GM had such a problem with run on when they boosted the timing to 14° before top dead center, and at this same time, made air conditioning standard on all the vehicles with a 305 V8 and of course the 307 and the V6s. This was prior to fuel injection and also included some of the TBI cars.
The run on problem was so bad from the factory that when you shut the engine off they had it set up that when you turned off the key it engaged the AC clutch.
It's a little bit down the rabbit hole, but because of the change to unleaded and having to use a lower compression engine this is what caused the problem. The car's base idle is running around 1,000 to 1,200 with 14 degrees advanced timing before top dead center, there is too much air, and the plates are uncovering the transfer slot, so too much fuel.
This is also why some of the cars had the solenoid on the throttle lever. The method to start the car was to depress the gas pedal three to four times. This gets a pump shot into the carb. With foot off the gas then you turn the key and the car should start. If it's idling too fast then you press the gas and release the gas pedal but when you do that that sets the solenoid on the throttle lever, now your base idle is running between 1000 and 1200 RPM.
After driving the car and the engine is warm you have the temperature valve that can import a signal to the solenoid so that when you shut the car off it disengages or while you're driving it at Cruise it disengages and steps you down to about a thousand RPM at base idle and somewhere around 850 in drive.
When you get ready to park and shut the car off transmission goes into Park, disengages all the clutch packs so the whole transmission is basically free revving in neutral, the RPMs go back up. You shut the car off the solenoid a lot of times would stick so your car is still at 1,000 to 1,200 RPM, your base timing is still at 14 before.
So you shut the car off and now you got run-on. To counter this, GM made it so that when you shut the car off the AC clutch engages which drags the engine RPM down and the car shuts off with no run-on or dieseling as they used to call it, ...problem solved.
What everybody seems to forget is that the earlier tune-up specs were zero to four to maybe eight before top dead center. You have the distributor set to manifold vacuum. Because once the engine starts you need to boost the base timing or you don't have any power. This method goes back to the 50's, and this is because we were using leaded fuel and compression ratios anywhere from 8.5:1 - 11.5:1 from the factory.
Once we go to unleaded fuel in the early 80s, you cannot run high compression now. You have Pistons now that are as low as 7.5:1 - 8.5:1, as static compression. That's at the point where the engine will barely run, let alone run correctly. But this was done because unleaded fuel does not have the octane rating needed for 9, 10 or 11 to one pistons.
So the solution was to open up primary jetting to allow a little more fuel at the same time reduce your air fuel mixture and cap the screws off, so there is no more adjustment at tune up. Increase your base timing to about 14 degrees before top dead center.
This starts your flame front as the piston is still moving up. This allows the fuel charge with has already been partially burned to create the head pressure needed to push the piston down even though with the unleaded fuel and it's poor octane rating running leaner.
This gives you a higher compression temperature and this higher compression temperature assists with keeping the engine running even though you're using poor quality fuel.
The factory keeps the base timing higher, the computer controls the reduction of timing based on when it senses a detonation event.
This is the basis of the C3 system, sometimes called CCC. When a detonation is perceived the computer backs down the timing until the detonation goes down and this would be at (lean) cruise and acceleration because you're no longer using higher octane fuel.
When you start the discussion about the restoration of an early to mid 80s car you have to understand the changes that happened at that time. You can't think about "now" when you have a Silverado in the driveway with a 5.3 fuel ejected LS engine. That didn't exist back then.
We had carburetors that were designed in the 60s and being updated to handle the lower quality low octane fuels. And then trying to get the engine temperature and the base compression pressures to work with the fuel that was available on the street.
All of these cars and all these problems was because fuel delivery was messed up, the engine temperature cooling baseline operation environment was messed up, and timing was messed up. Then you add soft cams, problems with metallurgy, Japanese steel, thrust weight ratios that were way out of whack for a 5400 lb car, four-speed automatic transmissions with a wide ratio first gear running about three to one, and a rear end gear that's somewhere about two and a half to one.
And don't forget that the engines are running leaner, the fuel is crappier the engine is running hotter so it's also overheating. And what did anybody really care about??? Keeping the EPA happy so you didn't get multi-million dollar fines.
So you just have to remember that the 80s automobile was at the point where brand new from the factory automobiles would barely run properly without a bunch of quirks and adjustments made by the engineers to get them off the line so they could be sold.
In my own history, I ran my Turbo Trans Am crankcase under full vacuum at all times I replaced the quadrajet with a Holly Double Pumper, reduced the thermostat down to 180, and added water injection. GM cannot sell a car like that because no one would buy it. 1982 I took that Trans Am at 75 miles an hour from Chicago to Springfield and got 25 miles per gallon running a 650 Holley double pumper.
In 1983 at College of DuPage when I finished up my ASE certification, I had my 1979 GMC Jimmy with 35O 4 barrel and on regular gas which was still available at the time. Hooked up to our equipment and beat the EPA standard for exhaust emissions. Not only for that year but for the next 5 years step. But you can't get the general public to keep their trucks their cars in tune.
It wasn't until about 1985 where GM was able to get the automobile to where it needed to be again and start to run again with some level of performance. I remember being at the dealership when we brought in a brand new normally aspirated Trans Am at the same time a customer brought in an SS Monte Carlo for some work.
I had my back turned, yet as those cars drove in, this was the first time, on that day that I heard a V8 with a cam. It was unfortunate that a lot of those cams were soft which later on caused a lot of runability problems. However it was in 1985 at the dealership when that Trans Am came in and that SS Monte Carlo came in after, and you could actually feel the floor that you knew you had a V8 that sounded and was acting like it should.
After that comes TBI, then TPI, then vortec then MPFI, and then LS. Gone are the days of the Quadrajet.
So, with all of that said, you have to understand what was going on when those 80's cars were being produced that they were messed up from the factory and they barely ran from the factory. And now, we're running out of the mechanics and technicians that understand those vehicles, because the mechanics are aging out. They have retired, they have died, and there are very few left.
The run on problem was so bad from the factory that when you shut the engine off they had it set up that when you turned off the key it engaged the AC clutch.
It's a little bit down the rabbit hole, but because of the change to unleaded and having to use a lower compression engine this is what caused the problem. The car's base idle is running around 1,000 to 1,200 with 14 degrees advanced timing before top dead center, there is too much air, and the plates are uncovering the transfer slot, so too much fuel.
This is also why some of the cars had the solenoid on the throttle lever. The method to start the car was to depress the gas pedal three to four times. This gets a pump shot into the carb. With foot off the gas then you turn the key and the car should start. If it's idling too fast then you press the gas and release the gas pedal but when you do that that sets the solenoid on the throttle lever, now your base idle is running between 1000 and 1200 RPM.
After driving the car and the engine is warm you have the temperature valve that can import a signal to the solenoid so that when you shut the car off it disengages or while you're driving it at Cruise it disengages and steps you down to about a thousand RPM at base idle and somewhere around 850 in drive.
When you get ready to park and shut the car off transmission goes into Park, disengages all the clutch packs so the whole transmission is basically free revving in neutral, the RPMs go back up. You shut the car off the solenoid a lot of times would stick so your car is still at 1,000 to 1,200 RPM, your base timing is still at 14 before.
So you shut the car off and now you got run-on. To counter this, GM made it so that when you shut the car off the AC clutch engages which drags the engine RPM down and the car shuts off with no run-on or dieseling as they used to call it, ...problem solved.
What everybody seems to forget is that the earlier tune-up specs were zero to four to maybe eight before top dead center. You have the distributor set to manifold vacuum. Because once the engine starts you need to boost the base timing or you don't have any power. This method goes back to the 50's, and this is because we were using leaded fuel and compression ratios anywhere from 8.5:1 - 11.5:1 from the factory.
Once we go to unleaded fuel in the early 80s, you cannot run high compression now. You have Pistons now that are as low as 7.5:1 - 8.5:1, as static compression. That's at the point where the engine will barely run, let alone run correctly. But this was done because unleaded fuel does not have the octane rating needed for 9, 10 or 11 to one pistons.
So the solution was to open up primary jetting to allow a little more fuel at the same time reduce your air fuel mixture and cap the screws off, so there is no more adjustment at tune up. Increase your base timing to about 14 degrees before top dead center.
This starts your flame front as the piston is still moving up. This allows the fuel charge with has already been partially burned to create the head pressure needed to push the piston down even though with the unleaded fuel and it's poor octane rating running leaner.
This gives you a higher compression temperature and this higher compression temperature assists with keeping the engine running even though you're using poor quality fuel.
The factory keeps the base timing higher, the computer controls the reduction of timing based on when it senses a detonation event.
This is the basis of the C3 system, sometimes called CCC. When a detonation is perceived the computer backs down the timing until the detonation goes down and this would be at (lean) cruise and acceleration because you're no longer using higher octane fuel.
When you start the discussion about the restoration of an early to mid 80s car you have to understand the changes that happened at that time. You can't think about "now" when you have a Silverado in the driveway with a 5.3 fuel ejected LS engine. That didn't exist back then.
We had carburetors that were designed in the 60s and being updated to handle the lower quality low octane fuels. And then trying to get the engine temperature and the base compression pressures to work with the fuel that was available on the street.
All of these cars and all these problems was because fuel delivery was messed up, the engine temperature cooling baseline operation environment was messed up, and timing was messed up. Then you add soft cams, problems with metallurgy, Japanese steel, thrust weight ratios that were way out of whack for a 5400 lb car, four-speed automatic transmissions with a wide ratio first gear running about three to one, and a rear end gear that's somewhere about two and a half to one.
And don't forget that the engines are running leaner, the fuel is crappier the engine is running hotter so it's also overheating. And what did anybody really care about??? Keeping the EPA happy so you didn't get multi-million dollar fines.
So you just have to remember that the 80s automobile was at the point where brand new from the factory automobiles would barely run properly without a bunch of quirks and adjustments made by the engineers to get them off the line so they could be sold.
In my own history, I ran my Turbo Trans Am crankcase under full vacuum at all times I replaced the quadrajet with a Holly Double Pumper, reduced the thermostat down to 180, and added water injection. GM cannot sell a car like that because no one would buy it. 1982 I took that Trans Am at 75 miles an hour from Chicago to Springfield and got 25 miles per gallon running a 650 Holley double pumper.
In 1983 at College of DuPage when I finished up my ASE certification, I had my 1979 GMC Jimmy with 35O 4 barrel and on regular gas which was still available at the time. Hooked up to our equipment and beat the EPA standard for exhaust emissions. Not only for that year but for the next 5 years step. But you can't get the general public to keep their trucks their cars in tune.
It wasn't until about 1985 where GM was able to get the automobile to where it needed to be again and start to run again with some level of performance. I remember being at the dealership when we brought in a brand new normally aspirated Trans Am at the same time a customer brought in an SS Monte Carlo for some work.
I had my back turned, yet as those cars drove in, this was the first time, on that day that I heard a V8 with a cam. It was unfortunate that a lot of those cams were soft which later on caused a lot of runability problems. However it was in 1985 at the dealership when that Trans Am came in and that SS Monte Carlo came in after, and you could actually feel the floor that you knew you had a V8 that sounded and was acting like it should.
After that comes TBI, then TPI, then vortec then MPFI, and then LS. Gone are the days of the Quadrajet.
So, with all of that said, you have to understand what was going on when those 80's cars were being produced that they were messed up from the factory and they barely ran from the factory. And now, we're running out of the mechanics and technicians that understand those vehicles, because the mechanics are aging out. They have retired, they have died, and there are very few left.
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