81 CC carb issue?
Thread Starter
Instructor
Joined: Jun 2008
Posts: 164
Likes: 0
From: Groton MA
81 CC carb issue?
What is happening is after cruising at a constant speed for a while, like being on the highway for several miles, the car stumbles badly when I stop and start rolling again. After a couple of times through the gears it's fine, but the first couple of starts are pretty rough if I don't keep the rpms up. Other than that it runs fine. I've had the car for 2 yrs and it has always done this. It is an all original 4-speed with the cc Q-Jet.
I spoke with an older mechanic who is supposed to be experienced with older performance cars and he blames the computer. According to him the computer leans out the engine while cruising and it takes a couple of times through the gears to readjust it. His suggestion is to replace the carb and dist with non-cc ones.
I don't buy this. I've read here about others who have cc carbs that run fine. Any suggestions? The car is in winter storage now, but I will be taking it out in a month and would like to have a plan to get this straightened out.
I spoke with an older mechanic who is supposed to be experienced with older performance cars and he blames the computer. According to him the computer leans out the engine while cruising and it takes a couple of times through the gears to readjust it. His suggestion is to replace the carb and dist with non-cc ones.
I don't buy this. I've read here about others who have cc carbs that run fine. Any suggestions? The car is in winter storage now, but I will be taking it out in a month and would like to have a plan to get this straightened out.
Race Director
Joined: Jul 2004
Posts: 12,696
Likes: 329
From: Scottsdale Arizona
Your instincts were right. The computer is certainly not doing what your mechanic suggested. It will lean out the mixture for cruising but will change dwell time (mixture) as soon as conditions change, like going to idle.
The real cause could be a number of things. The first that occurs to me is plug fouling. As an experiment, try checking the condition of a couple of plugs and then go for a short cruise that ends in your driveway with as few slow downs and stops as possible. Check the plugs again and see if they are more fouled. If they are then adjustin g to hotter plugs could solve the problem.
If not, we'll go from there.
Edit: I forgot to ask. Does your "check engine" light work and does it come on when it's stumbling?
The real cause could be a number of things. The first that occurs to me is plug fouling. As an experiment, try checking the condition of a couple of plugs and then go for a short cruise that ends in your driveway with as few slow downs and stops as possible. Check the plugs again and see if they are more fouled. If they are then adjustin g to hotter plugs could solve the problem.
If not, we'll go from there.
Edit: I forgot to ask. Does your "check engine" light work and does it come on when it's stumbling?
Thread Starter
Instructor
Joined: Jun 2008
Posts: 164
Likes: 0
From: Groton MA
Thanks for the suggestion. There is a rest area I can pull into and check the plugs. Like I said it will be 3-4 weeks before I take it out of mothballs, but I will try that and get back to you.
Race Director
Joined: Jul 2004
Posts: 12,696
Likes: 329
From: Scottsdale Arizona
DON'T BURN YOURSELF! I firmly believe that these cars never get fixed without some kind of blood sacrifice, but I think a skinned knuckle is adequate.
Drop me a PM when you try this. If it doesn't take care of it, I'll walk you through the CCC diagnostic on this post.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Good suggestions above... One of the more likely causes is the computer controlled mixture valve on the carb. It is adjusted via the computer and the mechanism can get sticky or the solenoid go bad. My advice is to get a copy of the Chevrolet Chassis Service Manual for your year and go through the check-out procedure for the fuel system and computer management system. A simple rebuild and/or readjustment of the carb may be in order.
Burning Brakes
Joined: Aug 2006
Posts: 1,058
Likes: 27
From: Eastern North Carolina
Good suggestions above... One of the more likely causes is the computer controlled mixture valve on the carb. It is adjusted via the computer and the mechanism can get sticky or the solenoid go bad. My advice is to get a copy of the Chevrolet Chassis Service Manual for your year and go through the check-out procedure for the fuel system and computer management system. A simple rebuild and/or readjustment of the carb may be in order.
Racer
Joined: Nov 2007
Posts: 318
Likes: 3
From: kuwait
Good suggestions above... One of the more likely causes is the computer controlled mixture valve on the carb. It is adjusted via the computer and the mechanism can get sticky or the solenoid go bad. My advice is to get a copy of the Chevrolet Chassis Service Manual for your year and go through the check-out procedure for the fuel system and computer management system. A simple rebuild and/or readjustment of the carb may be in order.
most of the time the couse is the computer
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Abby Normal
Joined: Dec 2001
Posts: 53,394
Likes: 92
From: Press "2" for English
probably the most overlooked cause of problems with the computer carb cars is the MAP sensor. generally, it doesn't go bad, just gets crappy connection. it's mounted on the firewall between the distributor and master cylinder. it's mounted on the bottom side of that stamped piece of steel. the sensor has a small tube and a 3 or 4 wire electrical connector. take it out and check the tube, also use a little dielectric grease on the connectors (won't hurt on the tube as well) and see if that does the trick.
Burning Brakes
Joined: Feb 2000
Posts: 770
Likes: 284
From: Middleton, Id
My 81 dosent do this - thats a goofy thing for a mechanic to say. I would check the float level and if that dosent do it go thru the carb using the manual. These carbs have lots of adjustments on them but I learned and set mine up great.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
lets face it. if you changed out the carb and dizzy and hooked up your loc-up to work by another means other than the ccc your problem would be solved. this is just the easiest way out for the mechanic and i cant blame him for wanting to go this route. he probably doesnt have the time or patience to be mucking around with 30 year old computers,sensors,wiring....i sure as hell know i didnt so i ripped it all out, fed it to my dog and put in non computer components. happy days 
Advanced
Joined: Jul 2009
Posts: 55
Likes: 0
From: Tucson Az
You may want to check your fuel filter as well. I have an 81 as well and I hate the electronic carb. It seems that the my carb is seasonal. I had my carb rebuilt, but it still seems to need adjusting twice a year. The suggestion about checking the MAP sensor was a damb good suggestion. You may want to pull off you distributor cap and instect the inside contact and you rotor contact. Before you remove your cap, number it according to the firing order.
Melting Slicks
Joined: Nov 2005
Posts: 3,323
Likes: 142
From: Green Valley Arizona
What is happening is after cruising at a constant speed for a while, like being on the highway for several miles, the car stumbles badly when I stop and start rolling again. After a couple of times through the gears it's fine, but the first couple of starts are pretty rough if I don't keep the rpms up. Other than that it runs fine. I've had the car for 2 yrs and it has always done this. It is an all original 4-speed with the cc Q-Jet.
I spoke with an older mechanic who is supposed to be experienced with older performance cars and he blames the computer. According to him the computer leans out the engine while cruising and it takes a couple of times through the gears to readjust it. His suggestion is to replace the carb and dist with non-cc ones.
I don't buy this. I've read here about others who have cc carbs that run fine. Any suggestions? The car is in winter storage now, but I will be taking it out in a month and would like to have a plan to get this straightened out.
I spoke with an older mechanic who is supposed to be experienced with older performance cars and he blames the computer. According to him the computer leans out the engine while cruising and it takes a couple of times through the gears to readjust it. His suggestion is to replace the carb and dist with non-cc ones.
I don't buy this. I've read here about others who have cc carbs that run fine. Any suggestions? The car is in winter storage now, but I will be taking it out in a month and would like to have a plan to get this straightened out.
Good luck
Thread Starter
Instructor
Joined: Jun 2008
Posts: 164
Likes: 0
From: Groton MA
I finally got to take it out of storage and drive it around between floods. I ran through the gas that sat in it this winter with carb cleaner added and re-filled it with fresh. I now have a different problem (I think). After warm up, when it goes into closed loop it runs lean to the point where it will barely idle and backfires sometimes when I let off the gas. It runs fine when warming up and when hot, if I shut it off and start it again, it runs fine for about 30 sec to a minute. If I keep the rpms above 2,000 it is smooth, but below that it runs rough.
I'm thinking O2 or MAP sensor, since they are what the computer looks at to adjust the mixture in closed loop. I cleaned the MAP and checked the line going to it. It looked good and that made no difference, so I'll be swapping the O2 sensor next. The only one in stock locally is a Bosch OE replacement. Does anyone have experience with these, good or bad? I know I'm not a big fan of their sparkplugs.
I'm thinking O2 or MAP sensor, since they are what the computer looks at to adjust the mixture in closed loop. I cleaned the MAP and checked the line going to it. It looked good and that made no difference, so I'll be swapping the O2 sensor next. The only one in stock locally is a Bosch OE replacement. Does anyone have experience with these, good or bad? I know I'm not a big fan of their sparkplugs.
Intermediate
Joined: Apr 2010
Posts: 29
Likes: 0
I have an 81 does the same thing... Friend who is a mechanic said to unplug that mixuture valve and see what happens..on the front of your carb.. I did and it dont did it.......check engine will stay on..
Good suggestions above... One of the more likely causes is the computer controlled mixture valve on the carb. It is adjusted via the computer and the mechanism can get sticky or the solenoid go bad. My advice is to get a copy of the Chevrolet Chassis Service Manual for your year and go through the check-out procedure for the fuel system and computer management system. A simple rebuild and/or readjustment of the carb may be in order.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
My replacement Carb was one pulled from a '79 cost me 40 bucks works perfect, bought a distributor (yes) off of Ebay brand new weysco for 40 bucks(no brand name , I did research and got it cheap.) No computer anymore, it is unplugged. So we have 80 bucks so far, it is all installed and runs better than it ever did, awesome. Add the B&M controller for the lock up and add another 159 dollars(can be done with a switch for ten bucks). I know the last poster showed his high dollar intake and heads, but the swap can be done very inexpensively. No disrespect intended..... everyone has thier own opinions,this is just mine
Last edited by 81pilot; Apr 3, 2010 at 08:52 PM.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
lets face it. if you changed out the carb and dizzy and hooked up your loc-up to work by another means other than the ccc your problem would be solved. this is just the easiest way out for the mechanic and i cant blame him for wanting to go this route. he probably doesnt have the time or patience to be mucking around with 30 year old computers,sensors,wiring....i sure as hell know i didnt so i ripped it all out, fed it to my dog and put in non computer components. happy days
Racer
Joined: Apr 2006
Posts: 477
Likes: 43
From: Millbrae California
If you can check the computer codes if any and get the dwell readings of the mixture control solenoid when the engine is warm (in closed loop) i can most llikely help you figure out what the problem is. The green connector on the harness that goes to the mixture solenoid is where you get the dwell reading with a dwell meter set on 6 cyl. a reading of 5 degrees is a full rich command while a 50 degree reading is a full lean command to the mixture control solenoid. When you unplug the mixture control solenoid it goes full rich as long as the metering rods are not stuck in the lean/down position.
I hope this helps Henry @ olescarb
I hope this helps Henry @ olescarb