Bosch DU4 problem

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Model T Ford Forum: Forum 2014: Bosch DU4 problem
Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth from NC on Tuesday, May 13, 2014 - 05:24 pm:

I just got my gears back for my Bosch Magneto Drive and have been trying to get the car back to running. I thought that maybe I was just having trouble picking the right teeth for timing it, but she's not firing on all cylinders now. I've got the timing about right, I can get it started, but #1 and #4 only fire intermittently. If I rev the engine up, then it starts firing on all 4 consistently.

I've taken the distributor face off of the magneto and made sure that the carbon brush is clean, the brass contacts are clean, the spark plugs are all gapped correctly, the brass contacts on the spark plug wires are clean, I don't know what the problem is. I wish there were a magneto guru near me instead of having to ship it all the way to Mainely Magnetos to have him straighten it out. On a positive note, I have another DU4, but it needs to go to Mainely Magnetos for an impulse coupler. This is frustrating because I'm ready to get back driving with this excellent weather we are having.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Wayne Sheldon, Grass Valley, CA on Tuesday, May 13, 2014 - 07:31 pm:

You say the plugs are correctly gapped. But are they correctly gapped between 028 and 032 (for standard ignition)? Or are they correctly gapped between 018 and 022 (for magneto ignition)? The Bosch DU4 magneto requires a smaller gap at low rpm (like starting) and will miss even at idle if the gap is more than about 024.
I think I recall a thread where this was discussed before and you know this, but I just wanted to be sure of that so that people better than I can give you the right answers.
Drive carefully, and enjoy, W2


Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth from NC on Tuesday, May 13, 2014 - 07:41 pm:

Yes sir, I have all of them gapped at 018.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Wayne Sheldon, Grass Valley, CA on Tuesday, May 13, 2014 - 08:19 pm:

I figured you did, I just wanted to clarify that.
Timing for magnetos is very tricky. There is a peak power within the magneto that fires the hottest and best ignition. It would appear to me that even Bosch was unclear on this back in the day as I have had several DU4 magnetos, and the internal timing marks vary quite a lot between them. Parts will cross-fit and work fine. I mention this simply as a way of pointing out how tricky they are. Your magneto re-builder probably has it right internally. However, with the multiple gear type drives, it is very easy to get the timing to seem correct, but actually be off that armature peak. For example, an advance of the armature, coupled with a retard of the points, will fire the plugs at the right time, but off the peak of armature fire resulting in a weak spark which could miss at low rpm. There are numerous ways to get a magneto "off peak". It only takes one gear being off one tooth to mess them up.

You could try carefully advancing and retarding before and after the armature to see what difference it makes. I am not an expert. But that is how I got mine to work about 40 years ago when I didn't have an internet forum or expert to help. Only small changes, like one tooth at a time, and keep track of your changes so you can put it back to where you began. It worked for me. Do be very careful with the final timing so as to not backfire and break anything (arm or car).
Good luck.
Drive carefully, and enjoy, W2


Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth from NC on Tuesday, May 13, 2014 - 09:08 pm:

I just figured it out. The points need adjusting. Forgive my ignorance as I don't know the exact names of the parts. Below the distributor cap is where the advance and retard control arm on the mag sits, on the face of this part is where you hook up the grounding wire. Inside, is a set of points that open when the mag rotates : there's an arm with one point on the end and the other end is what brushes against cams inside the advance arm. Well, carefully looking and slowly turning I saw that I'm barely opening a teeny tiny bit on one cam and almost imperceptibly on the other! Which, matches up with my 1-4 vs 2-3 firing imbalance. The 2 and 3 cylinders are hitting on the better cam and the 1-4 are on the bad cam. Which also makes sense that it runs better at higher speed because the brush is hitting the cams harder and faster and actually opening. I have 2 other DUs here for parts and I am now replacing the bad cam with a less worn one and then going to google what the points should be set to.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Wayne Sheldon, Grass Valley, CA on Tuesday, May 13, 2014 - 11:57 pm:

See! You figured it out all by yourself! It sounds right to me.
Now, drive carefully, and enjoy, W2


Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth from NC on Wednesday, May 14, 2014 - 12:13 am:

Y'all just call me Grease Monkey Sherlock. Points are to be gapped at "No more than 0.5 mm" which is about .018". I adjusted them, then went inside to watch the wife's TV show with her as well as put her to bed (then sneak out after she fell asleep) I got the mag back together, back on the car, and Eliza finally cranked up sweetly and purred at idle. My heart soared and all is right with the world. She'll be back together and cleaned up in time to take me to the Cherry Point airshow this weekend and we will get to see the new F-35B fly around some. The sussing out and correcting of this mystery problem has been immensely satisfying. Unfortunately I have to be at work in 6 hours.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Seth from NC on Wednesday, May 14, 2014 - 10:43 am:

So I've noticed no one has said anything but Wayne (thank you sir). I have to say I'm still rather pleased with myself, even if it turned out to be something of simple fix. T will be test driven today to make sure I haven't forgotten anything.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Larry Bohlen, Severn MD on Wednesday, May 14, 2014 - 11:12 am:

Seth,

Good job.
It is very satisfying when you can lick a problem yourself.

Have fun at the air show. I remember them well.
It was there in '79 that I met Richard Landis and got to know him and his dad, Carl. Both were a big help to me finishing up my '27 touring while my dad was stationed at the Pt.

Cheers,
Larry


Top of pagePrevious messageNext messageBottom of page Link to this message  By Craig Anderson, central Wisconsin on Wednesday, May 14, 2014 - 04:14 pm:

The points are set at 1/64" or .015".
A good DU4 would fire across an .080" gap anyway.
That's hardly advisable but it would.


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