darmah timing

[dcdrive]

Hello all
Been putting back together 1981 darmah slowly( not my choosing)and am to a point where i could use some input from this forum which has helped lots in the past with bike. Am dealing with poor running ,slow and labored throttle response up to 4000 rpm's after it's right on. Some backfiring thru carbs have done the rebuilding of carbs,new insulaters and exhaust gaskets. The pick-ups have been re-wired and set in cover by phil at road/race .Am currrently waiting for degree wheel from steve and in the wait time was wondering is some one could answer this . The rotor"is designed to automatically modify the ignition timing in accordance with the engine speed." according to repair manual .How does this happen or what does the rotor do to affect the advance and my rotor has two gouges to the inside high side like it some how came in contact with the gears behind it it looks like two fangs bite marks as it might be located where the pick-up contact point would maby come close to.Just hopping some one will take time out to share there knowlege/thanks/ Two years + working and this is last big hurdle.Bike has been sitting for a while but i am sure this problem was there when i parked it in 1986. Thanks for your time.. Dave...

[abmartin]

The igition timing is advanced electronically by the transducer boxes under the fuel tank. The rotor and ignition pick-ups generate the spark impulse but don't alter the ignition timing.

Bruce

[dcdrive]

Thanks Bruce,
Just as i thought, the repair manual states it have some value to the timing advance and i could not see how ,,,thanks again. Am still at a loss at why the rotor has the two gouges at the "raised ridge" where the picks-ups create spark. It is on the inside of the rotor ; Was wondering if this could affect the spark in any way? as i am having some issues with it running and pulling strong at low rpm's plus some back firing, but at 4000 rpm''s and up it rip's along .. no back firing and such..just trying to eliminating all potential problems while i wait for a degree wheel to check timing..
<> Dave//

[wdietz186]

The gouges probably won't affect anything. What comes to mind is the possibility of one of the pickup leads being reversed. When that is the case what will happen is that cyl. will be running at a fixed advance and the other is working normally,at higher revs things even out and the roughness is diminished. You can check the advance operation once you get the indicator from Steve. There are a couple of threads on this forum that deal with this problem. Another possibility is the Bosch ign. amplifier is going bad,rare but it does happen.

[machten]

What comes to mind is the possibility of one of the pickup leads being reversed. When that is the case what will happen is that cyl. will be running at a fixed advance

A couple of us are about to be messing around trying the bosch system on a 75 SS after having a hell of a time with DMC, so I'm interested in just clarifying this so I can recognise the symptoms if they occur. Do you mean the two leads from one pickup reversed causes that?

Kev

[wdietz186]

Yes the polarity makes a difference and unless you can see which wire has the black stripe the only way to tell is watching to see if the advance moves. They tend to kick back really hard when they are reversed too,had the lever fold and the end of the pedal went through the sole of my shoe. Causes a bit of a limp,for weeks!

[abmartin]

Hello all
Been putting back together 1981 darmah slowly( not my choosing)and am to a point where i could use some input from this forum which has helped lots in the past with bike. Am dealing with poor running ,slow and labored throttle response up to 4000 rpm's after it's right on. Some backfiring thru carbs have done the rebuilding of carbs,new insulaters and exhaust gaskets.

Following the theory that many supposed electrical issues are really carburation problems, I would carefully re-check idle and synchronization settings to see if that helps. I presume that jetting and float levels are correct after the work you have done. Take a look at the spark plugs as well as I've read that fouled plugs can affect ignition advance. Depending on what plugs you are using you may want to go to hotter ones.

Bruce

[machten]

Yes the polarity makes a difference and unless you can see which wire has the black stripe the only way to tell is watching to see if the advance moves. They tend to kick back really hard when they are reversed too,had the lever fold and the end of the pedal went through the sole of my shoe. Causes a bit of a limp,for weeks!

Thanks Bill. I've been a victim of "foot in bum" a few times myself!

The wires on the pickups have been replaced so no indication of which one is which. I might roll up to this garage session feigning a limp so I don't get starting duties...

Kev

[dcdrive]

Thanks Bruce,
Been completely through the carbs with a couple of extensive cleanings then all new bits on the inside new floats and needle and seats. Have increased idle jet to 62 as the 60's had mix screw out 3+ turns out now i am 1 1/2 . Have change out plugs a couple of times. Manual says champions but here i see talk of NGK's have not gone hotter ,,yet.Have black Dyna coils. Got rid of the air boxes.Have sync-pro for carbs . Have tried K4/6/16s at all the diff. heights.Got contact info for a duc guy on this site and he tried diff. carb. jets and needles but could not make go away. He didn't or couldn't check timing so.....Thats my next thing i feel i need to check to see where it's at. Will switch boxes while i wait for degree wheel as it is mostly on front cylinder. Back fires after idle to 4000rpm's. Don't think the pick-up wires are switched but will add to list to check it out (thanks wdietz) .Feel i have gone as far as i can go right now with carbs.After timing checks out will get back to carbs . Dave

[wdietz186]

Dave, Before you get neck deep in polarity and possibly aquire a serious limp,see if you can dig up another known good Bosch box. A year or so ago I had a friends Darmah acting up sort of like you describe and went through all the checks to discover the unthinkable,a bad amplifier! Up until then it was assumed they never acted up but there is always the first one!

[machten]

Hi Bill,

I suspect you aimed your last post at me (kev). Yes, I have a number of spare amplifiers and like you indicate, they almost never fail. I've never seen it happen in 30+ years of messing with bevels. My original question was more related to the DMC experience - which uses the bosch pickups (and not wishing to repeat the experience with Bosch amplifiers). I'd always assumed that the polarity from the pickups probably didn't matter. My thinking being that it was just an induced EMF on a coil and I never considered there would be reason in the amplifier to rectify the sine wave - which i assume is on possibility why polarity matters. Am I on the right track here?

Or is it feeding the wrong sides of a capacitor if it's reversed?

Not my strong point here. Why does polarity matter?

At the end of the day it doesn't matter why - but for some reason it would make me feel better to know!

Kev

[wdietz186]

Sorry Kev, I get confused sometmes[age,fumes,beer] and as to polarity I suspect it has something to do with the magnet being in the pickup and the field being disturbed by the metal rotor swinging by.Many years ago I had a serious interest in elecronics etc.but I got sidetracked by the late sixties and spent more time expanding my mind and trying to get laid so the electrical stuff fell by the wayside. I do know that the air gap to the rotor has a definite effect on the low speed spark/timing which makes some sense with what I remember about magnetic fields or was it about karma? Oh well. A friend of mine who had a shop told me he had a Darmah that apparently had the pickup polarity change by itself, one cyl. wouldn't advance and he switched the leads and all was good. He knew the bike to be unmolested and was baffled by it all. Things that make you go Hmmmmmmmm?

bill

[machten]

Thanks Bill.

I'll store pickup polarity in the box labelled "Things I live with that I don't understand" along with gravity, why my wife can find things in the cupboard that I can't, and why i didn't buy a green frame for $1,000 when I had the chance.

Kev

[Harvey.]

The mysteries, of flux movement.

There are two ways that a magnetic pick-up can be used to trigger a circuit. As the rotor moves past the pick-up a voltage is generated, this voltage will increase to a peak, then fall to zero to change to the opposite voltage as the reluctor moves away.
The shape of the reluctor depicts the shape of the waveform. A sharp edge reluctor will produce a wave that changes polarity, suddenly, to quickly cross zero to the opposite polarity. In this form the zero crossing point is used to produce a very precise trigger point.

The Bosch type uses a wide reluctor to produce a smooth rise and change to the opposite potential. So they use the rise of the voltage to do the triggering. As the reluctor moves across the pick-up, a positive voltage is rising, when the voltage reaches a set voltage, the circuit is triggered. Due to the voltage level produced, depends on the speed, and proximity of the reluctor, this is use to vary the trigger point as the speed rises.

If the leads on this pick-up are reversed, the waveform that is produced, is the opposite potential to the normal. A negative going wave is generated first, to reduce to zero, then rise to the positive wave that is to do the triggering. This has the triggering occurring on the second wave instead of the first. So the triggering, and the timing is delayed. <>

[machten]

Nice one Harvey!

And further explained here with some graphs...

http://www.megamanual.com/ms2/pickups.htm

I'll go and retrieve pickup polarity from the box.

Kev