Timing and Degreeing

In this article, we will attempt to explain how to setup and degree in a camshaft to insure it meets the makers specifications as well as the tech guidelines. You will first need some basic tools to do this. The major tool needed is a degree wheel. These are available from most all kart supply houses as well as local shops. They run the gambit from a 7" diameter plastic model to a much larger and more expensive 11". If you are just doing your own motors the plastic wheel will do just fine. The advantage of the larger wheels is durability(most are aluminum) and precision(easier to read the exact degree you are on). All degree wheels are marked for both TDC(top dead center) and BDC(bottom dead center) with 180 degrees marked in between each making up the full 360 degrees in a complete circle or one rotation of the crankshaft. The easiest method for bolting on the wheel is to use the bolt and washers you use on the end of the PTO shaft for your clutch and bolt the degree wheel up to the PTO side. You will also need a 1 " dial indicator and a bracket that will position the indicator over the valves. These can be purchased from most kart shops or supply houses. The bracket is around $18 and the dial indicator can run from $20 to $70. You will also need some new valves that have not been clearanced or some older valves with their seats ground off to allow the valve to sit on the lifter with 0 clearance.

You will need to make or purchase a pointer which will attach to the block and be used to reference the exact position on the degree wheel. I have seen everything used from a piece of coat hanger bent 45 degree with a loop on one end that is bolted to one of the head boat holes to very fancy stainless steel pointer attached to the magneto bolt holes. The deal here is to have a stable pointer that will not move while you are degreeing in the cam.

FINDING TDC - The first step in setting up to degree a cam is find TDC of the crankshaft rotation and make your degree wheel pointer reference TDC on the wheel at this point in rotation. Where is TDC? TDC occurs on the compression stroke of any 4 cycle motor. As you rotate your motor forward you will notice it goes through cycles of the intake and exhaust valves opening and closing. TDC is when the piston reaches it's most upward movement during the cycle after the intake valve closes. The most accurate way to find TDC is to use a positive stop. This involves stopping the piston slightly before TDC. There are many ways you can do this but the simplest is to take a 5/16 bolt and large washer, mounting it on one of the head bolt holes near the middle of the cylinder. Now take dime or any thick washer and place it on the piston so that it will hit the washer we just bolted onto the block as the piston is moving up. This should stop the piston and crankshaft movement prior to it reaching TDC. Take notice of the point at which this stop occurs on the degree wheel. Lets say it's 12 degree BTDC. Now rotate the crankshaft in the reverse direction until we again hit the washer and stop the crankshaft movement. Take note of where this occurs, say 14 degrees after TDC. TDC is exactly between the two points we have noted. In this case we are very close and only need to move either the point or degree wheel over 1 degree to split the difference between the two reference points(12 and 14). Keep doing this cycle until you have both markings the same number of degrees before and after TDC. At that stage your point will be referencing exact TDC.

DEGREEING THE CAM - Once you have found TDC, we can proceed on to degree the cam. Both WKA and IKF have the same cam timing reference points for a legal Briggs camshaft. Each point is at .050 height intervals as the camshaft raises and lowers the valve. These cam specifications give you some play at each .050 point. Following are the specs for both the intake and exhaust valves.

To be legal the camshaft must fall in between the degree numbers on the chart, so let's begin. With the intake valve in the closed position and the dial indicator positioned over the valve and set at 0, rotate the motor in the forward direction until the valve just begins to move or open. Now slowly rotate the motor until the degree wheel points at the makers recommended open point(generally somewhere from 35 to 16 degrees BTDC). Once you reach this point stop and look at your dial indicator and make note on the exact reading. This height will be the valve clearance you will need for the valve to open at this precise point (This should match your cam maker's specs). I like to take note of the degree wheel position at several indicator points, so I can change the valve opening, if I am going to later experiment with this. I will generally note the degree wheel point from .003 to .006. Most cams today have recommended clearances in the this range.

After finding the valve opening point continue to slowly rotate the motor until the dial indicator is at .050. Read the timing off of the degree wheel at this point. It should fall between 7 - 0 degrees BTDC. Now continue rotating the engine until you reach the next point on the chart, 100 in this case. Take the reading here and write it down also. Continue to rotate the motor and mark down the degree wheel points at each of the following reference heights(150, 200). After the .200 point slowly rotate the motor looking for the maximum lift of the cam. Most cams will be from .232 - .233. The legal limit is .233. Once you have noted the max lift, continue rotating the crank marking down each .050 point on the downward side of the intake valve. After you record the .050 point on the closing cycle slow down and begin to look for the easy spin beginning. Easy spin? What the $^##@@ is easy spin.

Remembering that what we are dealing here with is a tiller motor, Briggs developed a way to make cranking the motor a little easier for the general public. They did this by stopping the closing of the intake valve for a period of time just before closing the valve. As the close cycle of the intake valve is on the compression stroke of a 4 cycle, what this does is bleed off a little bit of compression making it easier to pull the starter. If the valve closed with no easy spin compression would have a point sooner within this cycle to build within the cylinder.

Back to degreeing the cam. As you approach 45 degrees ABDC slow down the rotation and look for the point where the dial indicator stops it's downward decent. This is a bit tricky and you may need to back the motor up and try again to find the exact point. If you have to back up, go back to at least .150 on the dial indicator and start again. This will take out any slack in the crankshaft movement. The is also true if you miss one of the .050 points you previously recorded. To be WKA legal the easy spin must start between 45 and 60 degrees ABDC and last for a minimum of 30 degrees. Lasting for 30 degrees means the valve can only move .001 during this event, up or down. Most easy spins on today's ground cams start around .015 - .016 lift remaining. The legal variance here is from .013 to .019 . After you find and mark down the easy spin starting point, rotate the motor until the valve drops or raises 1 degree from the starting easy spin point. Now take note of the degree wheel position. this will be the easy spin stop point. The difference between the start and stop must be the 30 degrees. Most cams have more than 30 degrees giving you some room for wear. This is the spot on the cam that seems to wear out the most so be sure to check this carefully on any used cam. Quoting directly from the WKA manual " If ez-spin starts at .015, it may drop to .014 and move around between .014 and .015 but not go above .015." Also " if ez-spin starts at .015 and rises to .016, it may move around between .015 and .016, but at no time fall below .015" At no time can the ez--spin or the .001 travel go above .019 or below .013". IKF specs are different so check their tech manual in this area.

After we are done with the easy spin, let's continue on looking for the point where the intake valve will close. This will be the point on the degree wheel where we encounter the same height we marked for our intake valve opening. If your cam let's say had a recommended clearance of .005 you will want to look for that height as the valve is closing and mark down this point. Generally this is somewhere between 85 and 65 degrees ABDC. If you marked other opening heights earlier, mark the same closing points now for future reference.

After finishing the intake valve, move your dial indicator over to the exhaust valve and zero it on top of a closed valve. Now you repeat the above process marking each .050 lift point as the valve raises and lowers. There is no easy spin on the exhaust valve. Well there you have it. Hopefully you will be able to degree in a cam at this point. It may take you a couple of times to really get the feel for the process but stay with it. If you have a cam that will not come into spec call the maker and ask for his advise. You will find small variances from the manufacturers specs due to the differences in blocks.

SETTING TIMING

Setting the timing of a Briggs is fairly straight forward. What we are going to do here is set the static point within the compression cycle at which we want to fire the spark plug. This is always BTDC as it takes a little bit of time(movement of the piston) before the ignited air and fuel actually reach their maximum point of expansion(power). While you still have the degree wheel on the motor and referenced to TDC, mount your flywheel and magneto. I generally mount the mag with .015 clearance. the best way is not to use feeler gauges but fine some hard paper that is the thickness you want for the clearance. Place this between the flywheel and the bottom legs of the magneto. Tighten down on the mag bolts and you have your clearance.

One of the things we need to know when setting the timing is where the magnet is on the Briggs flywheel. We will use the right most edge of the magnet as reference point in this process. The magnet is housed within the aluminum part of the flywheel. On an older rusted flywheel I like to sand the magnet area to highlight it. As the magnet it is not aluminum you can also blue the magnet to make it stand out from the lighter aluminum. You can use tool makers layout bluing or get some instant gun blue from your local Kmart or gunshop.

To set the timing we are going to use the rightmost edge of the magneto leg and line it up with the right most edge of the magnet. If you look closely at the magneto's leg you will see a slight indention on the bottom. This is the point I like to use for lining up the magnet. It is a little more exact that way. After you line up the magnet and magneto take a look read your degree wheel. This will be the static timing point. This is generally set from 24 - 30 degrees BTDC on a methanol Briggs. For restricted motors, I like to set it between 24-26 and on a stocker set it 28-30.

You can vary the timing by changing the offset keys used in mounting the flywheel or by just changing the position of the flywheel when you bolt it up on the motor. Some builder will use the offset key while others will simply mount the flywheel without a key. Either method will work fine if you torque the flywheel to at least 70 Ft. Lbs. I use the offset keys and have never had a flywheel slip timing. The offset keys are available from any kart shop or mail order house. They come in .010 increments which does not mean they change the timing by 10 degrees. The increments are the amount of offset on the key. You always want to mount the key with the lower side toward the valve side of the motor, or with the flywheel turned slightly clockwise in relation to the crankshaft. Keys vary from maker to maker and different flywheels will have their keyways cut different from one another, so don't put a .30 offset key in a motor and expect it time the same as the last one you did!!! The most common keys you will use will be from .020 offset to .050 for restricted and stock motors.

This method seems to be the standard among current engine builders. Some use other reference points but as long as you always use the same method on your motors you will have a reference point to experiment from.

You can also measure and set your timing without having the degree wheel set up. This method is called the 'in the hole' method of measuring timing. Mount the flywheel as you would normally with the offset key you want to try. Now set your dial indicator up over the piston. What we are going to do is measure the travel to TDC when the magnet and magneto are lined up. Once you have them lined up as described above, zero your dial indicator on the piston. Now rotate the crankshaft forward until you reach the upper most height of piston travel.(TDC). Read you indicator and match it's reading to the closest one on the chart below. This chart will give you the corresponding degrees for a given height.

Height Degrees

.1157- 22

.1262- 23

.1372- 24

.1486- 25

.1603- 26

.1725 -27

.1851- 28

.1981- 29

.2115- 30

.2252- 31

.2394- 32

.2539- 33

.2687- 34

.2839- 35

I generated this chart from an old BASIC program on my PC and will put it on the web page for everyone to use a little later. The calculations use a standard Briggs stroke of 2.437 and a rod length of 3.873(center to center)

 Good luck and happy tuning-- Jimmy Glenn