Performance Problems on 81
Thread Starter
Navigator
Joined: Apr 2013
Posts: 9
Likes: 0
Performance Problems on 81
I have an 81 vette that when it gets warm it runs like crap. We have replaced the exhaust, carburetor, fuel pump, timing chain and still after running for about an hour it suddenly runs terrible. When it is cold it runs great but after shutting the engine off and starting it when it is hot it will hardly run. Any suggestions?
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,753
From: Charlotte NC
Need much more descriptive information. You are leaving too many un-answered questions.
What type of carb???
What type of exhaust...and is the engine still needing the computer to run it...such as an O2 sensor in the exhaust...the TPS an mixture control solenoid in the crab??? Headers??? If you installed these new...you can run into severe heat issues that can effect many items if you did not take into account for this and protect items from excessive heat.
Are you using a timing light to set timing??? Do you know for a FACT that the outer ring on the balancer had not spun and you not having the timing correct???
Have you connected a scanner to the ALCL and see when the car starts running bad...has the car gone into "closed loop"??? This is important if you are still using a computer to make it run correctly.
Fuel pump??? Did you install one that still uses the fuel return line???
Like I mentioned...numerous issues that need answers to. The more detailed information you can give will be helpful.
DUB
What type of carb???
What type of exhaust...and is the engine still needing the computer to run it...such as an O2 sensor in the exhaust...the TPS an mixture control solenoid in the crab??? Headers??? If you installed these new...you can run into severe heat issues that can effect many items if you did not take into account for this and protect items from excessive heat.
Are you using a timing light to set timing??? Do you know for a FACT that the outer ring on the balancer had not spun and you not having the timing correct???
Have you connected a scanner to the ALCL and see when the car starts running bad...has the car gone into "closed loop"??? This is important if you are still using a computer to make it run correctly.
Fuel pump??? Did you install one that still uses the fuel return line???
Like I mentioned...numerous issues that need answers to. The more detailed information you can give will be helpful.
DUB
Thread Starter
Navigator
Joined: Apr 2013
Posts: 9
Likes: 0
Thanks DUB and Shark. Dub, It has a Rochester quadrajet stock carb. The exhaust is now straight dual with no cat, stock exhaust manifolds and maganaflow mufflers. It is still on the computer and the guy that installed the mufflers said he attached everything back. My mechanic put the new timing chain on so I am not sure how he timed it. I will ask him about the scanner. The fuel pump that we replaced does have the return fuel line. I wish I could be more helpful. My mechanic is pulling his hair out trying to figure this out. Thanks again
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
Post a couple pics of the carb and engine bay if you can. The 81 came with an E4ME electronic carb. If its not all connected and setup correctly it will not run right. Its not hard to do, but does have to be done right.
The O2 sensor should be very close to the exhaust manifold, close to the head.
Is the GEN light indicating any kind of code?
The O2 sensor should be very close to the exhaust manifold, close to the head.
Is the GEN light indicating any kind of code?
Racer
Joined: Nov 2011
Posts: 319
Likes: 12
From: South Chase WI
1st...Choke hanging up? Check the choke cold. Take off the air cleaner, mark it's position and start it. Make sure it is functioning properly. Check engine light work?
2nd....Hot engine performance issues, if the engine is not overheating points to ignition component failure / problems. Cap, rotor, ignition module, coil....plugs, wires....as old as it is, replace that stuff 1st. Cheap and necessary tune up so you know that is all OK. Fuel filter screen in the carb inlet, and the inline under the chassis.
2nd....Hot engine performance issues, if the engine is not overheating points to ignition component failure / problems. Cap, rotor, ignition module, coil....plugs, wires....as old as it is, replace that stuff 1st. Cheap and necessary tune up so you know that is all OK. Fuel filter screen in the carb inlet, and the inline under the chassis.
Intermediate
Joined: Feb 2013
Posts: 33
Likes: 0
From: Lake Placid Florida
I have the same problem on my 81. I replaced the coil pack cap & rotor but not the wires was thinking the carb next! I will take maxmachz's advice and try wires first.
SharkRacer where are the seals on the idle air? Will be humble myself and say I don't know what they are? Thanks in advance for any and all advice!
SharkRacer where are the seals on the idle air? Will be humble myself and say I don't know what they are? Thanks in advance for any and all advice!
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
A "stock" carb for a 1981 model year C3 is NOT the same as any other Q-Jet. The '81 carb had an electronic gizmo for adjusting rate of fuel dispensed by the carb; and the computer system and O2 sensor managed that (one year only) carb and matching distributor to set the fuel flow properly.
If you still have the ECM and dizzy from the original '81 installed and operating, but have changed the carb, it is not a surprise that it runs like crap. The only surprise is that it runs like crap ONLY when it is warmed up.
If the ECM has been disconnected and the carb and dizzy are units from a year other than 1981, you will have to look to the "usual and customary" reasons for your problem. But, if you have a 'Frankenstein' combination of parts on your engine and the ECM is trying hard (but failing) to manage the timing and fuel flow, you will have to decide whether you want a 1981 induction/ignition system on your engine or you want to revert back to a 'non-electronic' system for those functions.
If you still have the ECM and dizzy from the original '81 installed and operating, but have changed the carb, it is not a surprise that it runs like crap. The only surprise is that it runs like crap ONLY when it is warmed up.
If the ECM has been disconnected and the carb and dizzy are units from a year other than 1981, you will have to look to the "usual and customary" reasons for your problem. But, if you have a 'Frankenstein' combination of parts on your engine and the ECM is trying hard (but failing) to manage the timing and fuel flow, you will have to decide whether you want a 1981 induction/ignition system on your engine or you want to revert back to a 'non-electronic' system for those functions.
Last edited by 7T1vette; Jul 31, 2013 at 04:42 PM.
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Instructor
Joined: Aug 2010
Posts: 233
Likes: 18
From: Monrovia Md
I am with Max -- something in the ignition circuit. Coil pack or Ignition control module is breaking down with heat.
The 81 CCC carb does not run "bad" when the computer goes open loop, it runs rich (Feul jets wide open) and slugish (default timing advance at all times). If the engine is stumbling, probably ignition
The 81 CCC carb does not run "bad" when the computer goes open loop, it runs rich (Feul jets wide open) and slugish (default timing advance at all times). If the engine is stumbling, probably ignition
Last edited by wajulia; Jul 31, 2013 at 05:19 PM.
Intermediate
Joined: Feb 2013
Posts: 33
Likes: 0
From: Lake Placid Florida
I want to let you know and thank you! I replaced my ICM last night drove it to work today! Still acting a little strange but I think it's a timing issue will do some fine tuning and see where I'm at. Will keep you posted. Any other thoughts? Thanks again
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Timing on the '81 is set 'differently' because of the computer-control system. Check the internet for instructions on setting timing on '81 C3. You just have to disconnect some connector, if I remember correctly.
Burning Brakes
Joined: Oct 2011
Posts: 956
Likes: 86
From: McDonough Georgia
I'd rather invest in something trouble free with a great deal more performance. Not to mention looking better.
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,114
From: Crossville TN
Individual negative stigmas about an engine type that you've [probably] never had does nothing to help the OP. 
There is nothing 'wrong' with a working '81 powertrain system. The (electronic) fuel mixture module in the carb is readily available, as is the ECM. All components can be rebuilt or replaced for relatively small $$$, compared to buying all-new engine components. When working, the system has adequate power, good economy, and excellent start/idle/run qualities.....unless you just want to race the he!! out of it.
There is nothing 'wrong' with a working '81 powertrain system. The (electronic) fuel mixture module in the carb is readily available, as is the ECM. All components can be rebuilt or replaced for relatively small $$$, compared to buying all-new engine components. When working, the system has adequate power, good economy, and excellent start/idle/run qualities.....unless you just want to race the he!! out of it.
Burning Brakes
Joined: Oct 2011
Posts: 956
Likes: 86
From: McDonough Georgia
Individual negative stigmas about an engine type that you've [probably] never had does nothing to help the OP.
There is nothing 'wrong' with a working '81 powertrain system. The (electronic) fuel mixture module in the carb is readily available, as is the ECM. All components can be rebuilt or replaced for relatively small $$$, compared to buying all-new engine components. When working, the system has adequate power, good economy, and excellent start/idle/run qualities.....unless you just want to race the he!! out of it.
There is nothing 'wrong' with a working '81 powertrain system. The (electronic) fuel mixture module in the carb is readily available, as is the ECM. All components can be rebuilt or replaced for relatively small $$$, compared to buying all-new engine components. When working, the system has adequate power, good economy, and excellent start/idle/run qualities.....unless you just want to race the he!! out of it.
If constant diagnosing of the q junk, computer and assorted other add on components is your idea of fun, have at it.
A stock 81 is like driving a Vega that looks good. And the gas mileage is no better than any well tuned small block.
It doesn't take much to clean up the engine compartment, trash the junk and have a nice car.
You also don't have to build it to race. Just something that won't be embarrassed by all the Kia's and Hyundai's out there.
