VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 I cheated, but she runs fine

First of all, thank you to the guys that sent me email wondering where I have been. I hope to be back more often for a while.

Some will recall that I was having problems getting my '49 Chevy 4400 grain bed truck to run without a relatively bad but eratic miss. She runs well now, but I cheated. But there are some here who will know what the original problem is when you hear what I did to make it run smoothly. Tonight, I will test the gas mileage.

OK. This is not legit, and I know it, so don't beat me up.

I losened the distrubutor and started the truck. I pulled out the choke until it ran smoothly. The timing light indicated that the engine was firing ahead of the ball. The ignition was advanced. Then, of course, the engine was running many more rpms than one wants at idle. So, I tightened the distributor and adjusted the carb until the engine had a good idle speed.

I drove the truck to the gas station 5 miles away and the engine sounds great and runs great. Gas mileage tests scheduled for tonight.

The disadvantage is that there is that 'sweet spot' in how far I pull the choke out where the engine runs very smoothly.

It seems reasonable that this test indicates something isn't right with the carb. And the carb I'm running is a new carb (I took it out of the box myself) that a friend loaned me.

I did all the tests for leaks in the manifold-to-head joint, the manifold to carb joint, et cetera. No leaks. I did many other things as well including basically replacing all the components in the ignition system and so on. Vacuum at the wiper port is 18 inches mercury.

The long and the short of it is that the engine will run well under the present conditions.

Lee

I don't know for sure but the symptoms and "cure" highly suggest a poor or late spark at the plugs. The main reason is that it is much easier to ignite a rich mixture than a lean one. It could be wrong plugs, weak ignition, late timing or any one of similar situations. Of course late valve timing, weak valve springs and similar things can contribute to the problem.

You could have done the same thing by just moving the distributor around untill it runs the best.
At a speed over idle the timing will be advanced due to the vacuum advance is reveiving vacuum and that speed and the weights are beginning to advance the timimg.
Often you are better off just timing it where it runs the best and forgetting about the timing light.

Chev Nut, I did that ad infinitum and never could get the result that I have now. This time all I used the light for was to see how the timing was, not to actually time it. The engine essentially timed itself.

Lee

Thanks, Chipper. Of course it has new points, condensor, plugs, plug wires, distributor cap. All of which made no difference.

At first I believed that pulling the choke out like that could be compensating for a vacuum leak somewhere (manifold to head or manifold to carb). If the gas mileage is still way low (I'll test it tonight) then too much gas is going through the engine.

The head was done by a really good guy in Pipestone, MN. I would be surprised if it had weak springs, since they don't have but about 400 miles on them.

Lee

Ok, I got 10 mpg with the timing as I set it. Listening to the sound of the engine, it was clear that 30 mph was the point at which I would get the best gas mileage. At 30 mph, the engine was not over-revved and had good power. I drove 84 miles on 8.6 gallons. And the truck is loaded with stuff as I am moving from South Dakota to Oklahoma soon.

I need to do the timing the right way now. I'm an old tractor guy and I've never had timing problems like this.

Although I am running with the choke out, I adjusted the carburetor to compensate for the fuel/air mixture. So, I don't believe I am running with a too rich mix. But, I am concerned that cold-weather starting will be difficult when a 'normal' choke is required.

Lee

Glad to hear from you Lee.

Am not trying to tell you what is wrong, but when received my 48 Chevy car, had same problems with it. Would only run if choke pulled out also, but no good idle. Threw away the W-1 it came with and got rebuilt one from Chevs of the 40s. Installed it got the identical results. Needless to say, went through all the same things as you did as to vacumn leaking checks everywhere and jacked with timing, vacumn advance, etc. Called Chevs and they immediately sent me another carb and than installed it and to my dismay, it did the same thing. After fighting problem for at least 2 weeks, bought a used Rochester off e-bay, put it on and walla, all problems cured without doing anything else. Gave me the best service I have ever had for 20.00.

All I am saying is don't rule out the carb just because it is new in the box.

Hows the Allis back doing???

Jim.

I was not going to say "Get a Rochester model B!", but I have been thinking that, there are so many vintage Chevrolet owners (pre-1951) that all worship the Carter W1 carbs. The first thing I did back in 1952 when my dad gave me a 34 pickup was put on a Rochester "B" rebuilt. I have never got a W1 to run decent on any of my favorite 51, 52 or 53 cars or trucks. I have heard that the only reason that GM went to the Rochester B was that they were cheaper, but that isn't what is in all the literature.

Hmmmm.... I will verify what kind of carb I have on it. I believed it was the Rochester B. But, lemme look again and compare it to the drawings in the online manual.

Be back soon.

Lee

The carburetor is a rebuilt one from Canada. It has a metal tag on it with the number C-900 on it. The site that has the online chevy manuals is evidently down right now.

On the front of the carburetor just below the air cleaner is an embossed square with RP in the upper left-hand corner, GM in the lower right-hand corner and the word "Rochester" written from the lower left corner to the upper right corner.

I take this to indicate that it is a Rochester carb.

Lee

like this?

Yes, Dads, that looks like it.

Lee

The one on my car has no metal tag, but the same embossed square on front of carb just below air cleaner. The only difference, if any is that mine has a "C" instead of the "RP" in the upper left corner of the square. Mine sits on 216 intake/exhaust bolted to 235. Runs flawlessly.

Jim.

Thanks, Jim. That might be significant. I'll check on that some more......

Lee

I may have the wrong Rochester carburetor. Mine doesn't look like either the Carter downdraft carburetor:

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0649.htm

Or the GM Model "B" carburetor:

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0658.htm

The GM B carb has different venturis and throttle bore for 216 and 235.

I don't believe I have the right Rochester carburetor. My engine is a 216 block that has been bored to accept 235 pistons. So, it should have a 236 carb if there is a difference

Lee

I have the wrong carburetor. There is a Rochester RP GM, which is what I have.

http://shop.ebay.com/sis/_W0QQ_kwZV...woQ20CarburetorQ20sQ20CarbQ20forQ20Parts

Lee

Lee, I am sure you know the difference, but remember that a 235 carb will not fit your 216 intake. The mounting holes on a 216 are, I believe 2-5/8 center to center and the 235 carbs are, I believe 2-15/16 center to center. I am probably wrong on these measurements and someone here on Chat can give you the correct ones.
If you need more fuel for the bore job, you may have to re-jet a stock 216 carb, but I'm thinking that the way mine runs with stock carb, that may not be necessary.

Best of luck Lee.

Jim.

Jim.

The 1941-1949 235truck engines used the same Carter W-1 carburetor as a 216....it had the same jets, etc.
There is no reason a newer 216 Rochester would not work on an engine bored oversized.
The replacment Rochester sold for the above 235 engines was also used on a 216 with no mofifications.
I would doubt the condition of a new, out of the box 40 year old carburetor. From my experaince there are often passages and jets plugged with white fuzz from the pot metal casting. If it were mine I would do a simple rebuild on it.

Thank you, Jim and ChevGene.

ChevGene, The online manual indicates that the venturi and other things about the GM Model "B" carburetor are different for 216 and 235.
http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0658.htm

However, the same manual seems to indicate that the Carter downdraft carburetor is the same for the 216 and 235.

I was ignorant of the Rochester carburetor models, and still am to a lesser extent. The carburetor I am running is a Rochester RP GM carb. I don't know if that model is the same as the Rochester C or what the differences are. I've looked online for Rochester carburetor specs, but I haven't come up with anything yet.

Lee

P.S. I really love that Chevy truck. My lovely grandfather had a '52 1/2 ton with a visor (He believed it was a '52, but someone on this site said the visor wasn't available until '53). The cab is exactly the same on my 1.5 ton as his 1/2 ton. I didn't know I was going to get so enthusiastic about old Chevy trucks when I started.

Now Gene is the expert on this stuff and I'm sure he will be along to straighten all this out for you. But the manual you’re reading from covers 1948 thru 1951. The 1948 and 1949 engines used a Carter W-1 carburetor and the bore was the same for the 216 and 235. 1949 was the last year Chevrolet used the W-1 and it was a little different than the earlier versions in that it had fast idle linkage added to it. At the tail end of 1949 the Rochester G.M. Model "B" was introduced in limited quantities on some of the engines and became standard equipment on the 1950 models. Many improvements to the cylinder heads were made for the 1950 models and with them were two different versions of the Rochester “B” carburetors. The throttle bores and internal jetting for the 216 and 235cu in engines were different.
This is all covered in the Chevrolet Service News.
And again, 1950 was a transitional period for the engines and I’m sure Gene will have a lot more to add to this.
Denny Graham
Sandwich, IL

The orginal carburetor (I still have it) is a Rochester model "C" GM. When I get moved back home, I will have it rebuilt.

CheveGene, what is the difference between the Model RP and the Model C?

Lee

The RP stands for Rochester Products Division of General Motors....thats all.

So, it is the same as the Rochester "C"?

Lee

There is no Rochester "C" model Lee, the 'C' that you see on the casting is not the model of the carburetor. It does have something to do with where it was manufactured just as the RP stands for Rochester Products. I remember reading it somewhere and I printed it out, but I simply can't find it in the pile now. I've been looking for it since I started reading this thread and if I run across it I’ll post it.
What you do have it sounds like is just a version of the Rochester "B", this thread is getting drawn out so I may have missed it, but have you posted a picture of it yet???
I certainly no expert, there are some here that are, but as far as I know in this series there were only the “B”, “BC” and the “BV”. All of them looked basically the same from late 1949 all the way up into the 60’s except for minor changes in the air horn and choke arrangements.
Denny Graham
Sandwich, IL

His 1949 would have had a Carter W-1 when it was new.
Supposedly a limited number of very late 1949 engines made at the Tonawonda plant had the Rochester but I have never seen one and the parts book dosen't bare that out.
The 1950 and 1951 low cover was a model B. The 1952 and up with a hand choke and high cover was still a model B and the automatic choke jobs were a BC.
Up to 1954 there was the brass tag with the carb. model on it. Was under one of the cover screws. From 1954 and up no tag was used but the corner of the bowl cover gasket was exposed and the modle number was stampe into it. These are the numbers that had ID for the application.