'77 351M with 2150 carb, everything stock.
New ignition box, coil, cap, rotor, wires, plugs
Failed on HC's at idle. Everything else is good. Guessing I'm running rich, and my idle is bouncing around.


So.....


I have a new EGR valve that I'll install. Not sure the old one is bad, but since I have a new one, why not....
I can lean it out a little bit at the risk of making less vacuum. It's already only at around 15 inches, but I can't find any vac leaks
I need to do some choke adjustments I'm guessing. The truck doesn't idle worth a darn below 900-1000 rpms, and even then, the idle fluctuates quite a bit 200-300 rpm). Any ideas on cause?
I'll back off timing as much as I can. The vac advance comes into contact with the AC compressor bracket at some point and doesn't let me retard any further.
I'll run a couple quarts of 91% Isorbophyl (sp?) in the gas when I go through again.


It passed last year with a carb that had a horrible vac leak, but it idled smooth. Now it runs good with a different carb, but idles poorly. Looking for tips to smooth out the idle.

 

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Can you post up your sniffer results? They usually post your CO, HC, and etc. readings, along with the limits. The readings can point to where the problem really is.


You absolutely want that choke plate all the way open when you bring it in for it's screening.
Does your choke have an electric hook-up? I'm pretty sure it should have a hot air tube running to the manifold. If that tube is missing, the choke will keep closing, (and bumping up to fast-idle) whether the electric portion works or not.
For sniffer testing, you want a good fresh ignition, so you're good there. You don't want your timing too advanced either, I think that can elevate HCs. You also want to be running at stioch, or just slightly slightly lean. And if your engine has some miles, you want to have fresh oil in the engine too. Old oil which smells gassy can end up in the exhaust, and adding emissions.
Been a long time since I had to deal with any of this, and by the last time, I was running EEC IV EFI...

 

HCs were about 565 at idle. Standard is 350.


HCs under load, and CO all were way below standards.

 

No on electric choke, and yes, the hot air tube is connected.
oil is about 600 miles old but I did just get back from a week of 4 wheeling.

 

How about your idle mixture settings, where are they? They should normally be about 1.5 turns out from tightened/bottom/fully seated.
If your choke is indeed not staying open after warm-up, then you might just need to adjust it by turning the bi-metal spring housing. It probably doesn't need to move much, and you might found that you went too far the next time you start it in cold weather...

 

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Same D-Rat from Pirate?

 

They run a test at idle?

If you can't bring the rpm down to 650 then it won't pass. If the a/c compressor is in the way, you may have to move the distributor over a couple teeth.

To say it is "high HC" at 900-1000 rpm when the machine is looking for it to be at 650ish rpm is premature, it may be within specs at 650. The smog guy manually types in "650" because that is what his book says that your truck should be at idle. He should be checking it with a timing light.

You can see if your EGR valve is working by simply using a vacuum pump or connecting a longer vauum hose to it and sucking on it. You will hear and feel if it is actuating and holding suction. Even if it is functioning it may not be working, the egr spacer plate may be clogged. You would have to pull the carb and clean the plate, but I assume you did that when you installed this carb.

What is your rpm idling in gear? I am assuming your 900-1000 measurement is in park. You may get closer to 650 if he measures the gases in gear.

I don't know about AZ, but the shop can do a "pretest" for you that will give you an idea if you will maybe kinda sorta pass. They only shove the thing in the tail pipe, enter in your timing and rpm then the machine measures and calculates it. But if you tell them 8btdc @ 650, it wont be correct. If you tell them accurately, it will calculate it accurately.

The only tests that matter are at 15mph and 25mph.

 

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That's me.

 

I don't think so on the overly lean idle. My nose tells me otherwise...it smells rich. mix screws are out about 1 3/4 turns to get the most vacuum. What you're saying about overly lean would be true if I was getting a lean misfire, but I don't think that's the case.


It was tested on a roller, but idle test is irrelevant there as the truck is stationary. It passed the load test easily. WAY under standard.


Yes, I do need to get the idle down, but equally important I need to get the idle smoothed out.

 

yes, the idle testing is done after the load testing on the dyno.


Got a buddy with a tail pipe sniffer that I might tap as I work on the tune. This thing passed easily last year idling at 1k rpms but with a different carb. That carb was junk though...truck would cough and sputter at anything more than 1/2 throttle because the vac leak was so bad in it at the throttle shaft. Truck is a manual, not an auto

 

Interesting, maybe I just am not paying attention during an idle test.

High HCs are what failed me at 15 mph, we ended up turning the idle screws out to 2.5 turns to pass and it runs well.

How far out is your curb idle screw? Maybe back that off to zero and readjust.

 

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