I was just wondering if anybody can tell me what I need to set my timing to on a 1973 XLCH. First, what RPM to be at and what the actual degree BTDC. The book says set idle to @900 then in the inspection hole, look for the vertical line. Is that all there is to it or do I need to do more? Thanks in advance. Maybe this will get the attention of PinionGear. I've been reading many posts and I have to say that this website is by far the best for us diehard fanatics.

 

1. Checking vs Setting the Ignition Timing

You can check the timing without changing anything. Changing the timing can be done by one person but is easier with two. The ignition module or points is on the right side of the bike, the timing hole is on the left side. Easier with one person on each side than to go back and forth. Standard advance for the 1980 XL is 40`. I have mine set at about 35`. At 38` i get pinging.

2. Equipment That You Need

You need an induction timing light which you can buy at any auto supply store. Best to get the "dial back" type. These allow you to set a number [like 35`] into the timing light and then adjust the ignition module/points backplate until you are there.

If you have very well insulated spark plug wires the timing light may not sense the signal thru the wire. If this is the case then open the gap on the spark plug to 050 or more. Reset it to your normal gap [030 points or 040 electronic] when done with this procedure.

It also helps to have a throttle lock as the timing should be checked/set with the engine at 2500 to 3000 RPM.

You will need a piece of rubber oil line hose about 1 inch long. Press one end up against the side of a stone grinding wheel [or whatever] to make sure it is perfectly flat.

3. Front Cylinder: TDC Mark vs Advance Timing Mark

The TDC [Top Dead Center] mark is used with a dial back timing light to check the timing, or to set the timing at a specific degree, such as 37`. The advance timing mark is used to set the timing to the factory setting, such as 40`. Best is to use a dial back timing light to check or set timing.

4. To Identify Your Front Cylinder TDC Timing Mark

Remove spark plugs. Rear wheel off the ground, shift to 2nd gear, rotate wheel until it clicks, repeat until you get to top gear. Now by rotating the rear wheel you are rotating the engine. Almost impossible with the plugs in; very difficult in lower gears.

You want the front cyl to be at TDC. You can see in thru the spark plug hole, especially with a flash light; you can try inserting something soft like a pencil and watch it rise and fall as you rotate the engine with the back wheel but be careful it does not break or get stuck! You will have to hang on to the pencil with left hand while rotating the rear wheel with the right. Helps to have a good wingspan, or an extra person.

With the piston at precisely TDC look in thru the timing hole. You should see a drilled dot or a line. That is your front cylinder TDC mark. Once you know for certain what the TDC mark looks like you can check the timing.

5. Procedure, Checking The Timing

Begin with the engine at operating temperature. Remove the timing plug from the left side of the engine. Insert the 1" hose smooth end first, right tight up against the flywheel. I use a needle nose plier and twist it in there as firmly as i can. If it is not smooth and tight against the flywheel oil will spray out while you are working.

Start the engine, have it at about 2500 to 3000 RPM [it will be extra noisy with the timing plug out], set the timing light at 40`, aim it into the timing hole thru the hose. Click it down one degree at a time while you look for the timing mark. If you don't find it you may have to work up from 40`.

If it is in the 35` to 40` range and there is no pinging then you may wish to leave it as is. With pinging on acceleration you may want to retard it by about 1/16 of an inch or less [see Math below].

6. Procedure, Setting the Timing

Remove the points cover from the right side. You will be rotating the back plate so first mark it so you can get back to the current position if needed. Make a scratch mark or use a permanent magic marker.

To change the timing loosen the two standoffs holding the points/ignition module and rotate the backplate. Clockwise advances [larger degree number] counter-clockwise retards [smaller degree number].

7. Some Math, How Much to Rotate

The backplate has a 3" diameter = 1.5" radius.
The circumference = 2 * pi * radius = 2 * 3.14 * 1.5 inches = 9.42 inches
The circumference = 9.42 * 16 ~= 151 sixteenths of an inch.

One rotation of the backplate corresponds to two rotations of the engine, so it is like a circle that has 2 * 360 = 720 degrees. 720 / 151 ~= 5

Therefore a rotation of the backplate by about 1/16 of an inch results in a timing change of almost 5 degrees!

 

With regard to the ignition timing post:

1. You cannot believe what anyone or any book says your timing marks look like. You must follow the procedure above and look into the hole. That is, unless you are absolutely certain that the flywheels are original with the bike.

2. Many references suggest setting the timing at a lower RPM than do i. My experience is that at less than 2500/3000 RPM the timing mark will not be steady in the hole.

 

Here is the way I do it....
1) Check and set the points gap first. Should be .018.

2) Check the gap on both of the ignition cam lobes (front/rear
cylinder) Front cylinder is the tall narrow lobe and the rear is the fatter lobe.

3) The gaps will most likely be different. In a perfect world, you set one cylinder at .018 and the other should be .018 also.
Not a perfect world however.
The allowable difference between the two is .004 maximum. As I have described before, split the difference equally on both sides of .018 if they are different.

4) Now you are ready to start the timing setting. Place the breaker back plate in the middle of the slot if you have had it out installing new points. This will get any engine to start.

5) I like to use the plexiglas plug in the hole myself. Fire up the engine and warm it to operating temperature.

6) Clip the timing light pickup clamp on the front cylinder wire.
Clip the positive wire to the battery + and the negative wire to a good ground.
Run the engine at 2000 rpm. 900 rpm is the idle speed. 2000 rpm gets the centrifugal advance weights to fly outward and advance the timing.
This is where you will check the timing (in the full advanced state)
The engine fires at 40° BTC running in the full advance state on a 1971 and later engine.
This is 2000 rpm and the mark dead center in the hole. When the engine is shut off the timing retards 30° (advance assembly springs retract the weights) You cannot measure these figures, so just rely on 2000 rpm and the mark showing up in the hole.

7) The slash mark is hard to see, so don't expect it to be easy, because it is not. By angling your head you will be able to pick up the ghost image of it though. You may also see a ghostly double dot. This double dot is the mark for the rear cylinder. Sportsters fire both plug wires at the same time, known as the 'waste spark system', so ocassionaly you are going to see the timing mark for the rear cylinder. Pay it no mind. Your pickup is on the front cylinder..

8) Slowly turn the breaker plate using a screwdriver in the adjusting slot until the slash mark appears in the hole.
This is where a buddy is a big help. You can do it by yourself, but having a friend turn the breaker plate on the right side while you look in the hole on the left side is a great advantage and makes life much easier!
Dead center is perfect at 2000 rpm, but anywhere in the hole is fine, so don't beat your self to death trying to get it dead center. It is hard enough just to see the mark.

9) When you have done that, tighten down the breaker plate carefully making sure it does not move. If it moves...... you get to do this all over again.
Replace the steel plug in the hole and enjoy some riding............ pg

 

3. If you have electronic ignition with a VOES, disconnect the VOES while doing the timing procedure

 

Excellent. thanks Mick and Pinion. If I get a chance to get out to the garage, it'll be done 2nite. I did stop by the local Suckerpunch Sally's and grabbed one of those plastic timing hole inspection plugs the shop rags spoke of. I tried to put a piece of packaging tape over the hole last night. That was a laugh. Was spitting new oil all over the place.

 

do you use inch or millimeter measurement??

 

All of my measurements are in inches.
If you want to use millimeters, just do the conversion.
pg