BTH Components Ltd.

Comet Testimonial 1

We got this letter from Ken Tideswell who fitted one of our new BT-H magnetos to his Vincent Comet.  The magneto we make for  the Comet has a shorter end cap to allow it to fit under the narrower cowl fitted to the singles.

Here is an unsolicited plug for your ignition system. You may remember me, it was myself who asked you to modify your magneto for  my Comet.  Well at long last I have been able to go out and test the finished article.  What a transformation compared with the standard Lucas set-up. The analogue advance device is centuries ahead of a mechanical ATD.  The driveability of the Comet is a revelation.  You can accelerate from top gear at 30 mph if you want but changing down gives better results.  It has more torque mid-range and easily exceeds 85 mph in top with 65 available in 3rd gear, the original road test figures. This is with a 7.3:1 c/r, and a 30mm mark2 Amal carb.
Thank you very much and every success for the future.

 Ken Tidswell

Comet Testimonial 2

 From: Roger Bradley

 Dear BT-H,

 I carried home this magneto and fitted it next day to the Comet.

It was extremely easy to fit and fitted perfectly. I had worried about replacing the rearmost of the 3 magneto flange carrying studs in the Comet crankcase with the new setscrew but I needn't have as it was exactly the right length and the setscrew was very easily fitted with the tool supplied.   

Setting the ignition timing accurately with this magneto simply couldn't be easier. I did have to slightly shorten the locking bar supplied with the magneto for timing purposes so that it would clear the exhaust pipe when being inserted into or removed from the magneto. The position of the Comet exhaust varies because a new one has to be flanged by hand by the fitter and, therefore, the angle and exact length of the exhaust pipe when finally fitted is never the same on any two bikes. It was a simple matter to shorten the bar. Better that it be too long as supplied for this application than too
short.        

The bike now starts with one kick. It idles as steadily like a gas oil stationary engine. I confess I am sometimes tempted to push the petrol tap to "OFF" to see if the engine would continue to run without fuel on the spark alone ! It pulls reliably under any load from idling and runs perfectly up through the rev range to flat out.  I am totally satisfied with the engine's performance now.

It has not been easy getting to this point. When the bike was rebuilt at great expense some 5 years ago, I was persuaded to fit a big bore piston which raised capacity from 500 cc to 560 cc, and to have the combustion chamber modified with a small squish band being machined.  The advice from the engine-builder was to retard advance ignition timing from the original 38 degrees BTDC to 34 degrees maximum, perhaps even as far as 26 degrees BTDC. I timed it at 34 degrees BTDC fully advanced, that is in so far as you can ever be sure of accurately timing an auto advance Lucas magneto given that one needs three hands and separate pairs of eyes for each side of the engine.  The original Amal carb and the original auto advance Lucas magneto rebuild by a reputable firm, were fitted. It proved virtually impossible, however, to start the engine reliably. Compression was and remains enormous. It was and still is impossible to push the kick-starter through compression without use of the  de-compressor lever.  It doesn't help that the Burman kick-start crank is not the most  efficient and ergonomic of its type and was probably designed to cope with low compression engines in any case. Gearing leaves it impossible to push start a  Comet, or this one at least.             

I tried various other advance timing points between 34 and 26 degrees BTDC but the problem did not diminish. I thought the problem might be carburation so replaced the original Amal with a new Mikuni. No difference in difficulty of starting, i.e. it very seldom started.  I then realised that the retarding advance timing using the auto-advance Lucas magneto meant that I was trying to start the engine on kick-start at TDC or even after, whereas the original setting for K/S cranking on a Comet was 4 degrees BTDC. I acquired and fitted a manual advance Lucas magneto in order to be able to experiment.

With a manually set magneto the engine now sometimes started but it was a matter of trial and error on the advance lever. Unfortunately, it favoured firing up only on advanced settings, the resulting kickback doing my ankles and knees no good at all!  Once the engine did run and the bike was taken out on the road, 'pinking' could be a plague at higher speeds.  It is also a nuisance having to remember all the time to adjust the timing lever but the worst problem was never being sure that the bike would start so I could never ride it far from home in case it stalled and then wouldn't start again.  In addition, I discovered that that I had heart problems and simply couldn't afford to stand for ages working myself into a lather kicking away futilely at the thing.          

I had both magnetos checked and tested by the firm that had rebuild them who could find no fault with them. I have in the past had magnetos rebuild by this firm for other old British bikes although with unmodified engines, and they always worked perfectly so I had no reason to doubt their competence or advice. I began to wonder if the starting problem on the Comet was that that very high compression was
quenching the weak spark of a Lucas magneto at kick-start cranking speeds.  I switched to 12 volt electrics and fitted an electronic distributor ignition.       

The bike now started reliably and ran smoothly up the rev range without ' pinking'.  Ignition timing was easy.  However, whilst this system certainly worked well, some inconveniences became evident in practice.  First of all, a Comet doesn't really not lend itself to fitting a distributor.  It is not really possible to route standard wiring connecting supply to the coil or the coil to the distributor, down past the hot engine and into the magneto cowling, or underneath the engine, without risk of damage and shorting out. Secondly, additional wiring and a switch needs to be fitted and there is really nowhere on a Comet where a switch looks right. Thirdly, distributor ignition still depends on the battery for supply. The occasional use
made of old bikes nowadays introduces all sorts of potential unreliability here that don't pertain with a self generating magneto.

That's my experience, anyway.  The Comet engine was designed for use with a magneto.  No additional wiring or switches.  Fit and forget.  Get you home even if the battery's flat and the generator/dynamo has packed it in. That was my father's spec for motorcycle ignition and remains mine.  

I wish I had come across the BT-H magneto when the Comet engine rebuild was completed 5 years ago. I would avoided a lot of grief, unnecessary expense, about 40 engine ignition timings and instead gotten a lot of more miles ridden on what has turned out to be a very pleasant machine now that it is finally sorted.   

Many thanks,

R. Bradley

P.S. I'll be investing in one of those BT-H magdynos for Ariel's when I get my '59 VH rebuilt.

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Email: bthcomponents@gmail.com 

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