my webshots, I took some pictures of the carb inside and out. please give me some idea what to do with the excell pump.

One more thing, I was given this truck because it sat for some time and the guy isn't going to haul fruit on it anymore. I thought it is a piece of history that needs to be saved from the rust and from the scrapper. other than that, I'm repairing the major componants with the intention of selling the truck to somebody who will keep it in good shape.
My passion is the WW2 military 4x4's and 6x6's and the GMC V6's

Hi Travis, the best thing to do with that accelerator piston is to fill that cylinder with a good penetrating fluid like Kroal, and let it sit. That piston should come completely out of the cylinder it's in, and move freely up and down. Take your time and let the penetrating fluid work. You may have to push the piston down slightly before you bring it back up. Just go really easy to free that piston up. If you break off that control arm you may have to buy a complete carb to get another piston assembly at a reasonable cost. The longer you can let in sit in penetrating fluid the better. If the fluid stays in the cylinder, it has not penetrated long enough.
Otherwise, the carb is easy to rebuild. Remove all the jets, soak it in carb cleaner, blow out all the passageways with compressed air, make a new gasket and reassemble.

Travis - maybe I can help you understand the accelerator pump.

This is a VACUUM accelerator pump. There is no direct linkage from the throttle to the pump. The pump operates as follows:

(A) when the engine is running, engine vacuum is applied to the vacuum cylinder in the top section of the carburetor.
(B) the vacuum pulls the top vacuum piston up into the cylinder. Some models have an adjustment screw that will allow adjustment of the depth the upper piston is pulled into the cylinder; some do not.
(C) as the upper piston is pulled into the upper cylinder, the operating spring is compressed.
(D) when the operator increases the RPM of the engine, vacuum decreases (normally goes to zero under wide-open throttle conditions).
(E) as the vacuum decreases, the stored energy of the compressed spring overcomes the force of the decreased vacuum, allowing the lower piston (the pump) to be forced down into the well, and pumps fuel into the pump jet to be sprayed into the throttle area.

Note that since there is no direct connection from the throttle to the pump, pumping the footfeed prior to attempting to start the engine results ONLY in the exercise of the operator's ankle. A vehicle with an updraft carburetor should ALWAYS be cold-started using the choke. With a warm engine, no choke should be necessary.

Rusty is incorrect about buying another carb to obtain a pump. Pumps are available, but expensive. You do NOT wish to purchase one except as a last resort.

Rusty mentioned that this is a Zenith 63AW series. Assuming he is correct, the body of the carburetor will be cast iron. After removal of the float (soldered) and the venturi (diecast); heating the pump well with an acetylene torch will help to free a stuck piston. The principle is quite simple. The piston is stuck because of corrosion (oxide). Heating the outside of the pump well will heat the corrosion and burn the oxygen from the molecules of corrosion; thereby reducing the space occupied by the corrosion molecules. This will help the penetrating oil to penetrate.

And the Zenith 63AW series is one of the finest updraft carburetors ever made!

Oh, and one caveat! Try a magnet on the carburetor to make sure it is cast iron BEFORE applying the torch. Some of the replacement 62 series were diecast.

Jon.

Ya know, this is really just too darn cool to be able to ask a question and get so many answers from people trying to help a guy in need.

This carb is a Zenith part number 11646, there is a brass tag rivited to the top of the body. this is the part number the man at NAPA asked for when looking for a new gasket.

I now have a better understanding of the pump mechanism and will do as directed to free it up. Thank you all for the replys to my question...I'm sure I will ask a couple more regarding ignition by the time it's up and running.

CarbKing, thanks for a great description of that updraft carb. I too had a cabover at one time. I found it to be fascinating but a little frustrating to work on due to the contortions one had to go through to access it. Next time I need a carb rebuid it goes to you.

You guys should look forward to working on the cabover, because of all the room available :p

When I was much younger (and more foolish), and had not yet learned the importance of buying products produced in one's own country, I owned a Jaguar XK-120M. The lower radiator hose needed replacing. "I can do that :o ". Eight hours later the hose was replaced, and both my hands and arms resembled hamburger!

The car, and the joys of working on the car, are now memories!

Jon.

Carbking, I did as you instructed, heat along with penetrating fluid. it's been 3 days now and no go. I tried lightly tapping on the rod to jar it loose, still no go. I guess I'll soak it this week and see what happens.

I got it. It took some force to move it down then it came right out.