I first must admit this isn't for my 3rd gen, It's my mother-in-laws 86 1/2 ton chevy truck.
It has a 305 with a Q-jet. It has never ran real good but what can you really expect? I borrowed the truck the other day to haul some lumber. It really ran like sh**. It would idle fair and full throttle was about normal but part throttle it spit, sputtered, backfiring in the exhaust and smoking dead rich.
I went through and changed all the vacuum lines, pvc valve and replaced the vacuum switch on the water neck. None of this helped. I went to the parts store and got a rebuild kit for the carb and cleaned and rebuilt it setting everything per the instructions supplied. I put the carb back on but there was no difference.
I had an older ('79) q-jet that had ran decent prior to parting out the van it was on. I put it on connecting vacuum lines as best I could. Only one was unable to connect due to different diameters. I capped/plugged it. The engine fired right up and idled better and smoother than it ever had. I took it for a short drive and it seemed much better. I came back home and made sure everything was OK. I put the air cleaner back on and was going to deliver it to mother-in-law. The longer I drove the worse it got. By the time I got to her house it was running as bad as the original carb.
I'm guessing this problem is related to emission controls but not positive. Two carbs with the exact same symptoms tells me it's not the carb. The decent WOT performance tells me it's not ignition problems.
So I'm left guessing about emission controls. A friend suggested the charcoal canister could be screwing up but he's only heard that will mess up the idle which is still very smooth.
At this point I'm at a loss. My best guess is strip it of all emission control stuff and stick a holley on it.
I'm willing to listen to any ideas you people may have to help me out.
Thanks,,, Matt


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Dirt Late Model racer and Third gen lover.
91 V-6 bird with 406 in process.

 

It sounds like it's either icing up or the choke is AFU and closing while you drive. When it sputters and smokes, did you check to see if the choke had closed? If the choke housing isn't getting the heat it needs, the spring will close the choke even as you drive. It will still idle, and flooring it will force the choke open, but the rest of the time it will run really rich as you had diagnosed.

Q-Jets and any other carb with very small barrels will ice up if the hot air snorkel isn't doing its job and the temp/dew point is just right.

I would also suspect the cannister is getting raw gas somehow, but I still think that the choke is closing as you drive. BTW, does that truck have ESC (Electronic Spark Control)? Maybe that thing is AFU and messing up the timing. That will make it run like s**t, too!

[This message has been edited by jrr (edited November 27, 2000).]

 

The choke is working OK. I locked it open and drove it just to check but nothing changed. This problem has existed most of the summer, It has just gotten to the point it's not drivable now. I doubt icing is a problem although current weather conditions are favorable for that.
No ESC on this model, just a standard vacuum advance distributor.
Later,,, Matt


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Dirt Late Model racer and Third gen lover.
91 V-6 bird with 406 in process.

 

Matt, i had the same problem last summer. Ran great cold but once the eng. warmed up it ran like ****. I checked everything. One nite i had a buddy sit in the car, put the tranney in D, set the parking brake, press hard on the brake pedle, turn out the lites and brake lock the car while i looked under the hood. It looked like the forth of july. I replaced the plug wires and presto problem gone. Gained 2 mpg too!

-Auggie-

 

It has a new set of wires on it. I put them on back in July with a new cap, rotor and plugs.
Any other ideas?
Matt


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Dirt Late Model racer and Third gen lover.
91 V-6 bird with 406 in process.

 

This may be a wild hunch, but it the carburetor casting warped? It's happened twice to me. An old Rochester RFB was warped from age and being taken apart about six times in the past 28 years. My very own ECM-controlled Q-Jet has finally succumbed to warping after only three rebuilds and two over-torqued bolts!

I had experienced the same symptoms as you. As the warped casting warmed up with the rest of the motor, it would idle worse and worse. On the road and wide open, it ran great. Let things cool down and it had a fair idle.

Just thought I'd run this idea past you.

 

I'm thinking EGR valve...

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock... all the suspension stuff... 9-bolt posi disk is in... K&N filter... 93 octane...

-=ICON Motorsports=-

 

I don't know about warpage but the idle is fine all the time. Even after it's warmed up and running crappy. I initially thought EGR valve. I disconnected it so it should be shut all the time and plugged its vacuum line. It didn't run any better. I removed it to make sure it was shutting completely and it checked out OK. I was going to replace it since I had it off but they get $58 for it at autozone and since I didn't believe it was bad I put the old one back on.
Later,,, Matt


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Dirt Late Model racer and Third gen lover.
91 V-6 bird with 406 in process.

 

Do your plug wire test again-just because they are new don't mean they are good. I've got 2 brand new sets I won't run because the old ones are better.

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86 FB stock 305 Qjet South California emissions

 

check the fuel pressure... if its too high it can cause the prob you are having... fuel pressure should be 4.5-7psi...

zroc