Gents,        I picked up a poorly running, neglected '73 DT3 last year and am restoring it in my spare time.  First I've stripped it down to esentials and restoring it mechanically. Last year I went through the top end, timed it, checked the autolube, and cleaned the carb. It ran better, but still had performance issues.   This winter I did a more throrough carb rebuild with a nice Niche Cycle rebuild kit.   Havn't been able to test it again cuz its still winter here.     So, I'm testing other components during this downtime.     Today I pulled the ignition coil  which appears to be original.   I have the Clymer book.   The Secondary resistance gives me 6,500 and the Primary is 1.1 (see photo). The table in the book (see photo) says I should get 6,000 and  0.6 and Clymer doesnt provide a range of accceptable values.  I'm wondering if someone put a DT2 coil on this bike, as those values are closer to the DT2 table values.   

One of my thoughts is to simply get a new aftermarket coil, given that these old coils sometimes degrade over time, and despite bench testing OK, might weaken while the bike is running (I have heard).. I do get a blue spark, but its not strong.    I was thinking about the EMGO universal coil:     www.amazon.com/Emgo-Universal-Ignition-Coil/dp/B002EC389W    cuz I read the ignition coil thread at this forum which discusses it.    yamaha-enduros.com/index.php/forum/1968-...gnition-coil?start=0      

Your input would be appreciated. 

 

 

 

 

 

If you don't want your old coil, I'll take it.

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I seem to be on the way to answering my own questions.  

I just discovered that your forum has PDF copies of many YAMAHA OEM service manuals, including one that covers my bike.
I trust that info more than I do Clymer.

The proper Secondary resistance is 6500, so my bench reading is proper. (Clymer seems to be wrong)
The proper Primary resistance is 0.9 + or - 10%, which  is about 0.8 to 1.0, so my bench reading is only 0.1 out of spec.
The OEM manual identifies the proper iggy coils:  It is not perfectly clear, but since it groups the DT2 and DT3 together for coil specs, the proper coil seems to the HD-A4, which is what I have. 

So, my iggy coil seems to bench test as adequate, but I'm still concerned about whether it performs up to snuff on the road, as I mentioned, so I'm still interested in hearing your opinions on this. 


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My first thought on lack of performance is nearly always exhaust unless there's something else obvious. Removing the end baffle piece doesn't prove much as the problem is likely further down in the end in the spark arrestor. Spiral looking thingy & non removable although i 'have' got them out with a bit of smacking in the middle untill the spot welds break, then coax it back out the back end. Anyway--not recommended these days with spark laws in forests & all that, plus makes it twice as loud. So burn it out if necessary but will stuff the paint. Funny memory just came to mind. When i first started work in "62 we sold Jawas & Hondas Jawa manual made a big thing of burning out the mufflers--Chrome Mufflers--& said it will blue the chrome but with the increase in performance your customer will bless you.--yeah rite, that's after he's smacked you one for ruining his chrome lol. Have a happy day!!.

Actualy there's a few other things that could affect performance like the small Oring on the jet setter in the carb which if not sealing the fuel bypasses the side mounted main jet & goes straight up the spout & runs rich so won't rev out. Also if the points fiber heal has worn down the points gap will close retarding the spark to the point of not opening at all in a severe case but then of course won't run at all. We love video's so we can get a good feel of what's going on. Thanks.

& more. We had a Yamaha coil tester at work & on those single wire AC coils we could still test them & instructions were--if they draw less than half an amp-- to test them with a 6 volt battery instead of 12.Sounds odd i know but it worked. I guess we had to test on 6 volts initially & if less than half an amp then carry on with the adjustable gap to see what it'd throw. Same coil on 12volt would go off the scale but not what we were meant to do. Very rarely found a faulty coil & usuly something else. Also rarely found a faulty source coil in the mag on a points mag, unlike cdi source with its fine windings. & the extreme of fine was the encapsed grey coils on the YZ's with by memory 1427 ohms or something,. fine as Hair--my hair which is mostly gone & very fine lol.

Before you condemn the coil...  I'm pretty sure the "7mm gap test" is done with a special Yamaha battery-powered electro-tester, not the kickstarter on the bike. 

I might have just responded to the wrong thread...  I thought something was said here about testing the coil with a 7mm gap test...  might have been another thread.  I get confused easy sometimes.

I'll take a pic of my tester--fearsome thing but not used it since the "70's & if a coil 'was' faulty it'd barely throw a spark starting from the gap closed right down on the tester & if you could wind the gap out it was a very thin spark & not many mm gap before it gave up. Had to be sure of using a good test battery though.