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Makotosun

1974 DT125 timing problem

  • panikrev
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1974 DT125 timing problem was created by panikrev

I have a customers DT125 w/ the magneto-electric start currently with a ignition timing mystery. I rebuilt this motor from the crank up approximately 4 years ago, it ran like a watch up until last fall when I discovered the mag side crank seal started leaking. I was able to pull the mag and flywheel and sneak a new seal in without splitting the cases. After getting it sealed up, it had great vacuum and compression again, good fuel delivery and good fat spark but it didn't even make a pop when trying to get it started. 
Even though I hadn't changed anything with the timing when I pulled the mag and flywheel, I took the cover off and in visually checking it, the timing was WAY off- the points didn't open until the piston was mid stroke. Nothing had moved on the points plate, the gap on the points was at .012 and the governor key slot and the tab for that slot on the crank appeared to be fine. 
I set a dial indicator in the cylinder per the factory manual to set the timing at 1.8 mm BTDC along with a multi meter to detect the exact moment the points opened- it wasn't even within the stroke of the dial indicator at it's initial setting(which again I hadn't moved and there was no evidence that it had slipped). So in trying to set the timing, at full  rotation of the points plate the best I can get is about 4mm BTDC with the governor held in full advance per the manual. The governor appears to be working, although it's pitted and doesn't advance very smoothly. What could have happened? I'm at a loss- I could modify the slot in the governor to get the points cam back in range, but in searching eBay and Speed and Sport NOS I can't find another governor in case this mod went wrong. HELP!!!
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16 Mar 2021 08:12 #1

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Replied by MarkT on topic 1974 DT125 timing problem

Two things...  from what you've described you don't have a "magneto"...  You are working on a battery-points ignition with a generator/starter to charge the battery.  Magneto doesn't need battery for spark, the system you are working on does need a battery for spark.  In the scheme of things,it doesn't matter but it can get confusing if this thread goes on and you're asking "magneto" questions but have the generator system.  

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Pictures are always helpful but my first guess is the crankshaft key fell out or was damaged when you replaced the armature back onto the crankshaft.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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16 Mar 2021 11:49 #2

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Replied by RT325 on topic 1974 DT125 timing problem

yep i'm with Mark either crank key fell out-- or somehow you've forced the governor in & missed the locating tang. Not sure if 'that's' possible but if the tang had been previously bent in or down towards the long bolt could it do 'that'. Been 40 years since i've been there or 30 years with my DT125EA.
16 Mar 2021 14:16 #3

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Replied by Patroll330 on topic 1974 DT125 timing problem

Can you send me pictures of your points area? So what wires are you using for the volt meter to check timing?
16 Mar 2021 14:42 #4

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Replied by panikrev on topic 1974 DT125 timing problem

I can assure you that the armature key and keyway are fine. I checked it again this morning before I replied to this post. It slides right on and positively engages the crank. The alignment tab for the governor appears undamaged, and the governor positively engages the armature tab without an issue. So either the threaded hub (threads for the puller)in the armature that has the alignment tab for the governor has spun or the governor cam has spun. I tried to move the armature hub this morning while I had it off- although I didn't try to force the issue, it felt secure to me. 
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17 Mar 2021 08:26 #5

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Replied by panikrev on topic 1974 DT125 timing problem

I'm just connected to the points wire with the + and ground near the condensor. Just looking for it to lose continuity when the points open.
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17 Mar 2021 08:32 #6

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Replied by Andy L on topic 1974 DT125 timing problem

Hi Panikrev, from what you've described re: "looking for it to lose continuity when the points open" is not going to happen.  This is because there is still a path to earth through the ignition source coil, which is earthed down to the stator plate as are one side of the points and condenser.  What you should be looking to see (ideally on an analogue meter set to the LOWEST resistance value) is actually a change between continuity i.e. zero resistance reading when the points are CLOSED and then when the points are OPEN you will see a very small resistance value shown on the meter.  This is the internal resistance of your ignition source coil.  From memory I think my DT250B value was ~1.4ohms (anyway look in the Tech Library section of this forum for your particular bikes spec and tolerance).  Hope this helps, Andy. 

Apologies Panikrev, I have just realised your bike is a 12v starter/dynamo 74 DT125A, so all I've said above does not apply to your bike.  Like at school when they said 'read the question' I wish I'd read the thread right from the start.  Had I done so not only would I have seen yours is a completely different electrical system, I would alos have seen Mark T's warning that if this thread continued for any length, people might start thinking it's a magneto related article, which of course it isn't.  Sorry again.

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Last edit: 17 Mar 2021 11:55 by Andy L. Reason: My facts were wrong
17 Mar 2021 09:48 #7

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Replied by RT325 on topic 1974 DT125 timing problem

Gosh i dunno, out of ideas. Can the cam be moved round withing the weights, if so it would be half a turn= bdc to lock back into the weights not 1/4 turn. If it was totally in bits did you pick up the wrong crank as i'm sure someone found the keyway cut in a different place between electric start models of later years compared to the very early one. Ok not that as only did a left side crank seal this time. Bottom line is you need 1.8 btdc. If you run out of options you could bend the locating tang in on the governor & set it at 1.8 advanced & it shouldn't slip done up tight as that bolt holds the armature & all onto the crank so needs to be quite tight although it's a long bolt & not that big either. Think for the cam to be moved half turn in the weights there's two small screws involved to get it dismantled so isn't going to happen unless someone went in there to lubricate--ok i'm rambling. Goodluck.
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Last edit: 17 Mar 2021 16:12 by RT325.
17 Mar 2021 16:08 #8

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