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ET: Engine (Twin) Black Shadow ATD


Website User
VOC Member
Hi, Richard Gibson here, from Rockford, Illinois, USA. I’m new to the Vincent world, but have been wrenching for a considerable time; hot rods & motorcycles in the ‘50’s & 60’s, aircraft 70’s -> 2000, then British cars (Morgan’s) now a Vincent.

I purchased a 1951 Black Shadow, complete, assembled and unrestored at auction in October 2019. It has been in storage since 1991. I plan to clean it up and keep it in its original unrestored condition as much as reasonable.

While adjusting the valves, I discovered that the #2 (front) exhaust valve was not working properly, which led to opening up the timing chest and discovering a multitude of problems, which I have addressed and repaired. (hopefully) Basically, all moving parts in the timing chest are new, including the ATD.

While setting the ignition timing (using rear cylinder) for the rebuilt original Lucas mag, I discovered that the new ATD doesn’t have enough ”range” to allow me to use the ignition specs I got from the Rider‘s Handbook; retarded setting 4’ BTDC, max advance 38’ BTDC. The most range on my new ATD is 25’, so if I start at 4’ BTDC the new ATD will only advance to @ 29’ BTDC. (I’m actually working backwards from that, starting at 38’ full advance, but can only retard to 14’)

Compairing the new and old ATD’s I find that the new ATD has a rotational range of @ .225” measured at the outer ears, while the old ATD has @ .350”.

Something seems out of wack to me. Any suggestions? Any thoughts gratefully appreciated.

1st photo, bike as I bought it, 2nd, as it is now, 3rd, timing chest on initial opening


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Website User
VOC Member
solve the problem of the timed breather is to block it of put new breather top exhaust i have one on the rear rocker cover as well with a flap valve on each you wood think that wood do it but no next you take the dynamo or alton generator clean with cellulose and casing all clean put dynamo back in with silicon on both services line up the sprocket on primary chain making shore with a rag to block off round chain so you do not drop nut washer in casing clamp dynamo take sprocket of then fill with mastic dow corning type smelly one this is done because you have moved the dynamo to line up the sprocket seal up outer cover done no more oil leaks the reason why there is a timed breather is i m shore you no that it is trying to get rid of excess air in the sump negative sump pressure if you dont block of all air going to engine it will leak round most oil seals you can by a flattened pipe called a ducks beak fitted on royal enfield by on ebay for about £3.00 not tried it my self it is the same as a flap valve my ones cost £5.00 hope this will help someone


Website User
VOC Member
I took my new ATD apart (very easy), filed on the two large ears (very hard material) until I got 16 degrees of advance (32 at the crankshaft) reassembled everything, installed it on the bike, timed it to 34 degrees at full advance, 2 degrees at start. Hope all works.

I’m now looking for information on the correct routing of all the control cables and wiring for my bike, 1951 Shadow. I have the wiring diagrams, but what path do the control cables and wires take ORIGINALLY to get to from point A to B?

Best regards.


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