Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

I have a Sachs Prima G3 from 1979 (It was sold to me as a ‘79, but the manufacture date on the bike is December of ‘79, so prob a year model ‘80, perhaps?)

I tinkered with it. The wiring needed to be fixed. I re-wired it but couldn’t get a spark. I took it to a friend of mine who wires up airplanes for a living, printed up the schematics, and he wired it up real pretty. Got spark. Got compression.

Put it back together and crank, crank, crank, crank, vroom! Short vroom, like about a second or so. Poof of exhaust. Then sputter down.

Started leaking gas from the Bing carb. Had to replace a sinking float. Put it back together. The new float floats. Gas is getting to the chamber. All seems well.

Crank, crank, crank, crank, crank, crank, crank, vroom— But an even shorter vroom. Like 1/2 second. Some exhaust.

More cranking. Lots and lots more cranking. One final vroom for about 1/4th a second. Then, buttloads more cranking yet, no more vrooms. I pull the plug and grounded it out, crank the pedals in search of a spark (same test I initially did where I did see a spark). No spark.

I figure next to bypass the kill switch and see what happens. Maybe put a volt meter on the coil where the spark plug wire connects, crank it and see what’s cooking there, if anything. Not sure what the measurements should be... Re-test the spark plug cable.

What do y’all recommend I do to troubleshoot and fix the prob?

So close to riding this sweet bike! Thanks in advance,

-bob(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

One other note: The tail light lights up every time I crank it.

I’m a newb to mopeds, so correct me if I’m wrong, but the bike is creating electricity. So maybe that eliminates some areas as problematic. And maybe helps tell us something about where the problem areas are? The plug is brand new and the plug wire (from PO) looks to be newly replaced. I tested it around the same time my pal was wiring up the bike and it showed 5030 ohms of resistance (assuming my rudimentary knowledge of an ohm meter is up to snuff as it read 5.03 when set on the 20k setting).

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Points Condensor

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

I poked around in there and it was super-clean to begin with. I cleaned the points and when rotating, the parts are all moving as supposed to in conjunction with the timing and the gap/movement looks fine. If this were not working correctly, would we be getting power to the tail light?

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Correct. Condensor does not supply tail light, the windings of the stator do. Condensor supplies ignition coil for spark from the same windings in stator.

Check or replace condensor

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Thanks for the lead, themotorhead. I'm now scouring the web for how-to vids and such. If you happen to know of a good instructional page or vid, please let me know. I'll report back my findings.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

So I put a new condenser in there and cranked it. Spark! For about three seconds. Then the spark faded away. Cranked it again. Spark! For about two seconds, then nothing. Cranked it again and a smattering of infrequent sparks for about a second and then nothing. All subsequent cranks produced no sparks. Any idea what to try next?

I figure to give it some more cranks after letting it sit for a good stretch to see if the same thing happens. Any help would be greatly appreciated. Thanks!

-bob

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Do you know how to check a basic ignition coil?

That is also suspect.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Not offhand, but I ain't too shabby a doing research and figuring it out. If you have links, I'd appreciate the shortcuts, but that was kinda what I was thinking to test next.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

ground out your horn wires, lloookkk it up on myrons page.n test 4 spark.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

I actually have no horn. No left hand switch whatsoever.

I just wired the headlight into the brick per the schematics and left the left-hand wiring out of the equation altogether as the bike came without a switch. I was hoping that this would make it so that the headlight would stay on the entire time, but it hasn't come on at all. Perhaps I need a switch??? Or is there a way to just have it on at all times. Because I'd prefer that. I figured I'd trouble-shoot that after I got it running. I'm still trying to get the thing running for the first time (for me). There was no horn or head light switch when I got it.

Barring a horn to check the grounds, maybe I should unplug the headlight and see how that affects things?

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Headlight runs off of lighting coil. Headlight comes on when engine is spinning fast enough to create the needed power.

Check the resistance of the primary and secondary windings in coil with coil isolated from bike wiring. Resistance value wont be as important as the fact that it remains constant and does not fade.

Primary winding is low voltage power supply wire(s)

If only one wire use it in conjunction with grounded frame of coil.

Secondary winding is high voltage side that goes to spark plug. Use that lead with plug boot removed and ground of coil frame.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

i was lost earlier, spark now or not. always put a fresh plug in for good good n no bad bad

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

ggspeed,

The plug is brand new. AS for the spark, let me try to better explain:

On the first crank, I get spark. After about 3-4 seconds, the spark fades away despite continuous cranking. I pause for a few seconds and crank again.

Second crank gets me about 2-3 seconds of spark before fading away. I take a break.

Third crank gets me about one second of spark. And subsequent cranks get me no spark.

I replace the condenser and try again. I get the exact same results.

I wait 24 hours after the test with the new condenser and try again, getting the exact same results.

I’ve checked the timing and it looks good. I cleaned the points and they look great. The points move in conjunction with the piston as it should. I had the bike re-wired by a professional before any of these tests.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Does the ignition coil have a good ground?

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

themotorhead,

Thanks for the headlight explanation. I was assuming (hoping) that it was something like you explained.

I checked the resistance of the coil.

I’m no ohm-meter master and don’t pretend to know what this thing is telling me, but it is holding steady.

When connected to the spark plug wire/secondary winding and the #15 ground connector I get 5.64 with the setting on 20k.

When connected to the #1 connector/primary winding wire and the #15 ground connector I get 4.7 with the setting on 200.

Both are holding very steady.

I’ve included photos in case you care to check my math.

SachsPrimaG3-secondarywindingOhmTest.jpg
SachsPrimaG3-primarywindingOhmTest.jpg

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

The ground connectors are very shiny and clean. My buddy who wires up airplanes for a living re-wired the bike for me, so I'm assuming he did it up proper. And the nuts are snug.

And again, I really do appreciate y'all taking the time to help me with this. Gives me faith in humanity! And I look forward to being wise enough to help out someone in return some day.(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

This bike has a 2 bulb cev taillight. The ground curcuit feeding the magneto coil goes through both brake light switches and to ground via a hidden 6.5k ohm risistor in side tail light behind reflector. When this circuit is broken by applying the brakes which open one or both brake light switches, the coil is then supplied ground through the tail light bulb and it lights. Now if that hidden brake light resistor is bad and you have a bad brake light switch, connection to it or burned out bulb the ignition coil will get not ground at all and no spark. Garelli uses this setup too.

So, to simplify things for a quick diagnosis. You are going to temporarily by-pass all the afore mentioned by connecting the black/blue wire from the stator to a good solid ground and test for spark again. From the stator the black/blue wire will go to a plug and change colors to red/black.

Where did the replacemnet condensor come from?(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

I erroneously refferenced tail light bulb and ment brake bulb^^

Tail light and head light run off of lighting coil.

Ignition coil and brake light run off of magneto coil. Both are on stator plate under rotor or flywheel.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

ive seen this b4, your real close. so annoying to see intermittant spark. i wanna say on the two prong coil, one side is grounded to frame and the other goes to stator. doesnt matter which one. check your spark plug wire ends are clean an twisted in fully. u gotta check the grounds to make sure they clean/tight yourself and gap your new plug.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

♣Slew Foot♣ /

The oem bulbs were 2 w and 3w for the tail and 18w for the head. Too much draw with modern replacements.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

The condenser came from Treatland. The tail light works when I'm cranking it. Have not tried the brakes yet. But I will try this bypass tonight and see what intel we gather from it. Do you think the fact that I have spark (only a bit of it) on the first crank then less and less on each subsequent crank is a symptom that indicates a bad brake resistor?

I pulled the headlight bulb. Figured to remove that from the equation for simplicity's sake.(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

> Bob Ray Wrote:

> -------------------------------------------------------

> The condenser came from Treatland. The tail light works when I'm

> cranking it. Have not tried the brakes yet. But I will try this bypass

> tonight and see what intel we gather from it. Do you think the fact that

> I have spark (only a bit of it) on the first crank then less and less on

> each subsequent crank is a symptom that indicates a bad brake resistor?

>

> I pulled the headlight bulb. Figured to remove that from the equation

> for simplicity's sake.

Youre only worried about brake light curcuit being eliminated from equation since magneto gets ground through it.

Magneto sends power to ignition coil.

A bad resistor, ground or wires could supply intermittent connections to ground and thus intermittent spark.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

On the top right corner of the pics above, you can see where the grounds connect to the secondary coil. The lower of the two wires is coming from the #15 ground connector on the coil. The upper of the two wires comes from the wiring block inside the headlight assembly. On the other side of that connection in the block, there are two wires: one connects to the kill switch (In the stock schematics it would go to the speedo first, but there is no speedo, so it goes straight to the kill switch) and the second one connects to the hand-brake lever.

I continuously cranked the bike and the tail light lights up (with a very rapid, almost unnoticeable strobe), but when I pull the brake lever and/or use the coaster brake, the brake lights do nothing. Both of the depressed buttons connected to each of the braking mechanisms pops up as they should, but no brake light. There are two bulbs in the tail/brake light assembly and both have brand new 7.5w bulbs.

Maybe we're getting closer to the problem?

I reckon I could put a volt meter on the brake light and see if power is getting to it? Or is there another way to test the braking circuit?

I do intend to follow themotorhead's instructions when I have more time (tonight):

"Now if that hidden brake light resistor is bad and you have a bad brake light switch, connection to it or burned out bulb the ignition coil will get not ground at all and no spark. Garelli uses this setup too.

So, to simplify things for a quick diagnosis. You are going to temporarily by-pass all the afore mentioned by connecting the black/blue wire from the stator to a good solid ground and test for spark again. From the stator the black/blue wire will go to a plug and change colors to red/black."

I'll report back after I give it a go.(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

I cleaned the grounds and checked the connections. Cranked again and it did the same thing it did the last few days. Some spark and then diminishing returns.

I jiggled the spark plug wire while cranking and it didn't do a thing.

Here's a pic of an Ohm test on the spark plug cable. Not sure what the numbers should be, but here it is anyway.

SachsPrimaG3-sparkplugwireOhmTest.jpg

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Tonight I'll test/troubleshoot the brake resistor. Thanks, y'all! More info coming soon as I get it.

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Bob is that a copper core replacement plug wire in the pic? ^^^

Also, wire resistance is measured without the "boot" because many boots may have a built in resistor that will show more resistance.(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

> Bob Ray Wrote:

> -------------------------------------------------------

> On the top right corner of the pics above, you can see where the grounds

> connect to the secondary coil. The lower of the two wires is coming

> from the #15 ground connector on the coil. The upper of the two wires

> comes from the wiring block inside the headlight assembly. On the other

> side of that connection in the block, there are two wires: one connects

> to the kill switch (In the stock schematics it would go to the speedo

> first, but there is no speedo, so it goes straight to the kill switch)

> and the second one connects to the hand-brake lever.

>

> I continuously cranked the bike and the tail light lights up (with a

> very rapid, almost unnoticeable strobe), but when I pull the brake lever

> and/or use the coaster brake, the brake lights do nothing. Both of the

> depressed buttons connected to each of the braking mechanisms pops up as

> they should, but no brake light. There are two bulbs in the tail/brake

> light assembly and both have brand new 7.5w bulbs.

>

> Maybe we're getting closer to the problem?

>

> I reckon I could put a volt meter on the brake light and see if power is

> getting to it? Or is there another way to test the braking circuit?

>

> I do intend to follow themotorhead's instructions when I have more time

> (tonight):

>

> "Now if that hidden brake light resistor is bad and you have a bad brake

> light switch, connection to it or burned out bulb the ignition coil will

> get not ground at all and no spark. Garelli uses this setup too.

>

> So, to simplify things for a quick diagnosis. You are going to

> temporarily by-pass all the afore mentioned by connecting the black/blue

> wire from the stator to a good solid ground and test for spark again.

> From the stator the black/blue wire will go to a plug and change colors

> to red/black."

>

> I'll report back after I give it a go.

By doing the test like mentioned, you will eliminate everything including wasted time until the root of the problem is isolated. Then, if there is a problem found with the brake light circuit you can back track and hunt it down.(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

Tail light bulb should be a 5 watt 6 volt.

Brake light bulb should be 10 watt 6 volt.

Do not ground power wire for missing speedo bulb.

Also it would apear to me after looking at this wiring diagram that the resistor creates enough resistance to send signal through bulb until bulb blows. Then it goes through restistor to prevent engine dying when brakes are applied and bulb is blown.

Screenshot_2018-09-17-21-41-03.png

(edited)

Re: Sachs Prima G3 - spark probs, Intermittent or Diminishing spark

looks like the blue wire comes right from the magneto and the grey wire goes right to ground,mabe it is that little diode on the blue wire(connecting block),he could lift one side of it off. I use a cut off extension and a drill to crank it and a spark plug checker. should be a arrow which way it turns.

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