Can Someone Explain....
Thread Starter
Melting Slicks
Joined: Feb 1999
Posts: 2,473
Likes: 10
From: New Paltz, New York USA
Can Someone Explain....
I put a new crate motor in my 81, I also removed the computer. The new motor started right up, but I cannot get it to idle right. If you are driving its fine, but when you stop for any reason it idles rough and stalls.
I had someone look at it and determine that there are no vacuum leaks. He said that he has to re-curve my distributor, so, he had me buy an adjustable vacuum advance kit. Can someone please explain how this works, thanks. He is not able to install this kit for a few weeks so I was wondering that if I knw how it worked and what it did I may be able to do it myself.
I had someone look at it and determine that there are no vacuum leaks. He said that he has to re-curve my distributor, so, he had me buy an adjustable vacuum advance kit. Can someone please explain how this works, thanks. He is not able to install this kit for a few weeks so I was wondering that if I knw how it worked and what it did I may be able to do it myself.
Le Mans Master
Joined: Aug 2010
Posts: 9,272
Likes: 17
#2 says it best , your mechanic needs to be replaced. re-curving a dist. is usually done for quicker advance , like if you're sticking your foot in it ....at a drag strip, of course, shouldnt affect idle, unless you have a broke spring and it wont return to base dwell. it is possible that you have a broken diaphram in the vacum advance unit itself, that still wouldnt cause your issues, dist. vacum is ported , or should be. Could you post what you did during getting it running and then what you've tried to fix this. pictures of your intake may let someone see something simple that you overlooked.
Last edited by oldalaskaman; Mar 22, 2012 at 11:55 AM.
Melting Slicks
Joined: Feb 2007
Posts: 2,136
Likes: 62
From: West Kingston RI
I believe your original motor was throtle body or efi and know I assume you have gone to a standard carb. If so the fuel pump is putting out to much pressure and overridding your float. If it was efi or throtle body put a pressure regulator in line and reduce pressure to 6 to 8 pounds and make sure there is a return line to the tank or you will blow the pump, ask me how I know. 
Burning Brakes
Joined: May 2008
Posts: 1,116
Likes: 5
From: Middleburg Florida
No, the 81 originally cam with a E4ME Q-jet. It also has a lockup torque converter (if it's the original TH350C trans) and depending on how the computer and wiring was reomved, you won't get the use of that now. If you've modified the wiring for the lock up, and it's not correct, this could be contributing to your idle problem.
Chances are a re-curve of the distributor won't help, unless you have a big cam in it. I'd start with the initial timing, what's it set at? Then the idle settings, what carb is on it? How was the idle mixture set, was a vacuum gauge used? If not, initial timing first, then idle mixture (with a vacuum gauge), then idle speed.
Chances are a re-curve of the distributor won't help, unless you have a big cam in it. I'd start with the initial timing, what's it set at? Then the idle settings, what carb is on it? How was the idle mixture set, was a vacuum gauge used? If not, initial timing first, then idle mixture (with a vacuum gauge), then idle speed.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Did you re-use the carb and/or distributor from the original engine? If so, that's your problem. Those are 'special' components that are meant to be used only with the '81 ECU (first year for electronic engine management in the Corvette). The carb has an electronic fuel mixture device and is linked to the ECU and distributor [and O2 sensor] via a one-year-only wiring harness for integrated system control. The carb and distributor will not work properly unless all those components are hooked up together and correctly.
If you did not re-use those components, you've not supplied enough info about the specific symptoms you are experiencing, nor the components you have on the engine, for anyone to advise you of possible causes. From what you described, anything we would provide would just be a guess....
If you did not re-use those components, you've not supplied enough info about the specific symptoms you are experiencing, nor the components you have on the engine, for anyone to advise you of possible causes. From what you described, anything we would provide would just be a guess....
Thread Starter
Melting Slicks
Joined: Feb 1999
Posts: 2,473
Likes: 10
From: New Paltz, New York USA
OK, sorry it took me so long to get back, but I was looking through all my paper work to find some information. I bought the new crate motor from Pace Performance, its a 350/300HP motor.
I removed the computer so I got a Q-Jet 750 STG 2 carb, this care was recommended by Mark L. at Pace. The motor has a mild cam and Mark also gave me these timing numbers= 10 degrees with vacuum advance disconnected, total timing will be 32-36 degrees @ 3000RPM.
The paper work that came with the motor says the cam is: Intake Lift-.443, Exhaust lift-.465, duration @ .050-214, exhaust duration @ .050-224, Lobe seperation-112.
I installed the motor and did all the plumbing and wiring, had a friend with me at start up, all he had to do was turn the distributor a little and she started right up. Ran OK except for rough idle and from there I had someone else try all kinds of stuff until he finally told me that he has to re-curve the distributor.
I am NOT very good with the timing part. Sorry this was so long.
I removed the computer so I got a Q-Jet 750 STG 2 carb, this care was recommended by Mark L. at Pace. The motor has a mild cam and Mark also gave me these timing numbers= 10 degrees with vacuum advance disconnected, total timing will be 32-36 degrees @ 3000RPM.
The paper work that came with the motor says the cam is: Intake Lift-.443, Exhaust lift-.465, duration @ .050-214, exhaust duration @ .050-224, Lobe seperation-112.
I installed the motor and did all the plumbing and wiring, had a friend with me at start up, all he had to do was turn the distributor a little and she started right up. Ran OK except for rough idle and from there I had someone else try all kinds of stuff until he finally told me that he has to re-curve the distributor.
I am NOT very good with the timing part. Sorry this was so long.
Burning Brakes
Joined: Oct 2011
Posts: 1,093
Likes: 2
From: long beach California
well get a test light and set timing it is fairly simple procedure. then get your idle set right if your timing isn't correct your idle probly isn't either.
even if you don't know much but you honestly care about your ride and you want it to be right. you can usually learn/teach yourself, reach out for needed advice and come out ahead doing these things yourself. rather than letting someone who doesn't own or care about the car make decisions or uneeded repairs like this for you
good luck man hope you get it fixed on your own the guys here can help you step by step an make it easy enough for a child to do it if you ask
you wouldn't let someone else bend your girl over don't let them screw your vette over either
even if you don't know much but you honestly care about your ride and you want it to be right. you can usually learn/teach yourself, reach out for needed advice and come out ahead doing these things yourself. rather than letting someone who doesn't own or care about the car make decisions or uneeded repairs like this for you
good luck man hope you get it fixed on your own the guys here can help you step by step an make it easy enough for a child to do it if you ask
you wouldn't let someone else bend your girl over don't let them screw your vette over either
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Drifting
Joined: Oct 2007
Posts: 1,458
Likes: 1
From: Collinsville MS
St. Jude Donor '08
Did you re-use the carb and/or distributor from the original engine? If so, that's your problem. Those are 'special' components that are meant to be used only with the '81 ECU (first year for electronic engine management in the Corvette). The carb has an electronic fuel mixture device and is linked to the ECU and distributor [and O2 sensor] via a one-year-only wiring harness for integrated system control. The carb and distributor will not work properly unless all those components are hooked up together and correctly.
If you did not re-use those components, you've not supplied enough info about the specific symptoms you are experiencing, nor the components you have on the engine, for anyone to advise you of possible causes. From what you described, anything we would provide would just be a guess....
If you did not re-use those components, you've not supplied enough info about the specific symptoms you are experiencing, nor the components you have on the engine, for anyone to advise you of possible causes. From what you described, anything we would provide would just be a guess....
Le Mans Master
Joined: Aug 2010
Posts: 9,272
Likes: 17
double check your plug wires for proper location . fuel pump pushing past the seats, floats to high,floats to low, not likely with a new carb. huge vacum leak,vacum hoses mixed up. they had him bring in an adjustable vacum advance so he's ok on the dist, he got a new carb 'quad.' so he's ok on that , gets him down the road but wont idle and runs rough, make sure you check the plug wires
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Didn't hear from you on whether the stock carb OR the stock distributor were reused. But, from a later post, I think it indicates that the carb was new/different than the stock one.
If those stock parts were NOT used, it is most likely that you have a vacuum leak...somewhere; or you have a leak in one of the intake manifold gaskets or at carb gasket. It is also VERY possible that the setup on the new carb (or installation of internal components) is not as it should be. The problem rate for "new" carbs is much higher than it should be. Most builders [or rebuilders] don't even do a 'functional' test of the carb anymore.
If those stock parts were NOT used, it is most likely that you have a vacuum leak...somewhere; or you have a leak in one of the intake manifold gaskets or at carb gasket. It is also VERY possible that the setup on the new carb (or installation of internal components) is not as it should be. The problem rate for "new" carbs is much higher than it should be. Most builders [or rebuilders] don't even do a 'functional' test of the carb anymore.
