Timing the Triumph Pre-Unit (650cc & 550cc)

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Time: 02:00:00


People: Wes White




They say that timing is everything and with the magneto equipped Triumph 650 and 500cc motorcycles, it is certainly true. The magneto must be perfectly timed to fire at 38 degrees before top dead center (BTDC) on the compression stroke of the number one (right side) cylinder.

The best way to achieve this is to use a degree wheel to find true TDC and then find 38 degrees BTDC. If you do not have a degree wheel then you can use a graduated TDC finder that screws into the spark plug hole. By the side of the road you can use a stick and a tape measure as well. It is always preferable to use a degree wheel, however, since the pre-unit magneto is for the most part not adjustable after it is set. Remember, the more accurate the timing, the better the bike is going to start and run in all acceleration ranges.

If you are timing the motor on the bench before installation in the bike, it makes life easy, but if you have to time the mag while the motor is in the frame it helps to have some sort of lift so you don’t have to crawl around on the floor.

Our test bike is equipped with a Joe Hunt Magneto, but the same principles are going to apply with a Lucas K2F or K2FC or even a BTH.

Remove the timing cover and then get the motor in front of you to get a good view of each side. Remove the mag cap to view the points assembly. Rotate the cam clockwise (from the mag side) until you get a .002 gap in the points. You can also trap a piece of cellophane between the points and turn the cam until the contact points “release” the cellophane. With the points set so they are “just beginning to break,” don’t move the cam, otherwise you can disturb the timing. This can be difficult especially with a new Joe Hunt that has lots of magnetism. It helps to have someone helping you out on the primary side; remember to turn the cam in the direction of its normal rotation.

Next we must establish top dead center on the compression stroke of the number one (right hand) cylinder and then 38 degrees before top dead center. Attach your degree wheel to the crank and turn the motor until you reach the compression stroke. You can turn the crank from the pinion nut or the cam nuts as well, or you can put the bike in fourth gear and use the rear wheel to bump the motor through its stroke. Although you can only do this if the primary is hooked up. You can discern the compression stroke by watching for the valves to close through the spark plug hole or by putting your finger over the spark plug hole and feeling when the pressure builds up against your finger as the pistons come up. Remember on the exhaust stroke, the pressure will be sucking at your finger when it is over the spark plug hole. Continue turning the motor over until you reach what you think is TDC.

Notice the “float” of the piston when the it reaches TDC. Adjust the degree wheel to TDC or “0.” Set up your wire pointer to point to “0.” Now back the crank off about 40 degrees and put your piston stop into the spark plug hole. Make a mental note of 40 degrees BTDC or write 40 down on a piece of paper. Continue turning the crank backwards a full revolution of the crank and carefully “bump” the pistons against the piston stop you established. Take note of where you are on the degree wheel; you should be within ten degrees of the original 40 degrees.

Now “split the difference” on your degree wheel in other words turn the degree wheel, without moving the crank, to whatever the point is between your original 40 degrees and where the pistons stopped when you ran it backwards. For example if you came up with 48 degrees on your second “bump”, you would rotate the wheel (independently of the crank) to 44 degrees between 40 and 48. It is interesting to note that whatever your estimation of TDC was four degrees off in this case, enough to give you trouble starting or running.

Now turn the crank forward until you get to 38 degrees before TDC on the compression stroke. Take your advance unit and lock it open with a suitable instrument, I use an old spoke with a nipple attached because the small side of the nipple is just the right size to hold the advance unit open against the spring tension.

On racing machines, or machines you want to start and run at full advance, you can use a fibre or steel wheel, as shown here.

Place the wheel or advance unit onto the tapered shaft of the magneto and carefully turn the bolt down so that it holds the taper, making sure not to turn the rotating armature or the rotating magnet. Sometimes a carefully placed flathead screwdriver can help the wheels to remain fixed.

Remove your spoke and nipple and let the advance close. Replace the timing cover and the magneto cap and you are done.



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