Thanks everyone for your ideas.

I hadn't mentioned that before I bough the van the previous owner had a mobile mechanic come and get the engine to start and run. Apparently after nine years on a barn it wouldn't start or run. The carb was clean when I got it so I presume it was rebuilt or if not rebuilt than just cleaned out. Also in my endeavors to check out all possibilities, I'd done a flow test on the fuel pump, it was okay, and replaced the fuel filter in the carburetor. The old one was pretty clean, not much debris in the chamber either.

Also looking back at my failed DEQ records I see that I miss stated the fail. It was the HC (Hydro Carbons) threshold that failed not the CO. And, the threshold was 220 not 200. Last time the vehicle passed the HC level was 450 on 1st idle test, then after running the engine at 2500 RPM for 30 to 40 seconds a second idle test showed HC at 175.

Could very well be the old carb from the 305 is feeding fuel to the newer 350 engine. I don't really have a way of knowing if it is the original carb or not, but over the last year when I have had it running well, it runs just smooth as can be. Seems like the carb is working okay. When it goes into its missing and backfiring mode the change is abrupt, like flicking a switch. Doesn't seem like a carb can do that.

I did look at the timing. It was set to about 12 degrees BTC with vacuum advanced to distributor disconnected and plugged. vehicle idling in park. I changed the timing to 6 degrees BTC knowing that will help the HC numbers a little, and knowing that the mechanic who put weaker springs in the distributor was trying to get more advance because I had told him that the van seemed like it was a bit sluggish at the time. . . . And that was the timing setting for my '67 ford mustang I had in collage years and years ago. I've since found some advice that says the timing should be set with trans in drive with the e-brake on.

So I did some more sleuthing yesterday. Even though I had installed a new dist cap and rotor about 500 miles ago, I found some carbon on the rotor from the spring loaded center pin on the distributor cap. Cleaned that up. Also cleaned the ground connection for the coil. It was a new coil, and there was still some paint where the wire connected to ground the laminated core. Checked and re-seated the low voltage wires to the distributor. This morning I drove over to the DEQ for testing again. 10 mile drive on the freeway. All my missing and backfiring episodes had occured while driving city streets around the neighborhood.
Seemed okay on the freeway, though feel good that the freeway had good shoulders to pull off an wait for things to cool if I had problems. Arrived to find a long line out into the street. Took 50 minutes of idling to work through the line. The van never missed or backfired on the trip over or while waiting in line. Finally got to the testing booth and passed. Yea !!! HC was 714 on first idle test, but dropped to 213 on second idle test after the run up to 2500 RPM for 30 seconds. Drove the van around for another hour and no problems.

Still not convinced that the problem won't creep up again but in the mean time I'm happy that I have the new DEQ certification and can keep the registration current and continue to drive and test.

Thanks again for the responses. It's comforting to know there is a supportive community out there.


1983 GMC Vandura G2500 Starcraft conversion
Mr. Goodwrench 350
200K mi.