Question about 1979 Yamaha XS 650 SF

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Timing I was wondering how to adjust the timing on my XS 650 1983. It seems my left cylinder is back firing as i see blue flame coming from the exhaust. i replaced the pugs with new ones ( no change) and will be cleaning the carbs tomorrow. If this does not work I guess it would be a timing issue. If so how do I correct the timing. I cant see any air leaks as everything seems snug and tight

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Degree wheel bolts up flat against charging system rotor. Since the XS rotor is keyed to crank I just turn the motor over from the same side by hand with the rotor. The camshaft is installed with the flat ground factory index notch (white arrow above) on the camshaft (not on the cam sprocket) facing straight up. The motor is on top dead center on overlap on number 1 cylinder (nearest) in the above image. The two visable cam lobes are both up and the rocker arms, if installed would have both the intake and the exhaust valve open on this cylinder. Number 2 cylinder (rear of image) is on TDC on the compression stroke and both the rocker arms are on the cam base circle with both lobes centered and facing down..
I have discovered that you can check your stock cam timing for all years of the XS motor with the motor completely together and even installed in the bike. This can save many people lots of worry and problems. It can be as simple to check as removing a few screws! When the index notch ground into the camshaft is installed in the proper position as shown in both of my above images: (number 1 red arrow) above, and (the white arrow) in the previous image above it, NOTE the small hole for the ignition advancer at red arrow number 2 above is also facing up, and inline with the cam billet index blank!!!! It sticks out of the cam bearing end seal and is visible by removing the right hand camshaft end cover!! If you have points ignition, you have to loosen the ring nut and simply remove the advance mechanism to see the small pin in the index hole. If you have a 1980 or up with stock electronic ignition you have only to remove the two cover screws and look at the hole!!! It should be straight down (or straight up) with the pistons at Top Dead Center (TDC)!! Set the TDC mark on the rotor to the TDC mark on the left end of the crank with a 17mm socket (image below) , have the spark plugs out and see that both pistons are at TDC. It is that simple. This is a BIG problem for many novice engine builders, and now anyone can check the basic cam timing as I have shown here!

Posted on Jul 12, 2009

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