Truck is 67' but has a 71' 240 w/ 4sp manual.

When I first got the truck I took it for the test and it failed for high HC both under load and at idle. Under load HC was 848 and at idle was around 814 and both load and idle CO numbers were ridiculously low, like .20. I replaced the entire distributor (so it has new cap, rotor, points (set at 25), VA, etc), plugs (set at 35), wires, pcv, filter, cleaned and rebuilt carb, also did timing.

After all this I went back to retest and here are the results:



The tester said my idle was probably set low based on the fact that the truck did not idle smooth enough in his opinion. He said turn it up and see what that changes. So I went around the corner turned up the idle a bit and went through for another test which resulted in my idle HC being reduced some (not enough) but it also raised my under load CO to a fail level which was never a problem before.




Should I go back and try and redo my timing, is the timing the issue here? I set my ignition timing to -6BTDC at idle with the vacuum unplugged. With the vacuum unplugged it advances to about -15 under load. With the VA hooked up under load it advances to around -46. Do these numbers seem "normal"?

Any help would be greatly appreciated. I would like to learn and do as much of this stuff myself rather than just taking it somewhere.

 

Ignition timing and carburetor tuning is tricky - what gives the best performance, and by far the best mileage - will cause it to fail emissions testing. Bigtime. I'd be interested in hearing some more on this as well, as the strategies are different depending on what pollutant is excessive. Do they test for NOX? That's the stuff that, when exposed to sunlight, produces smog or that reddish brown haze that used to form over large cities when everybody had a job and nobody was on welfare.

 

It looks to me like you need to be a little leaner. That is easy at idle but the only way to change it under a loaded condition is with a smaller jet. Try enriching the carb with propane at an idle. Use a small hand held bottle like you would use to solder some pipes. Run a rubber hose into the throat of the carb. Adjust the mixture leaner, then add a little propane slowly until the idle speed stops rising. Lean out the idle mixture so that you get about a 50 rpm gain when you bleed a little propane into the carb.

 

No they only test for HC and CO. It is not a visual test either, just a dyno run with a sniffer.

 

Something else that comes to mind is that you might have a air leak. Use some carb cleaner spray to check around any gasket surface after the carb. Meaning the carb base gasket and the intake where it meets the head. Spray it while the engine is running and listen for an rpm increase. If the engine speed increases it is indicating an air leak where you were spraying.

 

I did try this and it did not seem to cause any change. When I took off and cleaned/rebuilt the carb I used all the gaskets in the kit, so all the carb seals are new. I sprayed a good amount around the intake manifold and did not notice any change? Would the manifold heat flap cause anything to be funky? I did notice the flapper under the carb is very very stiff, I had to use pretty good force to get it to move, I think I left it "stuck" closed for the test. My valve cover does leak ever so slightly in between the front bolt and the first bolt on the driver side, so right on that front driver corner. The rear bottom corner of the pushrod cover also has a small leak.

 

Who rebuilt the carb? Or, put another way how practiced at setting the idle mixture screws are you? It's not rocket science but sometimes folks aren't clear on the concept.

Get in the manuals on carb tuning. A mechanic's vacuum gauge will give a visual indication when carb tuning that is not audible, very fine adjustments of the idle mixture screws will make big changes in the AFR. I found just about a 1/8th turn or thereabouts of the screws will change it a full point when measuring exhaust with an O2 sensor. A vacuum gauge is a useful tool when tuning the engine and carburetor. Find out what the stock size jets are for your engine/truck at your elevation (shop manual) bounce that off what is actually installed. Consider too, leaning it out excessively can have the opposite effect on emissions, too! Want to have a selection of jets on hand to swap out. Probably want to jump two sizes at a time.

Fuel height in the bowl is affected by the float height setting, and this in turn will affect the idle mixture settings to some degree, as well as correct cruise jetting. Jet sizes are the factor determining the level cruise fuel mixture, but they also indirectly affect how much fuel is supplied under load, even though they are not in circuit at the time. Ain't it fun?

 

Lean out idle and pull more timing till it runs like complete poo, then drive it back through...
Write down what you did to pass next year. Good times.