Need Help With CCC Quadrajet Tuning

[CAChevy]

Just put a newly rebuilt SMI CCC Quadrajet carb on the 305/350 swap project and can't seem to get the thing to run right. I've read just about all the info I could find on the web about how to tune these things but can't seem to find out what I'm doing wrong.

The truck is a 1984 CA Emissions which originally had a 305 but swapped in a GM 260/350 crate motor with all the original smog stuff.

When I try to adjust the dwell for the MCS, (on the 6cyl scale) I can't get it in the 30ish range at idle. If I bring it up to around 1200 rpm it's right where it needs to be, bouncing between 27-32...but as soon as it goes back to idle the needle shoots down to about 9 and stays there.

I've put a vacuum gauge on it and adjusted the idle mixture screws to get the highest rpm/vac reading...ending up about 3 1/2 turns out on each of them. Manifold vac at idle is nice and steady at 20in Hg

My TPS voltage is about 0.46 volts at fully closed throttle.

The only way I can tell the truck has gone into closed loop to make my adjustments is when the smog pump starts pumping air into the air cleaner instead of the exhaust manifolds and I'm only guessing that is an indicator. I swapped out the original single wire O2 sensor and put in a 3 wire sensor with the heater because of the headers, the original wasn't getting hot enough at idle to make the adjustments in closed loop. I also put a piece of cardboard in front of the radiator to make sure water temp gets high enough...about 200 deg.

The truck idles just fine but sputters, (like it wants to die) and is very sluggish off the line. If I just mash the gas from a stop it'll try and die out on me. I have to be very light on the throttle to get it to accelerate smoothly.

I'm really not wanting to take this thing to a mechanic as I've already blown the budget on this build. Anyone have any ideas?

[CAChevy]

O.K....I messed with it some more after I got off work.

Cold start I had to keep "pumping" the throttle gently to keep it running for about a minute. Then as if someone flipped a switch it went on to fast idle and idled at 1500 rpm like a champ.

After it reached op temp I fiddled with the idle mix screws....now I'm out to about 5 turns both sides..gained about another 60 rpm and 1in hg vac. The MCS dwell at idle is still at 12 until I bring up the rpm's to 1200 or so. Then it jumps up and acts "normal" bouncing 27-32. Let it go back to idle...it seems to go out of closed loop. Is this normal?

Didn't get it on the road...I'll do that tomorrow.

[Gregski]

did you do the simple test where you screw in the idle mixture screw(s) one at at time till the truck stumbles and dies? it should do that it should stumble and die, if it doesn't you have a vacuum leak, wonder if this even applies to the newer trucks like yours with all the new electrics married to the carb, but it may still be worth a try

[CAChevy]

Originally Posted by Gregski

did you do the simple test where you screw in the idle mixture screw(s) one at at time till the truck stumbles and dies?

It'll die out no problem with just one idle mix screw turned in all the way. Going to take it out here in a bit and see what it does.

[CAChevy]

It's better, not great, but better.

Still had to feather the throttle at cold start up before it would eventually run fine on fast idle.

Idle's perfectly, it'll even break loose the rears with a little help from the brake pedal. Off idle hard acceleration, it still sputters and wants to die, but not as bad. Once it pushes through and the secondaries open up, it accelerates and revs better than it ever has.

I've got another issue now though, the truck wont warm up. It'll get to about 120f on the gauge and that's with a piece of cardboard stuck between the radiator and a/c condenser. After 20 min of driving I popped the hood and could leave my hand on the radiator. That means its not going into closed loop....guess I need to put another thermo stat in it and see what happens but the one in it now is new.

So, in open loop, what adjustments on the carb effect off idle throttle?

[Gregski]

Originally Posted by CAChevy

It's better, not great, but better.

I've got another issue now though, the truck wont warm up.

... That means its not going into closed loop....

So, in open loop, what adjustments on the carb effect off idle throttle?

Man this is exactly precisely why I went the pre SMOG (ie 1975) route on these trusty old trucks, I wanted nothing to do with this closed loop / open loop business.

[beamn7]

The majority of the CA emissioned feedback carbs in truck applications actually idle in open loop which honestly is better. They run much smoother in open loop. Your TPS voltage looks good (within range. 305 calls for .41v and 350 .48v) I tend to be about 3.5 to 4 turns out on the idle mixture screws. Factory called for 3 turns but with my cam and heads 3 wont cut it. Does sound like you have a stuck t-stat though....
What is your timing at? If you have the EST distributor, make sure you unplug the flat 4 prong connector on the pass side near the dist before setting the timing. Should be 6 deg. Don't adjust the idle air bleed valve. I'm sure Sean at SMI set it to the proper specs with the gauging tool. When you swapped in the 3 wire sensor, did you make sure it was still a narrow band sensor? A wide band wont work with this ECM and could cause multiple off idle stumbles, etc.

[CAChevy]

I hate unresolved threads so I thought I'd wrap this one up.

A new thermostat fixed the operating temp issue. The "old new" one was one of those fail safe units...and it failed...wide open.

Basically, I ignored the tps dwell all together when trying to adjust the carb at idle. As was suggested by beamn7, as soon as the truck went to idle, the ecu kicked it out of closed loop. So, I just used a tach and vaccum gauge and tuned the idle mix screws old school style. After I did, when it went into closed loop off idle, the MCS dwell was dead on. Off idle acceleration is perfect and I only had to tighten the secondary air door tension a smudge to tune the "bog" out of it.