New style BT-H mag

[Robert Watson]

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??

 

[vibrac]

the coils are the weakest point of the system I carry a spare
The system will easily cope with twin plugs (I have that on the racing Comet) so perhaps thats another way to prevent problems I had 4 coils on the racing twin.on the road twin I just run two as the other plug holes have RE decompressors (You wont catch me with all those leaky rods and rollers in my timing chest)

 

[b'knighted]

As I understand it we won't catch you anyway, however hard we try!

 

[BigEd]

Dear Robert,
I run a "B" Rapide with a similar set-up regarding 7.3:1 compression and MK1 cams but with the fuel supplied through 28 mm Mikuni carbs. I have run one of the new type BT-H magnetos firing 4 coils since I built the bike from a basket case in September 2008 and currently have over 30,000 miles. I have been trouble free in the sparks department although I can't say I've had the opportunity of riding at the kind of altitudes you mention.
A few magnetos were supplied with coils that had a plain "spike" that the plug lead just pushed on to rather than a "spike" with a screw. (The screw type were unavailable for a while.) If the lead works loose from the plain "spike" the spark has to jump two gaps and this can overload the CDI circuitry inside the magnetos and lead to a misfire. It is worth checking the security of the lead in the coil. The coils with a screw "spike" are much more secure
It is easy to carry a spare coil as they are small and rrelatively inexpensive. N.B. The voltage supplied to the coils is much greater than 12 volts so be careful if you trying an alternative coil.
Another thing that can cause an intermittent misfire is a short on the black wire to the kill button and sometimes the kill button itself.

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??

 

[Robert Watson]

Thanks for that. I talked briefly with BT-H (his phone died I think) but did get to the point where he explained that there are two cdi units and they do fire the coils seperately so if the whole system cuts out as it appeared to be doing, likely the kill wire. I have this on a very robust switch, not a push button, have inspected all the wiring and all seems to be in order, so must be some intemittent something. I did install new plug leads and they are the screw the wire onto a spike kind. Have to go and give it another try.

 

[Simon Dinsdale]

There is another possibility. The two seperate cdi units (one per cylinder) have a common power source I believe. So if the built in generator is failing intermittent then it would effect both cdi units. My BTH died as the generator part of the system failed.

 

[Hugo Myatt]

Dear Robert,
I run a "B" Rapide with a similar set-up regarding 7.3:1 compression and MK1 cams but with the fuel supplied through 26 mm Mikuni carbs. I have run one of the new type BT-H magnetos firing 4 coils since I built the bike from a basket case in September 2008 and currently have over 30,000 miles. I have been trouble free in the sparks department although I can't say I've had the opportunity of riding at the kind of altitudes you mention.
A few magnetos were supplied with coils that had a plain "spike" that the plug lead just pushed on to rather than a "spike" with a screw. (The screw type were unavailable for a while.) If the lead works loose from the plain "spike" the spark has to jump two gaps and this can overload the CDI circuitry inside the magnetos and lead to a misfire. It is worth checking the security of the lead in the coil. The coils with a screw "spike" are much more secure
It is easy to carry a spare coil as they are small and rrelatively inexpensive. N.B. The voltage supplied to the coils is much greater than 12 volts so be careful if you trying an alternative coil.
Another thing that can cause an intermittent misfire is a short on the black wire to the kill button and sometimes the kill button itself.

I had a great deal of trouble trouble with my newly fitted BT-H. It misfired and then would only fire intermittently on both cylinders and frequently going on to one cylinder at the most inopportune moments. I purchased new coils having heard that some had failed prematurely. It was only when I came to fit the replacement coils I discovered the cause. On one of the coils the HT lead had been screwed onto the threaded spike but had missed the copper core entirely and the spike was screwed into the insulation. I discovered the same fault at the plug cap end. I am amazed that it fired at all. Needless to say I had not done the original fitting. With the replacement coils (probably unnecessary), new HT leads and plug caps it works fine.

 

[craig]

I have had nothing but problems with my new Comet BTH since day one. Bike runs fine for 30-40 miles then total loss of spark. After it cools down for several hours, the spark returns...........by then ..............several times you have trouble shot and replaced everything you could think of, poor wire, mis threaded wire, poor kill button, bad plugs, rebore engine for fresh compression, new rings for least oiling, new plug cap, send unit to England to be told there is nothing wrong with BTH, reinstall ....run 20 miles then total loss of spark....cools down, spark returns.
I am planning on borrowing a standard Lucas magneto to see if a standard mag will run as required.
I am hoping BTH can provide a good solution as well.
This Comet has been stranded 6-7 times for this BTH.
I have ridden a standard Comet for thousands of miles during this same time. The owner is reluctant to let me pull the Lucas to trial on mine, but in just a few days.....

 

[b'knighted]

Hi Craig,

That sounds exactly like a failed capacitor in a Lucas magneto. Before I had experienced such a thing, I had a similar problem with ignition coil failures both on cars and on the Knight. I have no experience of new BTHs but believe that they use an external ignition coil. This would be easier to swap out as it wouldn't need any retiming. Can you borrow one of those to try before fitting a standard magneto?

 

[vibrac]

I replaced everything you could think of, poor wire, mis threaded wire, poor kill button, bad plugs, rebore engine for fresh compression, new rings for least oiling, new plug cap,

Yes I dont see replaced coil in that list.