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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Crankcase Breather
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<blockquote data-quote="davidd" data-source="post: 34384" data-attributes="member: 1177"><p>Phil,</p><p></p><p>I am not sure that I am following what is going on, so I will just mention what I think should be happening. I put a degree wheel and pointer on, without removing the timing cover, etc. Then I take the plug out to find TDC. Once the pointer is correct I pull the black rubber tube off the breather pipe and I slip on a long plastic tube so I can sit on the timing side while blowing in the tube. On a single, the window in the breather should align with the window in the spindle sometime just after TDC and it should close around BDC. So, as you are bumping the rear wheel forward the breather will be blocked until sometime just after TDC. It will be free flowing until sometime around BDC. This may vary somewhat as the timing position will most likely take into account the momentum of the charge of air that is being expelled. So it may open just after TDC and just after BDC to account for the momentum. Someone on the Forum may have experimented with the numbers, but I think the range would require you to move the breather gear one tooth once it is opening at TDC to see if it breathes better by opening later. I know on the twin the timing is on the rear cylinder and it opens 70 degrees after TDC and closes at 30 degrees after BDC.</p><p></p><p>Poking the screw driver in would reveal a blockage if one existed, but if there are no mud daubers nesting in there the shaft will hit nothing until it hits the top of the breather spindle as the windows or ports are 90 degrees to the shaft.</p><p></p><p>Finally, if the breather restriction is slowing the engine severely, it is most likely the top end that needs rebuilding, but it is good to know what your breather timing is before you begin drawing conclusions. A leak down test will tell you if anything is wrong with the top end.</p><p></p><p>David</p><p></p><p>David</p></blockquote><p></p>
[QUOTE="davidd, post: 34384, member: 1177"] Phil, I am not sure that I am following what is going on, so I will just mention what I think should be happening. I put a degree wheel and pointer on, without removing the timing cover, etc. Then I take the plug out to find TDC. Once the pointer is correct I pull the black rubber tube off the breather pipe and I slip on a long plastic tube so I can sit on the timing side while blowing in the tube. On a single, the window in the breather should align with the window in the spindle sometime just after TDC and it should close around BDC. So, as you are bumping the rear wheel forward the breather will be blocked until sometime just after TDC. It will be free flowing until sometime around BDC. This may vary somewhat as the timing position will most likely take into account the momentum of the charge of air that is being expelled. So it may open just after TDC and just after BDC to account for the momentum. Someone on the Forum may have experimented with the numbers, but I think the range would require you to move the breather gear one tooth once it is opening at TDC to see if it breathes better by opening later. I know on the twin the timing is on the rear cylinder and it opens 70 degrees after TDC and closes at 30 degrees after BDC. Poking the screw driver in would reveal a blockage if one existed, but if there are no mud daubers nesting in there the shaft will hit nothing until it hits the top of the breather spindle as the windows or ports are 90 degrees to the shaft. Finally, if the breather restriction is slowing the engine severely, it is most likely the top end that needs rebuilding, but it is good to know what your breather timing is before you begin drawing conclusions. A leak down test will tell you if anything is wrong with the top end. David David [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Comet Crankcase Breather
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