How to Time Your Motor Without a Timing Light
Jan 1, 2015 8:57:51 GMT -5
mesbe, nobodyjeff, and 4 more like this
Post by Scoutpilot on Jan 1, 2015 8:57:51 GMT -5
For this procedure, you will need your Vacuum Gauge, your Analog Dwell/Tachometer, your 1/2" and/or 7/16" short box/open end wrench (or your distributor tool), a 1/4" tool to adjust the Steel Idle Stop Screw, and a blade screwdriver to adjust your Mixture Screw.
We all want the best performance possible from our motor.
Back in the day, tune-up data was for low-compression engines, using low-octane fuel. If you were using 68 octane fuel, your timing was set for TDC. If you were lucky enough to have 72 octane fuel available, then your timing was set to 5º Before Top Dead Center (BTDC). Fuel does not explode when ignited, but instead burns at a predictably steady rate. Accounting for and depending on how clean and dry it is and its temperature at introduction to the cylinder. The Air/Fuel ratio must be correct as well.
The ignition process necessarily starts before the piston reaches TDC.
Why?
Gasoline burns and the pressure developed by that burning grows relatively slowly. Engineers knew they needed to get the combustion process underway early so the expanding, burning fuel would build pressure in the cylinder and begin to force the piston down as it passed TDC and not chase the descending piston down the cylinder, thus lacking power because the whole process started too late. This is the reason for the initial timing advance.
As engine rpm increases, the ignition (spark) and burning of the fuel/air mixture needs to keep up. This is the reason for an automatic, mechanical, vacuum, or electronic, timing advance.
Today, Regular gas is rated at 87 octane, not the 67 octane the engine was designed for. Higher octane fuel burns slower, not faster. Since modern engines have higher compression ratios, a higher octane is needed. The old shop tale of running higher octane fuel in lower octane engines for better performance is just that. You will get less power and poorer performance, at the same engine settings.
OK, you say. When running higher octane gas in our old motors we need to have the static timing advanced to get the same, properly timed cylinder pressure to maintain book performance. RIGHT?
Right. Let's begin. Hook up the Vacuum Gauge and Analog Dwell/Tach. Lay your tools out on the fender. Hopefully, you have a magnetic fender apron, or a magnetic dish, for them so you won't be chasing them when they fall.
First, check your oil level and your coolant level. Make sure that the Brass Idle Mixture Screw is set at 1-1/2 to 2 turns out from the bottom. When turning it in, do not force it past the light resistance you will feel at the bottom. Back off the Steel Idle Stop Screw just until it no longer touches the Throttle Body. Then turn it in three complete turns. Start the motor and let it warm up to operating temperature. While you're waiting, get a fresh cup of coffee.
When warmed up, loosen the distributor lock screw and while watching the vacuum gauge, slowly rotate the distributor until you have achieved the highest vacuum reading you can. Note: If you can't get the reading into the "normal" zone (15-21) by adjusting the distributor then valve timing is the problem. We aren't finished. You can't leave it here. It's too much advance and you'll soon start hearing the steady "knock-knock-KNOCK!-KNOCK!"
. If you shut it off, you may not even be able to start the motor without that annoying ” YUNH-YUNH-YUNH", and then it reluctantly fires off. You need to retard the timing a tad. Don't take your hand off the Distributor.
Your attention now shifts to the Analog Dwell/Tach. While observing the Analog Dwell/Tach RPM scale, Note the reading. Now, back the distributor off 100 RPM and lock it down. Still not done. Be patient, we're almost there.
Move around to the carburetor. Watching the Vacuum Gauge again, adjust the Steel Idle Stop Screw (on the right side of the carb, in the lever), to again reach the highest vacuum reading you can. The best is 21 inHg.
You will generally be adjusting (backing out) the screw to close the throttle plate to within a .008"-.009" opening.
When you've done that, shift again (DIZZY YET?) to the Dwell/Tach, and having gone to the other side of the carb, you will, with your blade screwdriver, adjust the Idle Mixture Screw (usually brass, sometimes steel) so that you set your Idle RPM to between 600 and 700. (NOTE: 700 RPM is when the Generator kicks in to charge the battery.) Another way is to lean the mixture to peak RPM and back the adjustment screw back to rich 1/4 turn.
Hopefully, having done all of this, your motor is now purring like a 65-year-old kitten.
We all want the best performance possible from our motor.
Back in the day, tune-up data was for low-compression engines, using low-octane fuel. If you were using 68 octane fuel, your timing was set for TDC. If you were lucky enough to have 72 octane fuel available, then your timing was set to 5º Before Top Dead Center (BTDC). Fuel does not explode when ignited, but instead burns at a predictably steady rate. Accounting for and depending on how clean and dry it is and its temperature at introduction to the cylinder. The Air/Fuel ratio must be correct as well.
The ignition process necessarily starts before the piston reaches TDC.
Why?
Gasoline burns and the pressure developed by that burning grows relatively slowly. Engineers knew they needed to get the combustion process underway early so the expanding, burning fuel would build pressure in the cylinder and begin to force the piston down as it passed TDC and not chase the descending piston down the cylinder, thus lacking power because the whole process started too late. This is the reason for the initial timing advance.
As engine rpm increases, the ignition (spark) and burning of the fuel/air mixture needs to keep up. This is the reason for an automatic, mechanical, vacuum, or electronic, timing advance.
Today, Regular gas is rated at 87 octane, not the 67 octane the engine was designed for. Higher octane fuel burns slower, not faster. Since modern engines have higher compression ratios, a higher octane is needed. The old shop tale of running higher octane fuel in lower octane engines for better performance is just that. You will get less power and poorer performance, at the same engine settings.
OK, you say. When running higher octane gas in our old motors we need to have the static timing advanced to get the same, properly timed cylinder pressure to maintain book performance. RIGHT?
Right. Let's begin. Hook up the Vacuum Gauge and Analog Dwell/Tach. Lay your tools out on the fender. Hopefully, you have a magnetic fender apron, or a magnetic dish, for them so you won't be chasing them when they fall.
First, check your oil level and your coolant level. Make sure that the Brass Idle Mixture Screw is set at 1-1/2 to 2 turns out from the bottom. When turning it in, do not force it past the light resistance you will feel at the bottom. Back off the Steel Idle Stop Screw just until it no longer touches the Throttle Body. Then turn it in three complete turns. Start the motor and let it warm up to operating temperature. While you're waiting, get a fresh cup of coffee.
When warmed up, loosen the distributor lock screw and while watching the vacuum gauge, slowly rotate the distributor until you have achieved the highest vacuum reading you can. Note: If you can't get the reading into the "normal" zone (15-21) by adjusting the distributor then valve timing is the problem. We aren't finished. You can't leave it here. It's too much advance and you'll soon start hearing the steady "knock-knock-KNOCK!-KNOCK!"
. If you shut it off, you may not even be able to start the motor without that annoying ” YUNH-YUNH-YUNH", and then it reluctantly fires off. You need to retard the timing a tad. Don't take your hand off the Distributor.Your attention now shifts to the Analog Dwell/Tach. While observing the Analog Dwell/Tach RPM scale, Note the reading. Now, back the distributor off 100 RPM and lock it down. Still not done. Be patient, we're almost there.
Move around to the carburetor. Watching the Vacuum Gauge again, adjust the Steel Idle Stop Screw (on the right side of the carb, in the lever), to again reach the highest vacuum reading you can. The best is 21 inHg.
You will generally be adjusting (backing out) the screw to close the throttle plate to within a .008"-.009" opening. When you've done that, shift again (DIZZY YET?) to the Dwell/Tach, and having gone to the other side of the carb, you will, with your blade screwdriver, adjust the Idle Mixture Screw (usually brass, sometimes steel) so that you set your Idle RPM to between 600 and 700. (NOTE: 700 RPM is when the Generator kicks in to charge the battery.) Another way is to lean the mixture to peak RPM and back the adjustment screw back to rich 1/4 turn.
Hopefully, having done all of this, your motor is now purring like a 65-year-old kitten.
This is the basic version. For a full and complete procedure check out TM 9-4910-477-10 in the library.