View Full Version : timing a 1924 j.d. mag engine. need help.

John Mertens
05-07-2011, 02:53 AM
I need help timing up my 1924 j.d.#74 WITH A BOSH ZEV mag.i have tryed timing on the rear cylinder top dead center. to fire on the front cylinder just after the front cylinder intake closes. but im not getting a pop out of it. I just installed a m51 linkert rebuilt by Mikes linkerts so i know that that ok. the mag is a new rebuilt . and fires real good.i belive my problem is in the timing of engine.could someone give me a step by step instructions on timing a mag j.d. engine. thanks guy. call me stumped. Johnny.:confused:

05-07-2011, 10:33 AM
how many teeth do you have on the mag gear?is your timing mark always in the same place when you recheck it?.are you sure you have a spark in the cylinder? a weak mag will zap your hand but not your plug under compression. call and ask ed strain.
813 521 1597.

05-08-2011, 03:04 AM
I thought someone may have given you more detailed advice by now so here goes
Firstly set the mag at FULL ADVANCE. That is with the interupter ring rotated as far as it will go in the direction opposite to the rotation of the magneto.
Then make sure that the pick-up marked 1 has its HT lead connected to the rear cylinder. This usually means that the rearmost pick-up HT lead will go to the rear cylinder and the front pick-up HT lead to the front cylinder
Set the mag points at 15 thou.
I find a bit of cigarette paper between the points is about the best thing to determine when they are just starting to open. You'll feel the cigarette paper release if you're lightly pulling on it as you rotate the mag.
Now set the rear cylinder at 5/16 of an inch before TDC on the compression stroke and then rotate the mag, ensuring the engine stays stationary, until the points are just staring to open.
Now tighten the nut holding the mag drive gear to the mag armature just enough to hold it firmly in place.
You now need to turn the engine over in it's correct rotation until the front cylinder is 5/16 of an inch before TDC on it's compression stroke and check that the contact points are just starting to open.
If they are you've timed your engine. If they're not you've timed it on the wrong breaker ramp so go back and repeat the process but time the rear cylinder on the opposite one you used the first time.
When you think it's right rotate the engine in it's running direction and stop when the pionts open, cigarette paper releases, and measure where your piston is. 5/16 BTDC.
By rotating and checking you take all the backlash out of the gear train and get a more accurate setting but it does mean you may have to adjust things more than once.
Some mag drive gears are keyed and some are not but I prefer to leave the key out of any that are keyed as I find I can get a lot more accurate timing setting.
If you prefer to keep the key you may find you need to advance or retard the mag drive gear up to as much as 3 teeth from the factory marks to get correct timing.

05-08-2011, 08:14 AM
That's good stuff Tommo. Printed off and filed for future reference. Thank you for taking the time.

John Mertens
05-08-2011, 09:10 PM
Thank you very much Tommo. is it best to remove the engine from the frame to get the 5/16.or is there a better way to find the 5/16 with the engine in the frame.what is the best way to do this.. the mag has a repop gear and it dozen't have any timing marks. i like your idea of removing the key im sure that i can get a more acccurate timing. i'm sure a lot a guy out there really appreciate your step by step instructions very easy to fallow.again i really appreciate you time. thank you very much. Johnny.

05-08-2011, 10:42 PM
All you need is a depth guage or a narrow scale to measure from the piston to the top of the plug hole or a head fin (something in a fixed position). I prefer a degree wheel on the sprocket shaft and a pointed rod fixed to a motor or frame bolt. If you don't have a degree wheel you can adapt a protractor to work. You will have to determine what 5/16" of piston movement is in degrees of rotation first.

05-08-2011, 11:41 PM
I prefer to do it before I install the engine in the frame and use the holes the two cylinder steady studs are screwed into. As they are directly over the piston they provide a ideal place for accurate readings.
Get a pair of old steady studs and drill 1/4 inch holes in them so that they can be screwed into the tops of the cylinders and have lengths of 1/4 rod dropped down them so that you have something that rests on the top of the piston and goes up and down as you rotate the engine.
Make sure you put a nut or something on one end of the 1/4 rod so as you don't lose it down inside the cylinder.
The rod that protrudes out the top can then be used to accurately measure where the piston is and thus help in getting the timing right.

05-09-2011, 12:51 AM
Here is a selection of my timing tools which may give you a better idea of what I'm talking about.
Sorry about the picture quality but my camera doesn't have a close-up facility.
I'm sure the Harley one is easy to pick.

05-09-2011, 09:31 AM

This is one of the adapters I made to attach a degree wheel to the output shaft. If there are a couple of threads showing on the shaft it will go on and hold just fine.

When I posted that about using the plug hole I never considered the location of the spark plug in that model and know it won't even work on a flat head. I also would never imagined there would be a through hole into the combustion chamber. I learn something new every day, and just wish I could rember it the next day.

Chris Haynes
05-09-2011, 01:26 PM
If you open a free account at photobucket here is how your photos will appear on the boards. No resizing necessary. Simply upload your photo then copy the IMG Code for the photo and paste it here.

John Mertens
05-09-2011, 02:32 PM
Thank you guys for the great information. it makes me proud to be a menber of amca. it truly has some very good members who truly want to help other members reach there goals . with all of this info i am sure i will have the old girl on the race track at Davenport.#24. again thank each and everone of you for your help. Johnny.

05-09-2011, 03:39 PM
As you're playing with a race bike does your cylinders have Ricardo chips in them?
If they do don't remove the steady studs unless you are prepared to take your cylinders off.
There is another way to measure the 5/16 BTDC but it is not as accurate and at this stage it will only complicate things if I try to explain it.
No "Riccy" chips just go for it but if you do have them you better contact me.

John Mertens
05-10-2011, 12:47 AM
Hi Tommo. thanks for all your help. no i am running the stock Harley Davidson cylinders. standard bore and stroke. however i wish i had Ricardo jugs.are Ricardo easy to find in New Zealand. we pay big money hear in the U.S. for them if you can find them. some of the guy i race against are very serious about racing and run very high compression cylinders . port the intakes. larger valves . grind the cams . stroke them. etc. my bike is a very much stock #74. with very little mod.i race to have fun and let the fan of board track racers injoy them. we are trying to bring back board track raceing.and the history behind them. to keep it alive so the younger people has a sence of history. it's a great thrill to leave the pits and hear the fans cheering.that in it self is worth it for us. o did i mention i'm 67 years young. my wife said she hopes i never grow up. me to. thanks again Tommo for all your help. Johnny.Mertens.#24

John Mertens
05-12-2011, 10:10 PM
I have a question on the braker ramps on a zev bosch mag.there are 2 of them. is the ramp at abourt 4 oclock for the rear cylinder and the other at 9 oclock for the front cylinder. and do they fire also on the exhaust.seams years ago i was told that older harleys fire on intake and also exhaust. thanks Johnny.

05-15-2011, 05:27 PM
Wasted spark is on the electric models that have what is known over here as the "Two spark coil". Mags are not "Wasted Spark."
It's very hard to describe things when there are so many variables in play and you have never seen the thing you are trying to describe but generally a Bosch ZEV 45 degree twin mag will have the points ramps at about 2 o'clock and 4 o'clock in the fully advanced position so long as a standard interrupter ring is fitted.
Some 45 degree interrupter rings have advance slots in two places at 180 degrees to one another to enable you to choose which side you want the advance lever on and others have no built on advance lever and instead have a band with the lever attached so that you can position the lever where you want it.
The most important thing for you is to make sure you have a 45 degree interrupter ring as there are 42, 50, 60 and 180 degree rings that will fit but will give you all sorts of trouble if you try to use them on a 45 degree machine.
It's also important to ensure you have a 45 degree armature in your magneto.
The easiest way to decide which ramp is firing which pick-up is to remove a pick-up and turn the mag to which-ever ramp you choose and look to see if the brass strip on the slip ring is visible or not.
If the brass strip is visible that ramp is firing that pick-up. If it's not visible then the chosen ramp is firing the other pick-up.
Does this help?

05-16-2011, 09:12 AM
here comes a can of worms tommo.why cant i take a 45 degree mag and pull the key out of it? rotate the the mag gear and make it a 42degree? see the can of worms? later tonight i'll post photos of some mag parts so everyone can see what we are talking about.

05-16-2011, 03:23 PM
I took myself up this garden path didn't I and yes Rob it's a real can of worms that can lead to serious arguements as opinions vary greatly on this subject.
My first observation would be the question, why would magneto manufacturers make several different versions of the same magneto if it wasn't necessary?
Now to your question Rob. The ramps in the interrupter ring decide when the mag is going to fire and by changing the position of the mag drive gear only changes the relationship of that firing to the position of the piston and does nothing about changing the mag from 42 to 45 or whatever degrees that the mag fires at.
A 45 degree v twin will run on a 42 degree mag and visa versa but staying with the first example, 45 with a 42, once timed on one ramp the second cylinder will be 6 degrees out ( 3 degrees at the mag = 6 degrees at the crank). So someone says my Harley doesn't run as good as it should, gets hot on one cylinder and at starting it sometimes spits back on one cylinder. Reason; one cylinders timing is not correct.
If you put a 45 degree interupter ring onto a 42 degree mag you can overcome most of the problems and it will work well so long as the mag armature is in excellent condition but if the armature is at all dodgy you will start to have trouble.
Reason armatures are built so that the maximum magnetic flux occurs at the time that the points break and that is why Bosch stamped all their armatures with the degrees that that particular armature was intended to be used at.
It's early in the morning here and there's a hell of a storm happening but as soon as I can I'll take some pictures of armatures and interrupter rings and put them up for you guys.

05-16-2011, 07:08 PM
I've taken shots of a 45 degree armature and one for a 180 degree flat twin Harley.
I don't have armatures for 42,50, 55 and 60 degree mags that are out of mags at present but feel sure these two can illustrate the differences.
The 45 degree armature is on the top and notice how the pole shoes have notches out of them and that how the notches are at opposing ends of the armature. One pole shoe has the notches at the drive end and its mate has its notches at the points end. Best seen in the second photo.
The 180 degree one has parallel pole shoes.
The pole shoes as they pass the magnet determine the point of maximum magnetic flux and now I'm out of my depth as I don't understand how the pole shoe shape effects this but experience has shown me that it does.
I don't think my photo is good enough but you may be able to see 45 stamped on the pole shoe in the first shot and take it from me all the other respective armatures will have their respective degrees stamped on them and all their respective pole shoes are subtly different.
Please note that we are talking Bosch mags and Bosch mags only here.
After my previous post two words came to mind that help when playing around with mags.
When; The points determine WHEN the spark occurs.
Where; The mag drive gear and it's associated gears determine WHERE the spark goes.
I know one relies on the other and it is simplistic to make a statement like this but if you bear this in mind it will help a lot in getting the best out of your equiptment.
Sorry about the quality of the photos but it's the best I can do with the gear that I have.

05-16-2011, 07:39 PM
Now interrupter rings.
Top left; 42 degree v twin notice the long ramps.
Top right; 45 degree v twin with small short ramps
Bottom; 180 flat twin with ramps dead opposite one another

One of the most important points is the positioning of the ramps in relation to the advance slot and I'll show this in a further photo but by now you will be starting to see that can of worms that Rob opened up and realise that when we get a mag that over the years has had all these components mixed and matched up it is no wonder that trouble occurs.
In most instances they spark and appear ok but in reality they are far from satisfactory and cause more trouble than they are worth.
Most good magneto overhaulers will have a machine with a 360 degree disc that will tell them exactly when the mag is firing and if you're really fussy they can stone the interrupter ramps so that the two sparks occur in exactly the right place.

05-16-2011, 08:01 PM
Advance slots
I've positioned all the advance slots to 12 o'clock and from that you can see how the positioning of the ramps vary.
Keep in mind that you are looking at them from the back but it must be obvious how mixing and matching opens up Rob's can of worms.
I don't overhaul mags but have a very good friend that does.
50 plus years of playing with them has taught me a lot but I don't know it all and if anyone disagrees with me go for it because I feel I'm never too old to learn.
One thing I do know about electricity is you don't see it coming and when it arrives it bites and generally arrives at the most unexpected times.
Enough one finger typing for now I'm out to brave the elements

John Mertens
05-17-2011, 01:41 AM
thank you Tommo. boy i never know there was so much to mag.judging by your pictures and all the info i'm sure i have the correct mag for my bike. i got my Mag from Marks new rebuilt. had problems with it right away. sent it back to them and had them rework it. they said the condenser was no good. the company that made them for them went out of business. and that they got a batch of bad condensers. now Marks gets there condensers from a company in China.it's seams to work fine. but i'm still worried. made in China.????????? again thank you .Johnny.

05-17-2011, 08:46 PM
thanks tommo this is how a forum should work.don't put that coffee can away yet.i'm gonna fill with some more worms.if i ever get this mad apart