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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Ignition Advance
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<blockquote data-quote="Martyn Goodwin" data-source="post: 128533" data-attributes="member: 2433"><p>Charles,</p><p></p><p>There was two things got got me pondering the situation.</p><p></p><p>Up front - I have a standard Comet, Mk1 Carb, original (rebuilt) magneto, Amal Mk1 30mm concentric carb, single spark plug head, been using Carello 7.2 :1 pistons for years and 92 octane fuel </p><p></p><p>With the last piston replacement (piston had overheated resulting on the top ring groove opening up to almost twice its width) around 12 months back I decided to fit a decompression plate under the muff. In doing so I had expected a reduction in performance but was prepared to accept that if it meant saving the next piston. To my surprise there was actually a seat of the pants improvement in performance even though by lowering the CR I had reduced the combustion efficiency. </p><p></p><p>The second thing was that around 2 months back before taking my bike out for a run (there was just a short window when the covid restrictions were eased) I decided to check the points and timing. Well I needed to adjust the points and as a result needed to reset the timing. After doing so I found the bike reluctant to start - it has been a first kick starter for years. Anyway got it started and went on my ride. It was a revelation suddenly I had significantly more power (torque) on the hills and also under acceleration. Hills where I needed to drop down a gear I was sailing over in top. Overall the motor sounded and performed much sweeter. Now this is with the decomp plate still in place.</p><p></p><p>On returning home I resolved to recheck the timing and was surprised to find I had made a mess of it when I set it prior to my ride. Normally I had been using 4 BTDC at full retard (gives very easy starting) and 34 BTDC at full advance. What I found was I had set the timing at TDC full retard and 30 BDTC full advance.</p><p></p><p>I reset it - but based on that experience I set 2 BTDC retarded and 32 BTDC advanced. Bit easier to start but it took the 'edge' off performance.</p><p></p><p>As I said, it was this that got me thinking about timing and what was going on and why. </p><p></p><p>Anyway I have some old ATD parts that I am now in the process of modifying so that I can have 4 BTDC full retard for starting and 30 BTDC full advance.</p><p></p><p>Martyn</p></blockquote><p></p>
[QUOTE="Martyn Goodwin, post: 128533, member: 2433"] Charles, There was two things got got me pondering the situation. Up front - I have a standard Comet, Mk1 Carb, original (rebuilt) magneto, Amal Mk1 30mm concentric carb, single spark plug head, been using Carello 7.2 :1 pistons for years and 92 octane fuel With the last piston replacement (piston had overheated resulting on the top ring groove opening up to almost twice its width) around 12 months back I decided to fit a decompression plate under the muff. In doing so I had expected a reduction in performance but was prepared to accept that if it meant saving the next piston. To my surprise there was actually a seat of the pants improvement in performance even though by lowering the CR I had reduced the combustion efficiency. The second thing was that around 2 months back before taking my bike out for a run (there was just a short window when the covid restrictions were eased) I decided to check the points and timing. Well I needed to adjust the points and as a result needed to reset the timing. After doing so I found the bike reluctant to start - it has been a first kick starter for years. Anyway got it started and went on my ride. It was a revelation suddenly I had significantly more power (torque) on the hills and also under acceleration. Hills where I needed to drop down a gear I was sailing over in top. Overall the motor sounded and performed much sweeter. Now this is with the decomp plate still in place. On returning home I resolved to recheck the timing and was surprised to find I had made a mess of it when I set it prior to my ride. Normally I had been using 4 BTDC at full retard (gives very easy starting) and 34 BTDC at full advance. What I found was I had set the timing at TDC full retard and 30 BDTC full advance. I reset it - but based on that experience I set 2 BTDC retarded and 32 BTDC advanced. Bit easier to start but it took the 'edge' off performance. As I said, it was this that got me thinking about timing and what was going on and why. Anyway I have some old ATD parts that I am now in the process of modifying so that I can have 4 BTDC full retard for starting and 30 BTDC full advance. Martyn [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Ignition Advance
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