81 Corvette Ignition Problem
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Joined: Sep 2004
Posts: 92
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From: Cambria California
81 Corvette Ignition Problem
New crate engine, rebuilt carburetor, plugs, ignition wires. new CO sensor, etc. With the computor connected and controlling the carb. it runs rough and has a very poor idle.Timing jumps to approx 40 degrees and the timing mark dances. No codes other than 12 which I assume is normal. With the plug to the distributor disconnected it idles much better and cant feel any missfire. timing set to 8 degrees advances to 15 degrees and goes no higher when the throttle is advanced.I dont notice any differences in power connected or disconnected. I dont have any way to compare how it was originally because it has been a work in progress. With the computer connected the dwell does swing but only about 5 degrees from the 30 degree mark. I am in over my head on this and dont want to just throw parts at it. I would appreciate any ideas before I have to give up and take it to a tuneup shop
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Joined: Sep 2004
Posts: 92
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From: Cambria California
81 ignition problem
The HEI has about 80,000. miles on it. because the timing advance immediatly jumps way up when the 4 pronge connecter is hooked up and drops back to base when not connected makes me think it is the computor but I dont want to just throw parts at it. The last time I drove it with the computer connected, at 60-70 mph the ignition started to break up and miss and buck. I thought it was a bad coil that was being affected by heat so I changed it out. no improvement.
Safety Car
Joined: Dec 2004
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From: San Diego CA
In addition to what sixfooter asked, what are the specs of your new engine? Compression, etc. If your compression is higher than 9.2:1 and you have iron heads, you should be using 91+ octane and if you aren't the computer may be trying to prevent detonation by changing the timing. The O2 sensor may also be giving it a bad reading and it could be changing the advance strangely because of it.
On the bright side, at least your wiring and stuff is unmolested.
On the bright side, at least your wiring and stuff is unmolested.
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Joined: Sep 2004
Posts: 92
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From: Cambria California
The engine is the 50 states ,Good wrench crate engine, Low compression ratio, designed to pass all emissions(I live in California so everything needs to be stock) I recall a post awhile back about a bad pickup coil in the distributor. I dont remember the details .but I think it would be a problem in all modes. I am not clear on what it does.
Le Mans Master
Joined: Jan 2006
Posts: 9,192
Likes: 27
From: Boca Raton Florida
OK, do you have the 81 shop manual? There is a flow chart in there to start diagnosing problems. I don't have mine running at the moment or I would do some checking on the timing and see whats up. A1sensei has a breathed on motor in his 81, maybe he will chime in. If you have the old sensor, try swapping it in. Don't try useing a ohm meter on the sensor, it will die. You can use a voltmeter on it though, the book tells you what it should read. With the low comp motor detonation shouldn't be a problem. Step by step, you will get there
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Thread Starter
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Joined: Sep 2004
Posts: 92
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From: Cambria California
My primary clue is the way the timing goes from 10 degrees with the distributor unplugged to set base timing to over 40 degrees also at idle when it is connected.. what controls this ? I have replaced the computor but am using the old prom. any connection ?
Racer
Joined: Jul 2006
Posts: 337
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I was reading my 81 manual....and I think there is a temp switch that alters timing depending on engine temp....IE cold or warmed up.
I also found that my distributer had a problem with the tiny plastic bushings for the wieghts. While they looked OK to the naked eye, they were actually so worn they were allowing the wieghts to **** sideways and jam in various positions. After replacement, I noticed a nice improvement in off the line and mid throttle performance. You might want to freshen these up for the little they cost....if you have not already!!
I also found that my distributer had a problem with the tiny plastic bushings for the wieghts. While they looked OK to the naked eye, they were actually so worn they were allowing the wieghts to **** sideways and jam in various positions. After replacement, I noticed a nice improvement in off the line and mid throttle performance. You might want to freshen these up for the little they cost....if you have not already!!
Melting Slicks
Joined: Mar 2006
Posts: 2,465
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From: Orange Park Fl
I don't think the ECM is capable of adding 32 degrees of advance even if it is receiving bad sensor data, so my guess is that the problem is a shorted or open wire or connector. I would start by going to Radio Shack and buying a DeoxIT power booster pen. It is like a felt tip marker of Caig labs contact cleaner, good stuff. Disconnect and clean the connectors at the ECM, distributor and ignition module (go ahead and hit the TPS while you're at it). This is time well spent even if it does not fix this particular problem. While you have it disconnected, check the wiring harness to the distributor for shorts (interconnection or ground) and continuity. If you have questions on how to do this, pm me. God bless, Sensei
Melting Slicks
Joined: Jan 2005
Posts: 3,028
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From: Madison Ms
Once again a victim of the 'puter. 
Now the hard part is figuring out how to make it run right with the 'puter, not that it can't be done by sensi or einstein.....
I'd toss it.......and get something you can control, like a new carb and distributor.
Now the hard part is figuring out how to make it run right with the 'puter, not that it can't be done by sensi or einstein.....I'd toss it.......and get something you can control, like a new carb and distributor.