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I just took my '47 1/2 ton (216.5 w/Carter W1 carb)through my local Air Emissions Test and failed on high carbon monoxide readings at speed (idle CO was fine) High CO at speed is a problem with carburetors apparently. Has anyone had success with lowering CO levels? Any suggestions greatly appreciated. Thanks.

Give her a tune up, and a tank of premium. Works for me.

Lower the float level a little, and/or go with a slightly leaner main jet. The high speed system is too rich, and it's not adjustable like the idle circuit. You'll need to find someone with a 4-gas exhaust analyzer to get an accurate reading on the high speed mixture. One of the ones I have in my shop can be operated on 12 volts DC, so I can actually monitor mixture during a road test under load, as well as high RPM operation with minimum load in the shop. It's a good idea to check both, so you don't end up too lean, and burn pistons and valves in a lugged-down situation.
Jerry

A wide bank oxygen sensor will also work. You will have to put a bung in your exhaust pipe a few inches below the manifold in order to have a place to put the oxygen sensor. There are a couple of brands on the market now. You don't have to leave the unit in place after you've tune your truck. But a tail pipe sensor like Jerry described above is probably the best bet for a one time tuning episode.

What is amazing and what people may discover because of the high price of gas, is that you can run an engine at cruise speed a lot leaner than you may have thought. These old engines only make about 30 to 50 horsepower at cruise, right? A lean mixture at low power doesn't hurt a thing.

Joe
Joe

Wow, emissions testing required on a 47. Just when I thought there was somewhere to escape Arnold and the land of fruits & nuts I find out older vehicles in Canada require testing.
It's required on 76 & up here.
Fred

That is only in BC where they do such things. I don't know anywhere else in the universe where such foolish harassment is rained down upon classic vehicle owners.

don't blame BC nix, it's only the Vancouver area that has 'Air Care'

Bill

At what speed did they run your truck? And in what gear? Are vehicles classified and tested by a Weight Class? They may have run your vehicle at a higher weight class than it should have been. Does that system require a retro-fitted PCV system? If so, change your engine oil before the next test. Did you have your choke on during the test? Even a small amount of choke during a running test will cause a large increase in CO. Make sure the choke is fully open when the knob is fully against the dash. It was not uncommon for the choke wire to break at the carburetor attachment, then someone would pull choke lever far enough to re-attach the choke wire. The 'little bit' may leave the choke slightly off the fully open position. What was the CO level from the test?

Before taking your carburetor apart, make sure the air filter is clean. Otherwise, adjusting the float level down a little, or going to smaller main jet may help. I think those carburetors typically ran with about 3%-3.5% CO at cruise- that is about a 13:1 air-fuel ratio, just above the rich-best-power mixture. If I remember correctly, your carburetor has a metering rod setup for power enrichment. If that spring is weak, the power circuit will turn on sooner than it should. The inspector may have been running your truck at such a condition that it was operating in the power circuit, in which case, they should be able to run it at a lighter load. You may have to argue with someone about that. And that may be like talking to a wall- but sometimes they can be reasonable.

Harvester

The truck was run in 3rd gear(out of 4). The technician had it in reverse the first couple of times) and it ran at approx. 25 m.p.h. The weight as tested was about 3170 pounds (1438 kilograms). It took a while for them to figure the Cu.in. conversion. They figured at 3.6 Liters. The standards are for pre 1960. The allowable amount is approx. 4.9% my reading was at about 7.3%. Should I play around with fuel changes etc.? Will a new carb (Chevs of the 40's - $160 plus core) bring levels to where they are supposed to be?

Without reading all the posts, I'm going to blame it on running too rich. You could compensate for that at idle to some degree, depending on the carb. Unburned gas could cause that. Also hydrocarbons might be too high.

But why in the world do you need to pass emissions on a truck that old? Do you live in California?

Rusty, I don't think you need to get into a new carburetor. The 4.9% CO is pretty lenient, and it sounds like they ran the test properly. Did the inspector let the speed stabilize before taking the reading? That is usually under computer control, so as to prevent any manipulation of test results, but a brief acceleration at just the right time during the test might have gotten the accelerator pump discharge involved in the reading. That shouldn't happen, but depending on how the sample system is timed and programmed, it is possible. What CO level did you get at idle speed?

Possibilites for your condition are: restricted air cleaner, choke not fully off, float level is too high (good possibility), main metering jet is too large (possible, but not the first thing to get into), carburetor base gasket is improperly installed, or is the wrong gasket. Either situation with the gasket will block the vacuum passage to the metering rod piston, which will keep the metering rod in the rich position, and would put the CO in about the 6% range.

With "fuel change," I'm assuming you mean regular or high-test gasoline. No, that won't help. There is something going on with your carburetor.

Harvester

Put studded tires on the back and go back to the testing place. They will only do an idle test, they will not run studded tires on the dyno and cant make you remove them either.

My friend did something to his car every time he went into air care he either advanced the timing or retarded it. It ran like SH>> for a while but then he put it back to normal..does that makes sence..Peter

R S, Here's a fast simple almost stupid little trick you could try.
Create a small vacuum leak at the intake manifold. This will lean out the motor. Might run a little rough but they're not going to care about that.
I did this on a car I had which would never get through a sniff test because of having one bad cyl. I tuned that thing and did everything I could to past the test, after the fourth try I said screw them and pulled a vac line and went back and passed.
Dan

I like the studded tire idea.

Harvester

One major flaw in the studded tire idea, in many places they are illegal. Unless you can prove that you are a rural male carrier, and then they're only allowed to have them during certain months of the year. So if you have them on during a time they are not supposed to be on, they can make you remove them and maybe even fine for running them during an unauthorized time of year.

Not a major flaw here. You can run studded tires till May in BC. I still have 1.5 ft of snow in my yard and it snowed two days ago. Welcome to Canada!

Here's a quick and dirty fix that might get you through the immediate problem. Run about 50% alcohol in the gas tank. Get the gas gauge down to almost empty, only a couple of gallons left, and dump two gallons of methanol into the tank. It's sold at most hardware stores as "shellac thinner". It's got a much lower specific gravity than gasoline, and it will lean out the mixture accordingly. Just don't get into the habit of runny alky, as it's rough on carburtetors as a steady diet.
Jerry

I wasn't sure about Canada. Here they are illegal except for the mail carriers, I think they're only allowed to March 31 to run them.