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Triumph TR3 FAQ page

Distributors and Timing discussions
(Discussion on problems & tricks regarding points, cap & rotor)

One of the problems with the stock distributor is that the shaft is easily bent and this leads to an alternate point gap for opposite cylinders. Checking you will see that the static timing for example the number one cylinder is perhaps 10 degrees, but when you turn the engine over to the number four cylinder the timing is as much as 2 or 3 degrees less or MORE. Therefore you have an engine that is operating with either too much timing on alternate cylinder or not enough. In the end you find that though you have programmed only 30 degrees of total timing into the engine you are in fact carrying 32 degrees or more on other cylinders, and those cylinders then are more prone to detonation.

In the end the check for detonation is the window to the combustion chamber, the spark plug. Install clean (not necessarily new) plugs or even a couple cleaned plugs and run a lap or test length, pull the plugs and look for carbon specks and worse yet is tiny aluminum balls. If you find the tiny aluminum balls on the center insulator of the spark plug, unless you immediately do something about the timing the piston in that cylinder is not long for this world. If you are doing this check on a race track without air filters, do not confuse the tiny BLACK shiny balls for aluminum. The tiny black balls are RUBBER. They don't appear black until you look very closely and are easily confused in appearance with aluminum which are also very shiny.


Basically the early Lucas units, up to the late 60s, had a symetrical cam lobe & a tendency to bounce the points at medium-high rpm, with the standard points. There are/were different points available with more spring tension, or you can add a spring strip, to raise the limit using this unit. The later assymetrical lobe design has a ramp that opens them quicker & increases the coil saturation time, allowing a more reliable spark at high speed. Then they came out with the 45D unit which was designed for the HEI system. It has the larger body, cap & rotor, & the cap has walls inside, all designed to prevent spark-scatter from the HEI.
A point type distributor based on the 45D body, assymetrical cam, good points with a locked braker plate, is good for over 7000 rpm in a 4 cylinder, reliably. (example- my girlfriends POS 1275 Sprite will pull reliably to 8k). For max reliability or regular use over 7000, I normally will fit an optical trigger unit such as Allison or Lumenition. 6 cylinder engines are a different story since the saturation time is obviously decreased by having 6 impulses per distributor rev vs 4. Dual point systems were really designed for 8 cylinder engines, where there is absolute minimal saturation time. It goes without saying that the distributor has to be in good shape re shaft, bushings, breaker plate, etc & a critical point is the indexing of the breaker plate to the body. If it's not locked in the correct orientation, the
rotor tip won't line up with the contact in the cap. You can see where the spark is going by looking at the carbon track on the tip of the rotor.
Safety FasTR,

Well, it looks like you absolutely know how to make a lucas distributor work. My tolerance is far less than yours.

The distributor I tried to use is the 45D. What finally put me over the edge is the realization that in this "racing" distributor the point plate is the same as the vaccum advance unit and is "fixed" in place with a tab that lets it shake about two degrees. Yes, I could fix that, and I could probably properly index the distributor with a distributor machine, or I could buy a Mallory and start building a catapult.

Bill Babcock


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