I'm hoping that someone can help me with the timing on a Bosch DU4 Magneto that I have on a car, and where the correct timing position is supposed to be.
At the moment the car runs well, not hot and has plenty of power, but I've never been certain that the timing is right.
I've attached a couple of pictures, the first one with the timing lever fully retarded, the second with it fully advanced if they're of any use. The pictures were taken with the engine at TDC.
Joe what model number is your DU4? It'll be stamped on the side of the base. Where is your magneto mounted on the engine and what drives it? Does it have an impulse coupler?
First - I would say that your initial statement about how the car runs: by definition your timing has to be right. If it runs that well the timing has to be correct. If it wasn't correct it would run hot, or not have power, or both, or not run at all.
I'm by no means an expert, and especially in this case with what you've said I would fiercely adhere to the "if it ain't broke don't fix it" rule.
Having said that, your points look really odd to me. Not so much the points mechanism itself, but the spark advance arm they ride inside. Specifically the cams on yours are crazy long. Obviously it works, but the DU4s I've fooled with and the pictures I've seen, normally the cams that open the points are much shorter. Essentially inverted from what you have there.
Here is a pic of my spare unit's spark advance lever and cams.
Again, with the way your car runs you can be confident your timing is correct. It'll take a DU4 expert to explain to both of us how and why the spark works on yours and mine both when the points run open/closed completely opposite from one another.
Lastly, if your DU4 runs on the speedster in your profile then k say MANY more pictures of your engine and car are necessary!
Why not use a timing light "strobe" to check it? You should be able to put it right where you want it. Dave in Bellingham, WA
If it's running fine and not overheating why monkey with it?
Anyway >>> http://www.oldengine.org/members/diesel/magnetos/ABDURep1.htm
Well, I am not a magneto expert either, but have used a few DU4 mags. The Bosch DU4 was manufactured for (I think) over twenty years, with quite a few changes over that time. They began about 1910, and were used on several marques of cars, trucks, and eventually many tractors. They were also sold after-market as replacements (magneto fit was almost sort-of standardized early, so off-make replacements were used on a lot of cars, and a lot of other early magnetos were troublesome at best)
From the photos you show (very close up, not a lot of information), it appears to be an early version (at least for the points mechanism). I have seen other early DU4s with the long internal cams to actuate the points. They changed a couple of times over the years the DU4 was manufactured. The points timing is critical both in its relation to the piston position and the armature position. Points are also right-hand or left-hand rotation, and will work the wrong way, but not properly. Again, timing is also affected.
To some extent, the points and the points housing with the internal cams are interchangeable with other magnetos, including other models of American Bosch, German Bosch, and a few non-Bosch mags. But interchanging them can get tricky, as that can affect the timing relative to the armature.
There are timing marks also on the internal gears of the magneto, both on the armature gear and the distribution gear. These marks also changed over the years. Mixing and matching parts can get tricky. One magneto I had years ago had been a victim of such mix and matching. It started hard, and ran poorly, but had one of the hottest armatures I ever used. By comparing several magnetos, I was able to sort out the change in a timing mark, and re-timed the distributor gear inside the mag. Ran great after that.
In general, magneto's firing gets hotter with increasing rpm. So, usually, the first clue to trouble is at low rpm. If you do not have an impulse for starting (I never used one)? Hard starting usually is the biggest clue to mag troubles.
Magnetos can be wonderful to have and use. But they can be difficult on several levels. It sometimes really is a matter of "if it is working well? Don't mess with it." Sometimes, if there is a problem? It is best to consult someone that really knows what he is doing.
And that really isn't me.
Drive carefully, and enjoy, W2