Timing Impulse Drive Magneto

[MochetVelo]

Any tips for timing a magneto with an impulse drive? I have a 1910 Model 10 Buick with a Bosch DU4.  

I thought I'd timed correctly, but it's a non-starter right now. Mag & carb were recently rebuilt.

 

Phil

[Oldtech]

Time it so when impulse trips you are just at or slightly past top dead center. Then it will be suitably advanced when running. That of course assumes the mag is set up properly.   The rebuilder likely set things as they were so the question is, Is it the one off the car?  There are several things that can be adjusted in the mag itself, such as rotation, right or left, amount of delay on the impulse- read advance when running, 

[MochetVelo]

Yes, this mag was on the car when I got it, and the engine ran well. I understand "regular" (no impulse) mag timing, but the impulse confuses me. Bosch says: "Bring No. 1 piston up to TDC of the compression stroke. Revolve the magneto armature backward until the contact points would be just about to open if the armature were revolving in the correct direction. Then securely lock the adjustment of the adjusting driving member."  The 2-pin impulse connection allows only a 180-degree adjustment. Anyway, I'm working on it!

 

Phil

[Oldtech]

The instructions may well be for a Mag with no impulse coupling.  Does it have an advance mechanism for the spark lever on the steering wheel too?  if so the above may work if it is installed in the fully retarded setting.  

[Terry Harper]

 

[Mark Shaw]

I don't recall where I got this, but it is a very complete description:

Timing Your Magneto Ignition

If you do major engine work, or just want to clean and paint your engine, chances are you have to remove the magneto and spark plug wires. And, you probably dread when you reach the point where you have to put it all back together and make it run. You might try carefully making a diagram of the wires, tape the magneto in the position it comes off, and be careful not to turn the engine over while you have these components off.  But after the cleanup and painting is done and it is time to put the magneto back on and start it up, you usually find that the tape didn't stick too well and your diagram is long gone. You could bolt it on as near as you can remember and hook up the wires according to the diagram (if you can find it). But the best you can usually do is crank till you have massive blisters and all you can get is an occasional pop or sputter.  Sound familiar?  You won't usually have the luxury of keeping the components aligned correctly.  Magneto setup is a simple process that every old car guy should understand.

Rough Timing

Aligning the Cap

There is a gear on the magneto shaft that drives a gear on the rotor. These gears must mesh at the proper point. On some magnetos, there are 2 teeth with bevels on the magneto side and a tooth with a timing mark on the cap side. The marked tooth meshes in between the bevel teeth. The intent of this process (in case all the marks are gone), is to get the points to open and close at the correct times to provide voltage out to the plug wires. Knowing this, it is possible to guess the meshing in the case of missing markings.

Lining up the Engine

The engine should be at the top of the compression stroke on the number 1 piston. There were many ways that this can be marked on engines, and these timing marks are often missing. There are a couple of sure-fire ways to know. Make a tool using small fittings with the same thread as the spark plug on one side and a nipple for a hose on the other side (These are commonly made for compression testers). With this screwed in to the number 1 spark plug hole, attach a hose to the other side, & crank the engine until air begins to rush out the hose (put a balloon on the end of the hose if you want to get some visual feedback). When the air starts coming out, it means you are headed into the compression stroke. Slowly continue turning the engine until the number 1 piston reaches its highest point.  (You can use a long thin plastic rod or straw to feel & see how high the piston is.

Bolting on the Magneto

To get your magneto to mesh to the engine, you must turn the magneto (in the direction it was designed to operate in) until the rotor is positioned over the number 1 plug wire terminal. When positioned correctly, the dogs or slots on the mag should line up with the corresponding dogs or slots on the engine and slide right in. Loosely bolt on the magneto so it can still be turned while being held in place. Connect a grounding wire or kill switch in the off position to prevent the engine from starting unexpectedly during the final timing.

Plug Wires

The plug wires should be connected in the order specified in your manual (Note: if you are not using what your manual, hook the number 1 wire to the cap terminal that you timed to be number 1). The firing order and direction of magneto rotation are the keys to hooking up the rest of the wires. Some engines use 1-2-4-3, others use 1-3-4-2. If no engine manual is available, you can determine the firing order by watching the order in which the intake valves open (the rocker arm will go down when the valve opens). Determining the firing order this way will work for modified engines.

Final Timing

At this stage, you have rough timing but don't try to start it this way. Make sure the magneto is grounded or you might just break your arm or worse. In this state the engine can backfire and run pretty ragged. If you are using a hand-crank, it can furiously spin backward and seriously injure you.

To get that final timing, rotate the body of the magneto counterclockwise (Note: whether counterclockwise or clockwise will depend on the rotational direction your magneto is designed for, the rotation you want here is the opposite of the normal rotation). Slowly turn the engine through its strokes till you have the number 1 piston at the top of its compression stroke again. Now gently rotate the magneto clockwise until you hear the pronounced click of the impulse coupler. This indicates that the magneto is right at the point where it will fire the number one piston. Tighten up the bolts, remove your temporary safety ground wire (if you put one on) and try starting the engine.

This is the point where, if you are hand-cranking the engine, you must follow hand-cranking safety precautions.  Since you are trying to start an untested engine, there are several factors involving timing that can cause your engine to backfire, making this the most dangerous time for hand-cranking.

After this process is complete, it is up to you to figure out if any further minor adjustments will help your engine run at speed. In general, your timing should be very close at this point.

 

 

 

Problems

You may have timed your magneto correctly but the engine still won't start. There are many things that can go wrong that are beyond the scope of this article.   A few related problems are as follow.

No Click?

An audible click was mentioned that should occur when the engine comes to the top of any compression stroke. This click is made by an impulse coupler that allows an engine to start at cranking speed with an extra hot spark by accelerating the action of the magneto.  If you do not hear that click, your magneto may not have an impulse coupler, or it may be broken. It is possible to start an engine with a dysfunctional Impulse Coupler, but to do so requires a starter that can crank the engine at fairly high RPM.

Confused on Number 1?

Some caps have a #1 at the contact where, by convention, the number 1 spark plug wire should plug in. In some cases, this conflicts with what the operators manual may indicate is the number 1 plug contact. In this case, if you time the magneto according to the manual but hook up the wires according to the number on the cap, the machine might not be timed correctly. What's important is for you to hook up the number 1 wire to the contact on the cap with the contact you are timing the rotor to inside the magneto.

Got Spark?

Before you spend too many days cranking, check the spark. If the magneto is functioning, you should be able to get a spark. Most auto parts stores carry a spark tester that makes it pretty easy. It looks like a spark plug with a large alligator clip. Hook this tool to a plug wire and connect the clip to ground. When cranked, it will flash if you have sufficient spark. If you can't see a spark, you are wasting a lot of energy cranking.

Grounded?

If you aren't getting spark, disconnect your ground wire at the magneto and try again. Remove all the plug wires from the plugs to keep the engine from starting since you are testing for spark, not trying to start it. The kill switch could be shot and permanently grounding the magneto. Pulling the ground wire off is the same thing as disabling the kill switch or more directly "turning the key on". When the engine does start, ground the magneto immediately with a jumper to kill the engine. This is critical since, depending on the wiring of your electrical system, you may be running with it disabled (since the ignition switch is the component in question). This could damage the generator if you have one.

[Terry Harper]

5 hours ago, Mark Shaw said:

... If you do not hear that click, your magneto may not have an impulse coupler, or it may be broken. It is possible to start an engine with a dysfunctional Impulse Coupler, but to do so requires a starter that can crank the engine at fairly high RPM.

We just experienced this. We had a magneto that had an occasional or no spark. When it was tested by our magneto guy it tested hot. Re-installed we had no spark again. We came to the conclusion that the impulse coupling was occasionally not doing what it was supposed to do. Unfortunately being a 964 CID T-head turning it over at a "fairly high RPM" was not an option.

[MochetVelo]

My magneto also failed when installed after rebuilding. The impulse didn't click at all. I discovered that it must have accurate alignment with the connection to the engine or the impulse will not work.