VCCA Home Forums TECHNICAL - GENERAL Commercial Vehicles COE Downdraft Carburetor Conversion

We talked earlier about converting a COE to a downdraft carburetor. My 1951 COE ("Diesel Dan", as he is better known) was converted to a Rochester single barrel downdraft carburetor back around 1961. My 1951 COE 2 ton slide back has a factory 179 inch wheelbase with a single speed rear axle. The engine on board is a 1957 with a 235 cubic inch displacement.

One advantage to the downdraft carburetor on this truck vs. the original updraft carburetor is the increased power and performance. Another advantage is the major increase in gas mileage. With the downdraft carb. "Diesel Dan" gets 12.6 MPG, and he was only getting around 6 MPG with the original updraft carburetor. The main disadvantage of the downdraft carburetor (in this particular setup anyway) is that it is isolated from the engine due to the floor boards, therefore the carburetor heats up much slower than normal, and the gasoline is not atomized as well as it would be under normal operating conditions with the updraft carburetor. In the summertime this is not a problem, but in the wintertime the truck's idle is a little erratic in the midrange of the heating process until the carburetor reaches full normal operating temperature.

Here is what the downdraft setup looks like.





As you can see, the air intake for the carburetor is through a flexible hose from a special air cleaner that is located on the firewall under the hood (next photo). The air intake hose, and the hose (black in color) for the vacuum powered brake booster are connected to a special homemade carburetor air horn adaptor. A special breather hose (red in color in the photo) is also connected to this adaptor and the other end is attached to a nipple on the valve cover cap. (Note: The portion of the lever that is shown in the lower right hand corner of the photo is the lever for the PTO that operates the Tulsa winch that is located between the cab of the truck and the slide back bed. The Tulsa winch has a 20,000 pound line pull and it is used as part of the slide back operation.)





This is the air cleaner used for the downdraft conversion. It is located on the passenger side of the firewall with a flexible hose in the back of the air cleaner that runs through a hole in the firewall to the special adaptor on the carburetor air horn.

This entire setup is kind of crude, but it works, and it works well!!

You must need an offset in your right leg to operate that hummer!

Not a problem! Junkyard Dogs have four legs! :eek:

If I read correct you said the vacuum brake hose is connected to the air horn before the carb. This being the case there would not be much assistance on the brakes as the better vacuum is below the carb direct into the manifold. If the hose above the carb is the booster breather then it is 1 way of keeping contamiation out of the booster.

Yep, that's how it is hooked up all right. I noticed that myself. Amazingly, the vacuum booster does work quite well hooked up this way, but not as well as it should work if the hose had the proper connection. Hey, don't tell "Diesel Dan" that because he doesn't know the difference! :eek:

I see you attached a brake pad (?) to the gas pedal or would that now be called a "gas pedal extention" device?

I would be sleepy one day and forget and kick the carb off of the intake.

I guess it also makes the cab a non-smoking area.

Nope! That is the actual brake pedal that you are looking at. The brake pedal is located above and to the left of the accelerator, but it looks different in the photo because of the angle of the photo.

Yes, this cab is definitely a non-smoking area, and I usually have to drive the truck with the windows down and the cowl vent open......even in January! :cool2: :cool2: :cool2:

I would venture that the accelerator pedal isn't much of a problem, you just curl your paw over and around the carb, then when you reach cruising speed (40mph) you set the cruise control or a broom handle cut exactly to the correct length and Hang on,and enjoy the ride!

Exactly! Not shown in the first photo, but above the carburetor and attached under the dash is a chrome handle. That handle activates the cruise control......or actually, it is an accessory throttle with a throttle lock for cruising.

Ya, ain't old "Diesel Dan" a cute little beast with this super cool carb. setup? Actually, the manifold is a stock 1957 downdraft manifold with a Rochester Model B single barrel carb. I'll tell ya, this set up sure turns a lot of heads when dudes look inside of the cab. Most can't believe that is the carburetor.......especially when I tell them it is stock! Ha ha!

Ya, the air hose could be rerouted, but then if I did that old "Diesel Dan" wouldn't know what to do and he would probably protest by not running until I put it back the way it was. He is really set in his ways you know. :eek: :eek:

Glad that you liked the photos!

Hey JYD, Since that down draft works so well, how about going ahead and sending that nos updraft my way. I sure could use it.

Not a problem. If you purchase "Diesel Dan" and his buddy the COE parts truck will get the new old stock carb. for free! It's in the original box you know. Now, how can ya beat a deal like that anyway?

Would you say this is "function over form" at its' zenith?

Must keep your sinuses clear, now about the lungs.....

I like the idea of 'if it aint broke don't fix it', you know what happens when you start.....

Of course if we had COE here the driver would not have a problem with the carb sitting above the floor it would be the passengers problem. You lot all drive from the wrong side of the vehicle and on the wrong side of the road.

Or put in duel steering & controls like the driving schools.

JYD, now that you posted a pic of the carb, how about the rest of the truck and some other treasures you have out there in God's country.

Standby!

JYD, I think it would be cool if you found a Chamber pot, cut the bottom off and a slot for the air intake and slipped it over the Rochester, lookers-in would think you have semi-indoor plumbing! Just a thought! That carb looks like a most modern Rochester BCV? I bet that is some weird pedal linkage!

Been there and done that.....but then I didn't have enough room to get my paw on the accelerator pedal. :( :(

Ya, there is some weird carb. linkage, but it does work great. And, "Diesel Dan" absolutely loves being "weird" too! :eek: :eek:

JYD, does this mean that you will start posting pics? Man-o-man are you going to be busy!

Well, no not really. I will be posting photos from time to time, but only when I have a digital camera available since I don't own one. :( :( :(

You have all of that neat Chevy stuff that the majority of us will never get to see. What can we do????

Send me a digitial camera!

I don't have one either. Wouldn't know one if I saw it, still use my Kodak.

JYD you are nuts man !! [ but I like that quality in a person ] how da hell can you operate that thang with the carb in front of all the pedals ??
Ya got me thinkin' though...........