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GM Coil Modification

FYI to this mod. I found out when tightening my own coil that one of the towers partially cracked because it was up against the frame previously giving it stress. I just used a soldering iron to melt the plastic to close it off and slide a silicone boot over it. However, to prevent this from happening again the fix is simple. Just install that Dynacoil upside down. Similar to how it was originally mounted:

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Finally got around to testing this with the DC11-2, set the gap to 0.038 and started right up. Idles and revs up just fine. I expected as much with double the output voltage, but now it is confirmed. (y)
 
Maraakate said:
Finally got around to testing this with the DC11-2, set the gap to 0.038 and started right up. Idles and revs up just fine. I expected as much with double the output voltage, but now it is confirmed. (y)
Click to expand...
Wow, .038" gap? Largest I've heard of was .036" with the GM coil and that had some misfire issues at times.
 
I have to take it out for a proper spin. Let it idle on the centerstand until it got pretty warm and revved up just fine. Locked the rear and gave it some hell seemed OK. I'll take it out in a bit and report back.
 
Just took it out for 15 miles highway and steep hill with 0.040" gap. Runs just fine. Throttle response is a bit better, not earth shattering, but there is a difference.

With regards to the misfire issues, I'm not trying to toot my horn but I'd imagine a new coil and new CDI probably helps with that. Bigger current draw means the ignition has to be up to spec with fresh components.

You should try 0.040" on the CDI I gave you with the GM coils, let me know how you make out.
 
Maraakate said:
Just took it out for 15 miles highway and steep hill with 0.040" gap. Runs just fine. Throttle response is a bit better, not earth shattering, but there is a difference.

With regards to the misfire issues, I'm not trying to toot my horn but I'd imagine a new coil and new CDI probably helps with that. Bigger current draw means the ignition has to be up to spec with fresh components.

You should try 0.040" on the CDI I gave you with the GM coils, let me know how you make out.
Click to expand...
That CDI is on the T1 with the original coil and NOS stator. Roadbike is an Ignitech module with the GM coil that's been running .030" gap for years.
 
LongDistanceRider said:
That CDI is on the T1 with the original coil and NOS stator. Roadbike is an Ignitech module with the GM coil that's been running .030" gap for years.
Click to expand...

Ah yeah, too much work unless you swap the coils and see what happens. I'll drop it down to 0.035" and just let it be so the components don't have to work as hard. It probably makes no real difference except ease of starting and masking imperfect carb tuning.

FWIW, these are plugs from last year. Haven't changed them yet. Also haven't synced the carb this year. Haven't really taken the 400A out that much this season except for test rides with CDIs and the coil.
 
Maraakate said:
Ah yeah, too much work unless you swap the coils and see what happens. I'll drop it down to 0.035" and just let it be so the components don't have to work as hard. It probably makes no real difference except ease of starting and masking imperfect carb tuning.

FWIW, these are plugs from last year. Haven't changed them yet. Also haven't synced the carb this year. Haven't really taken the 400A out that much this season except for test rides with CDIs and the coil.
Click to expand...
Plugs should be good for 10K or more, especially now that you have real good spark.
Once the carbs are sync'd they really don't loose it. The adjustment joint would need to wear a significant amount for it to change. It's a set and forget item until the carbs are split for rebuilding.
 
LongDistanceRider said:
Plugs should be good for 10K or more, especially now that you have real good spark.
Once the carbs are sync'd they really don't loose it. The adjustment joint would need to wear a significant amount for it to change. It's a set and forget item until the carbs are split for rebuilding.
Click to expand...

Fair enough, I'll just drop to 0.035" and let it be.
 
Small update, when testing ToniAnn's 81 CM400A with the 0.035" I found after it got really good and hot that it had a tendency to stall out in gear if you gave it a slight blip with the brakes on at a stop light. Put it back to 0.025" and problem went away. So for the Hondamatics you still need the stock gap. I think on the manual CM400s it would work fine if you did this with the clutch pulled in. I'd guess it probably happens with the GM coil as well since specs are practically the same.
 
FYI, I was wrong about the 0.035 being a problem. Real issue was the idle mixture. Running 0.040" on my CM400A and dialed in correctly and absolutely zero issues.
 
Cross-post, when it's all said and done this is what it will sound like with 0.040" spark gap and going two thou over on the valve adjustment with the DC11-2. Smoother idle, different idle exhaust note. Hundreds of miles with this configuration and no misfires or strange running.

 
@Maraakate I'm looking to replace the coil and cables on my CM400e, and I really like your setup with the Dyna coil. Thanks for all the testing and documentation. Can I get your opinion on cables? What are my options if I don't want to purchase a crimp tool and an entire coil of wire? Also, I think I'll try 3D printing the bracket. Can you imagine any issues with that? Would it be folly to rely on plastic? Thanks and cheers.
 
For cables do you mean the coil wire or the harness? Also I do have options for just the harness and coil wire. Practically at cost. I think 3d printing may not hold up from weather and vibration in the long run.

For ignition coil wire to spark plugs you want 7mm or 8mm copper/solid core wire. Do NOT use carbon wire for cars, it will degrade quickly and cause problems. For the harness you need the right crimp connectors and tools.

Im on mobile, but you said "cheers" which makes me think you're in the UK or AUS. If yes, I dont know your options but if you're willing to pay shipping ill sell you what you need sans the coil. I'm always willing to help forum members and just want everyone to ride these bikes reliably. 🙂
 
Thanks LDR for a great writeup, and for everyone contributing to this thread. I completed the mod without issue as a part of a project on my 1981 CM400. I used a Duralast C849 coil and a Taylor 45409 plug wire kit. I have some detailed pictures in my project thread here, see post #14.

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