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
Comet Testimonial 2
From: Roger Bradley
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.
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.
Telephone: 01234 880605
Copyright © 2013 E. Grew