B.T.H Mag-Dyno

[billirwinnz]

I've recently fitted a modern BTH magdyno to my A twin. The bike idles and runs perfectly and starts easily when hot, but is very difficult to start from cold. I've experimented with timing and fitting the carbs from another bike that starts easily on a traditional magneto without any improvement.

The BTH instructions say that I should see and hear a cracking spark when the plug is held 1/4" from the head. I see this when running on one cylinder but never at kicking speed. It's possible to see a spark at kicking speed but only infrequently. I did try a test with pieces of paper between the plug points. No holes appeared unlike they do on my traditional magneto. Is that because of lower current and higher voltage not burning the paper or because I can't get it to spin over fast enough when the oil is thick and cold? It's winter here in NZ!

I'd appreciate input from anyone who is using a BTH successfully on their A.

 

[vibrac]

Try a piece of thin cigarette like paper then hold it to the light you should see pin holes I dont think I have ever seen a spark on a BTH its so thin.

While on the Magdyno subject Has anyone fitted an Alton generator to a Lucas Magdyno? I took an ex Comet one and fitted it to a magdyno but I took it off as I thought the resistance to turning would cut up the big fibre gear I see Alton do a specifically Magdyno generator I wonder if they turn less lumpily?

 

[Robert Watson]

Hi Bill. I put one on my A twin a year ago but have only started it to test. I will tell you that my experience with BT-H CDI Ignitions is this. They do NOT like to start cold with a cold plug such as an NGK 7, but generally start much easier with a 6 or even a 5,

I have one on a post war B and had to resort to push starts on a cold engine, switched from a 7 to a 6, and even after a layoff of several moths started on the 2nd prod.

Tony's Big bore Comet runs one as well and on a couple of occasions he has brought it over as it would not start. Switch the plug from a 7 to a 6 and voila! This is a 92 X 100 engine with a squish head and pretty high compression. The second time it came over was because he had changed the plug and forgotten that it needs a 6.

The A started out with 6's and although always a bit cantankerous, having to reach into the depths to tickle the front float it responded in the proper manner!

 

[billirwinnz]

Try a piece of thin cigarette like paper then hold it to the light you should see pin holes I dont think I have ever seen a spark on a BTH its so thin.

While on the Magdyno subject Has anyone fitted an Alton generator to a Lucas Magdyno? I took an ex Comet one and fitted it to a magdyno but I took it off as I thought the resistance to turning would cut up the big fibre gear I see Alton do a specifically Magdyno generator I wonder if they turn less lumpily?

I used thin tissue. Same result. Holes from old mag. None from BTH.

 

[billirwinnz]

Hi Bill. I put one on my A twin a year ago but have only started it to test. I will tell you that my experience with BT-H CDI Ignitions is this. They do NOT like to start cold with a cold plug such as an NGK 7, but generally start much easier with a 6 or even a 5,

I have one on a post war B and had to resort to push starts on a cold engine, switched from a 7 to a 6, and even after a layoff of several moths started on the 2nd prod.

Tony's Big bore Comet runs one as well and on a couple of occasions he has brought it over as it would not start. Switch the plug from a 7 to a 6 and voila! This is a 92 X 100 engine with a squish head and pretty high compression. The second time it came over was because he had changed the plus and forgotten that it needs a 6.

The A started out with 6's and although always a bit cantankerous, having to reach into the depths to tickle the front float it responded in the proper manner!

Hi Robert

I’m using Champion NC5 plugs which cross to NGK B6ES. I’ll try a hotter plug.

If that fails I’ll refit the old mag which failed when the epoxy holding the capacitor let go. I’ve re-glued it with a special 3m vibration tolerant epoxy. This is on my Neal/Rodney replica. I decided on the BTH for total reliability.

B

 

[Robert Watson]

Hi Robert

I’m using Champion NC5 plugs which cross to NGK B6ES. I’ll try a hotter plug.

If that fails I’ll refit the old mag which failed when the epoxy holding the capacitor let go. I’ve re-glued it with a special 3m vibration tolerant epoxy. This is on my Neal/Rodney replica. I decided on the BTH for total reliability.

B

I thought they had put a German ignition system on them, or did you order without sparks?

 

[LoneStar]

I don't understand how the plug's heat range could affect the intensity of the spark?

The heat range is based on the shape of the insulator in the shell, allowing a better (cold plug) or worse (hot plug) heat path from insulator to shell. It should have no bearing on the quality of the spark, especially with hot / cold plugs at the same temperature during startup.

Perhaps there were other differences between the plugs when this disparity was seen?

 

[billirwinnz]

Yes. It had a PowerDynamo. Starting wasn’t good with that either and it needed a battery because it wouldn’t spark at low revs. My old A usually starts first kick hot or cold so I decided I’d prefer a mag. I built one from parts as a lockdown project. After removing the Powerdynamo I discovered that the dip switches were set for 40 degrees of advance so it was firing 6 degrees ATDC at full retard. By then the mag had been installed and I’d sold the PD back to Neal.

 

[Robert Watson]

As I said in an earlier post on another "A" thread, Building an A (even a brand new one apparently) ain't for the faint of heart!

 

[Vincent Brake]

Getting a longer kickstart.
To get a bit more starting revs, seems a good cure.