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Ace, Aceca & Greyhound Forum / Bristol Centrifugal Ignition Advance
« on: September 10, 2007, 15:42:53 »
In tuning my recently rebuilt Bristol engine, I found a problem with the centrifugal ignition advance. It was stuck at an intermediate setting at all engine speeds.
The advance mechanism has two springs, a light and a heavy one. The heavy one is 0.054” wire and a little longer. The light one is 0.027” wire. This arrangement intends to produce a fast advance rate initially with the light spring. Once the slack in the heavy spring is taken, the advance rate slows. In my distributor, the light spring was stretched and had no “spring”. The heavy spring is so heavy that it didn’t move at all. I set total advance at 35 degrees BTDC at 3000 rpm, but had 35 degrees at all engine speeds. I don’t know if these springs are original.
I purchased a set of springs from Moss Motors for a Triumph TR2. There are five of springs in the package for $27. It’s a little pricey but the only source I could find. I used the 0.028” wire and the 0.035” wire. I stretched the 0.035 a little to give two advance rates. Now the engine has 8 degrees BTDC at idle and about 30 degrees at 3000 with more to go. The mechanism has 30-34 degrees of possible advance.
Performance is vastly improved. The idle is much smoother even with a cam that is more radical than the Bristol Sport Cam. The low and mid range torque is great as a straight six should be.
If you are setting the timing by the engine speed method in the manual, you may want to check out the centrifugal advance. The same problem could occur.
The advance mechanism has two springs, a light and a heavy one. The heavy one is 0.054” wire and a little longer. The light one is 0.027” wire. This arrangement intends to produce a fast advance rate initially with the light spring. Once the slack in the heavy spring is taken, the advance rate slows. In my distributor, the light spring was stretched and had no “spring”. The heavy spring is so heavy that it didn’t move at all. I set total advance at 35 degrees BTDC at 3000 rpm, but had 35 degrees at all engine speeds. I don’t know if these springs are original.
I purchased a set of springs from Moss Motors for a Triumph TR2. There are five of springs in the package for $27. It’s a little pricey but the only source I could find. I used the 0.028” wire and the 0.035” wire. I stretched the 0.035 a little to give two advance rates. Now the engine has 8 degrees BTDC at idle and about 30 degrees at 3000 with more to go. The mechanism has 30-34 degrees of possible advance.
Performance is vastly improved. The idle is much smoother even with a cam that is more radical than the Bristol Sport Cam. The low and mid range torque is great as a straight six should be.
If you are setting the timing by the engine speed method in the manual, you may want to check out the centrifugal advance. The same problem could occur.