Idling issues, intermittent revving when warm and fails emissions test

[Paulco]

Finally got my truck in drive-able condition by fixing the brakes etc, and was probably a bit too enthusiastic and took for an emissions test. This was the longest run the truck has had and when I got there the engine was revving every 2 seconds (between approx. 1000 and 1200).

It failed the emissions test with a 90 CO reading, the pass level is 65 in Colorado.

When idling the truck does sound a bit rough but I am not sure if V8's are not supposed to sound smooth

The carburetor is new (maybe a year old, previous owner installed and I assumed was working correctly. I was planning to run through the carburetor set up from scratch to see if it fixes the problem. Unless someone has  better options. I did take a peak at the carb and have attached as picture with the horn removed.

Fuel filter and Fuel pump are new.

Thanks
Paul

[Gary Lewis]

The changing idle speed may be one of several things.  My first thought is a cracked vacuum hose that is opening and closing as the engine moves a little.  So I'd try moving the hoses around as the engine is running to see if you can make it do it.  Or, spray carb or brake cleaner in some places where there might be a crack and if so the speed should pick up as the cleaner gets sucked in and burned.

But if it really is regular, like every two seconds, then I wouldn't think a cracked hose could do that.  Probably something to do with an electrically-controlled device, like the EGR that shows in your pic right behind the carb.  To test that you could pull the vacuum hose off and plug it.

[BullnoseLuvr]

Try unhooking the throttle kicker solenoid.

[old55pete]

You might want to pull the EGR and see if it is stuck open and to see if the EGR port is not plugged.
When was the last time the spark plugs were changed? How about the distrubiter cap, rotor and sparkplug wires, how long sense they were changed? How about the PCV valve and the mounting gromet? Are they in good shape, does the PCV valve rattle when you shake it?

As for the intermittend reving, watch the throttle kicker selnoid with the engine running, see if it is moving the throttle linkage.

With the engine running, spray some brake cleaner around the carb base plate and see if the engine speeds up or dies. If it speeds up, there is a leaking gasket between the intake manifold, the EGR spacer, and the carb.

[old55pete]

Paul, when was the last time the oil was changed? If it has been in the engine for to many miles, it can cause high co and hc, just a thought.