Thursday, May 8, 2003

Setting Ignition Timing

By Tom Cole
Almost every day someone calls or emails us asking how to set the ignition timing on their engine. It is an important topic because as little as one degree can be the difference between an engine that runs up front and an engine that sputters and pops its way to last place. In this article, I am going to describe what I believe to be the most accurate and reliable method to set the timing on a Briggs and Stratton™ Engine. If you are using an ARC adjustable hub flywheel, begin by setting the hub index mark in the middle of the degree marks on the aluminum body. This will give you the maximum amount of adjustability after you set the timing based on the cam manufacturer’s specifications. The adjustable hub gives you an “at the track” advantage, because it allows you to easily advance or retard the ignition timing to tune for variable conditions.
The first thing you must do to set the timing is to identify the exact position of the flywheel’s magnet in relation to the coil when ignition occurs. To do this right, you need a plain old induction timing light and a car battery. Some folks will tell you about aligning the trailing edge of the magnet with the center of the little button just in front of the left leg of the coil, and for the most part, they are correct. But there is no such button on the Animal coil and factors such as coil gap make this method only a close approximation.

Below is the technique I use to find the exact trigger point using a timing light:

  1. Install only the crank, its bearings and its timing gear in the engine block and put on the side cover.
  2. Install the sheet metal guard that goes behind the flywheel.
  3. Install the flywheel with a standard key on the crank and snug it up with the starter nut. (No need to torque it, you are going to take it back off)
  4. Install the coil with the proper gap. (It will not be removed so tighten it up)
  5. Attach a spark plug to the plug wire and tape it to the block so as to ground it and create a spark.
  6. With the magnet at 12 o’clock under the coil put a white line on the outside rim of the flywheel at about 3 o’clock. This line needs to be plainly visible when looking at the block from the front.
  7. Attach the timing light to the battery and clamp the induction lead on the plug wire. Be careful that all wires are away from the flywheel.
  8. With the timing light pointing at the front of the block, turn the crankshaft clockwise with a drill or starter and you will see the timing strobe light up the white line. You need to spin it faster and more consistently than possible with a pull starter because a magneto has a retarding effect at higher rpm, and you want to compensate.
  9. While the strobe is flashing and the flywheel is spinning, make a white mark on the sheet metal guard or block that aligns with the white mark shown by the timing light on the flywheel. You can now place the magnet exactly where the spark is triggered when and if you ever remove the flywheel. BUT, if you move the coil or the metal guard, you have to start all over again.
  10. Remove the flywheel, side cover and crank, and install the piston, rod, crank, cam, etc (leaving off the cylinder head) getting to the point where you are ready to install the flywheel and set the timing.

You are now ready to set the timing. Truthfully, it is more accurate to set the timing by fixing the piston at a measured distance before it reaches its highest point on the compression stroke. But, although everyone knows the “in the hole” distance for a pure stock setup, (30deg is .2115”) it is difficult to calculate the distance needed by different length rods since the distance traveled by the piston per degree of rotation varies with rod length and/or stroke. You can calculate it, but it just really isn’t worth the effort.

So, since the cam manufacturers generally provide you with a recommended ignition timing expressed in degrees before top dead center (BTDC) of the compression stroke, it is going to be best to set your timing using a degree wheel. (This is a good time to degree your cam too) On to the dreaded degree wheel…
  1. Using coarse grit sand paper, rough up the tapered part of the crank and then make sure it is clean. If you are un-willing to lick it, it isn’t clean enough!
  2. Similarly, rough up and clean the inside of the hole in the flywheel hub.
  3. Set the crank so, according to the degree wheel, you are at the cam manufacturers specified degrees of ignition timing before the piston reaches the highest point of its compression stroke BTDC.
  4. Put a few drops of Loctite™ on the tapered part of the crank and install the flywheel with the white timing lines aligned. DO NOT USE A KEY! Keys do very little to prevent a flywheel from spinning, and they will hinder your accuracy.
  5. Carefully tighten down the starter nut. Check and recheck the timing several times as you tighten to be sure that the white timing lines are still aligned and that the degree wheel is still where it is supposed to be. Then tighten the starter nut, A LOT. You are shooting for tight enough to hold it together, but not so tight as to split the flywheel.
  6. If you have an ARC adjustable hub flywheel, you can fine-tune your ignition timing using a dyno or a stopwatch and a Digatron CHT/Tach at the track.
That’s it! Once you have the timing set and are done with any track adjustments, measure the distance from the deck to the top of the piston (with the white timing lines aligned) and record the measurement. As long as you refresh the engine with the same parts, you can just set it in the hole and go when you refresh. If you re-deck the block, just subtract the amount removed from your recorded depth figure.