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81 rough idle and shaking

Old 10-10-2018, 04:08 PM
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Stingydonut
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Default 81 rough idle and shaking

hey guys my 81 has a shake to it at idle. like enough that it makes the mirrors vibrate a little bit, recently had a tune up so new spark plugs wires and distributor. to give you an example it kind of feels like how a you would picture a muscle car with a cam to feel ( the car is bone stock) and with a wopping 190hp i really doubt this engine has so much power that its supposed to be this shaky. my mechanic told me i need all my belts replaced as they are slipping, and i need a new carb that doesnt have the computer unit(he left the computer unplugged on purpose as he said it was causing issues). he told me to get one a few years older that still has an electric choke. if anyone can suggest a good year for me to buy a rebuilt one that would be great. also it feels like at idle and low speed like going up hills theres these little bumps with in the engine. like the engine skips a beat about once a second. could this be the belts slipping or the carb issues my mechanic told me about? or does this issues sound more like a motor mount issue? if so how expensive can those get to replace? id assume the issue isnt ignition related due to the fact that plugs wires and distributer were replaced and this issues existed before that was done. no smoke or knocking noises from engine so im doubting its internal. thanks in advance!
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Old 10-10-2018, 05:10 PM
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Monterey C6
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It's a possibility that has something to do with the computer being disconnected. You should also consider that you might have a vacuum leak somewhere or a vacuum hose disconnected.

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Old 10-10-2018, 05:36 PM
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My 80 had a rough idle when I first drove her home. I fixed a few vacuum leaks and she perked right up. 93 octane gas was also required. Make sure timing is correct, and you don't have crossed ignition wires.

Only the 1980 carb has both an electric choke and no computer. Search for "lars" on this forum. He can help you ID a rebuildable core, if you go that route, perhaps to include adding an electric choke to an older carb.
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Old 10-11-2018, 05:06 AM
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Originally Posted by Stingydonut View Post
hey guys my 81 has a shake to it at idle. like enough that it makes the mirrors vibrate a little bit, recently had a tune up so new spark plugs wires and distributor. to give you an example it kind of feels like how a you would picture a muscle car with a cam to feel ( the car is bone stock) and with a wopping 190hp.
So you took it in to the mechanic with a shake and replaced plug wires and distributor but it did not change the shake, rattle and roll?

i really doubt this engine has so much power that its supposed to be this shaky. my mechanic told me i need all my belts replaced as they are slipping, and i need a new carb that doesnt have the computer unit(he left the computer unplugged on purpose as he said it was causing issues).
Why do mechanics have this penchant to replace carbs when a rebuild will accomplish the task? I would replace the mechanic.

he told me to get one a few years older that still has an electric choke. if anyone can suggest a good year for me to buy a rebuilt one that would be great. also it feels like at idle and low speed like going up hills theres these little bumps with in the engine. like the engine skips a beat about once a second. could this be the belts slipping or the carb issues my mechanic told me about? or does this issues sound more like a motor mount issue? if so how expensive can those get to replace? id assume the issue isnt ignition related due to the fact that plugs wires and distributor were replaced and this issues existed before that was done. no smoke or knocking noises from engine so im doubting its internal. thanks in advance!
I'm with Monterey...vacuum related or the electric choke disconnected. Sounds like your auto-mechanical skills are limited but why not connect with a local Corvette club that caters to C3s and get some help there. These guys are more than willing to help solve problems. I would have to do some digging to understand what the computer does in 81 but know that by 82, it was managing that fuel/air ratio and could cause the effects you're describing.

You did not say how many miles on the odometer. How many miles?
If you intend to keep the car, I would suggest:
  1. picking up a 1981 Corvette Shop Manual and study the troubleshooting guide as well as the computer operation.
  2. Get carb rebuilt
  3. Connect with local Corvette club
My mechanically neglected 78 Pace Car ran like you describe. I sent it to the guys below and asked for a rebuild. Got it back, installed and car started right up, running smoothly.

http://allamericancarburetor.com/contactNow.html
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Old 10-11-2018, 04:47 PM
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Originally Posted by Stingydonut View Post
my mechanic told me i need all my belts replaced as they are slipping, and i need a new carb that doesnt have the computer unit(he left the computer unplugged on purpose as he said it was causing issues). also it feels like at idle and low speed like going up hills theres these little bumps with in the engine. like the engine skips a beat about once a second. could this be the belts slipping or the carb issues my mechanic told me about? or does this issues sound more like a motor mount issue? no smoke or knocking noises from engine so im doubting its internal. thanks in advance!
The 81 computer only controls idle. So that could be an issue and a non computer controlled carb may fix it. I would not rule out an internal issue because the low speed bumps as you call it where it feels like the engine skips a beat could be a burnt or stuck valve. Since you say there are no noises from the engine that would rule out a valve being stuck because you should be able to hear that. The slipping belts would not have anything to do with rough idle or a miss (engine skipping a beat) neither would a bad motor mount. Before you spend money on another carb I would take it back to the mechanic and have him do a compression check which should be a lot less than a replacement carb.
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Old 10-11-2018, 07:39 PM
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81 computer controls idle and light throttle cruise. That carb is worthless without a WORKING computer. And I think that computer also controls timing. Get a 50 buck HEI off eBay. They run good. Carb? I would try to get Lars involved in guiding your search for a core to send him. Then take that 81 carb and see how far you can throw it...
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Old 10-11-2018, 10:56 PM
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thanks for all the solid advice guys! my buddy who has a few vettes has a brand new edelbrock 1406 performer 600 cfm carb with an elecric choke hes willing to give me for free. is it worth getting a rebuilt core if i have this carb? will it still cause issues that there is a computer but its not hooked up to the carb? im not looking for amazing performance just a smooth running car, i understand the gold standard would be to rebuild my carb or rebuild a factory core. but will it matter that much if i replace it with a aftermarket?
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Old 10-12-2018, 12:12 AM
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Are the electrical connections to the carb disconnected? or did the mechanic disconnect the plugs into the computer? (the computer sits in front of the battery)
Do you get a check engine light with the ignition on but the engine not running?
Either way assuming you still have the factory distributor then you have no timing advance. (there should be a 4 pin plug that connects onto some wiring at the back of it)
Vacuum leaks confuse the computer and cause a rough idle like you are describing. An easy check for a vacuum leak is to disconnect and cap every line coming out of the carb and see what it runs like.

The computer carbs are a bit different to set up and tune than the non computer carbs and a lot don't like them for that reason once they are set up they run just as well as any other carb.
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Old 10-12-2018, 01:04 AM
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Is there a hint of mis-fire? If so you could have bigger problems than rebuilding a carb... like me with bent valve stems and whatnot. If not, then troubleshooting the carb is the way to go.
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Old 10-12-2018, 11:57 AM
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Run it till almost empty and put in a tank of ethanol free 93 octane. I had an '81 Z28 with essentially the same engine, computer control and carb. Even when new it ran best on Shell SU2000 gasoline. As it aged that was the ONLY gas with which it started perfectly and idled smoothly. I'm not sure if this true but I've read many times (and agree by experience with that Z28) that the first generation Computer Command Control system lacked feedback to automatically adjust as things naturally wore and strayed from their "factory perfect" specs.
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Old 10-12-2018, 12:19 PM
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Sounds like your mechanic did not know how to work on carburetors, especially an electronic carb. Even if you have all electrical connections disconnected from the carb you should be able to tune it to run like a non-electric Rochester carb. Without knowing the history of the carb, if it was recently rebuilt etc. it could be very dirty, only some electrical items working etc.

The ECM controls the lean/rich mixture, timing and lock up torque converter. It does pull information from the O2 sensor, map sensor and controls the ECG valve. You could have a stuck ECG as well. I went through all of the same on my 81 when I first got it 4 years ago. First find any vacuum leaks and fix, set timing to factory 6 degrees at idle and move it up if you want a little more power after problems are fixed. The ignition system needs to also be in good shape before tackling setting the carb properly. If you keep the electrical carb, pick up a Monitor 85 scan tool to set up all carb settings, TPS voltage and check how the computer is running, if there are any codes etc. Oh, also the factory system is set up to run on 87 octane. I run mine all the time on 87 and every once in a while I put in ethanol free 89 octane.

If you decide to remove the carb and ECM you'll need to find a way to control your lock up torque converter and change out the distributor.

Good luck. Let us know how things go.

Last edited by Reaper19; 10-12-2018 at 12:21 PM.
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Old 10-12-2018, 08:44 PM
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"If you decide to remove the carb and ECM you'll need to find a way to control your lock up torque converter and change out the distributor."

if i just replace the carb and distributor but leave the ECM will the torq converter still work as it should?

Last edited by Stingydonut; 10-12-2018 at 08:44 PM.
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Old 10-12-2018, 09:24 PM
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Originally Posted by Stingydonut View Post
if i just replace the carb and distributor but leave the ECM will the torq converter still work as it should?
No it wont work. The ECM looks at the Throttle position sensor in the carb as well as vehicle speed to control the converter.
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Old 10-12-2018, 09:42 PM
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gotcha thanks. whats the downsides to having no lockout on the transmission?
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Old Yesterday, 12:02 PM
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Originally Posted by Stingydonut View Post
gotcha thanks. whats the downsides to having no lockout on the transmission?
Higher temps and poorer fuel mileage.
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