New style BT-H mag

[HRD888]

Bth Magneto

I have had a BTH magneto on my 1948 "B" for ~22k miles. I had two issues one was a coil that failed because probably because I bent one of terminals on the coil . I replaced it which fixed the issue. The one other issues I had was with the BTH spark plug wire. Which has a carbon core. I had miss firing and the engine would run on only one cylinder or was intermittent. Took me a few days to diagnose the issue. What I found was part the carbon core in the BTH plug wire had vaporized and created a open ~1/2 of a inch. I replaced the spark plug wires with solid metal core wires, which in my case fixed the issue.

So I was out riding on the weekend and going to head over the going to the sun road in Montana. JRO 101 is an early B rapide with 7.3:1 Mk1 cams and 1 1/16 original carbs. It is running a new style electronic BTH mag. Pulling up a long hill it started to misfire, my first thought was that it was just getting over rich as we were probably at 4000+ feet. However the misfires were much more pronounced that just rich mixture. Not just an odd misfire on one cylinder but like some one was turning the sparks on and off, both cylinders, and sometimes very pronounced, like 1/4 to 1/2 a second. This seemed only to happen on hard pulls. This setup has been over 8000 feet on road rides and up to the top of Mt Evans in Colorado a couple of years ago --- 14,000 feet, and did not act like this.

I turned around and headed back to Waterton and only got a couple of very small misfires. If I wound it out in low gear as the revs climbed, it missed a bit.

The system has about 8000 miles on it.

At one point it would start, rev up for about a second and then switch right off, I didn't even have time to pull in the clutch and engage a gear. It did this several times in a row. I pulled off the mag cover and inspected all the wiring etc which seemed to be fine. After a few minutes doing this it started up and ran OK. Then I put the mag cover back on and headed for home.

I suspect that the coils are starting to break down and that is what is causing the misfire, when quite warm and placed under a heavy load.

Anyone experienced this??

 

[BigEd]

I think that the new BT-H magnetos were supplied with solid copper wire plug leads. One or two people had problems after fitting there own plug leads/caps.

I have had a BTH magneto on my 1948 "B" for ~22k miles. I had two issues one was a coil that failed because probably because I bent one of terminals on the coil . I replaced it which fixed the issue. The one other issues I had was with the BTH spark plug wire. Which has a carbon core. I had miss firing and the engine would run on only one cylinder or was intermittent. Took me a few days to diagnose the issue. What I found was part the carbon core in the BTH plug wire had vaporized and created a open ~1/2 of a inch. I replaced the spark plug wires with solid metal core wires, which in my case fixed the issue.

 

[HRD888]

I installed the BTH Magneto on my bike in 2003. The coil wires that came from BTH with the magneto setup were carbon core wires.

 

[BigEd]

I've been out this evening with Tony who made the new BT-H magnetos up until recently when he sold the company to the current owner. I mentioned your problem with the carbon plug leads. He said that he never supplied anything other than solid copper wire plug leads. Did you buy through a third party who may have changed the leads?

I installed the BTH Magneto on my bike in 2003. The coil wires that came from BTH with the magneto setup were carbon core wires.

 

[HRD888]

I ordered the magneto directely from Tony at BTH and it came with the those leads.

 

[BigEd]

Tony was pleased to hear that you'd had good service from the magneto. It was made quite a few years ago in 2003 so maybe he did fit some with carbon leads and doesn't remember. Your post highlights you should check the easy things first if you have a problem.

I ordered the magneto directely from Tony at BTH and it came with the those leads.

 

[craig]

October of 2012 I received my, repaired 2nd time, BTH back from England. I installed on my Comet, but winter set in and riding delayed until April of this year 2013. I have done 300-400 miles on this Comet with the repaired BTH in cold weather. All seemed fine until yesterday, when a hot summer day(88deg F) was the riding weather. The BTH failed again to provide spark after 20 miles and left me stranded to call a tow service. Once the magneto cooled down to ambient (75 deg), it now provides spark again. Am I the only Comet rider having this BTH problem? This is insanity, Craig

 

[vibrac]

no trouble with mine but then I guess we havent seen 88F since 2010 :-( also we can be shot if we dont use centigrade.

to be serious did you fit new coils?-they have just revised the design and were selling the old ones cheap at the stafford show in April

 

[Howard]

I've also done 3-400 miles on my recently acquired Comet with B-TH. It works well in high wind, rain, hail and snow - don't know about hot weather I've only ridden it in Spring and early Summer!!!!

H

 

[davidd]

I have ridden in some incredibly hot weather. At VIR it was 98 degrees F and after several laps the race was red-flagged. Then re-started. Also, at ECTA we had several members leave due to the high heat. I still set a record. So, no bad experiences. David