Motorcycle Repair: NO THROTLE RESPONSE, shifting gears, minimum compression

Question
I HAVE A 1973 SL70 . I AM NOT GETTING THROTLE RESPONSE . THE ENGINE STARTS AND IDLES BUT WHEN I TRY TO GIVE IT GAS IT STUMBLES AND HAS NO RESPONSE. I HAVE ANOTHER CARB THAT I PUT ON AND I GET MORE OR LESS THE SAME THING HAPPENING.IT DOES REV UP EVERY SO OFTEN AND WHEN IT DOES I CAN KEEP IT REVING UNTIL THE REVS DROP FROM SAY SHIFTING GEARS AND THEN IT BOGS DOWN AGAIN . WHEN I DO GET IT TO GO I CAN HOLD FULL THROTLE AND ITS CLEAN FULL SPEED. IGNITION TIMING IS OK. COULD IT BE THE TIMING CHAIN TENSIONER OR THE CAM CHAIN ? ANY IDEAS WOULD BE GREAT MY KIDS KEEP ASKING WHEN CAN THEY RIDE THE BIKE THANKS TERRY

Answer
Terry, I would pull the carb off and clean the main jet, should be a #65, and set the float level to 7mm. Look for water and debris in the bowl, needle valve and through the petcock. The whole fuel system has to be CLEAN.

This particular model has a limited mechanical spark advance, which adds another 10 degrees at 4500 rpm. Be sure that the point faces are clean, timed to open at the F mark alignment. In order to check the mechanical advance, you will have to get a flywheel removal tool from a m/c shop and remove the flywheel to check the point cam and weights inside.

You may want to do a compression check as these bikes often get ridden into the ground. 150psi is minimum compression spec. Check the valve clearances... .002" cold on both intake and exhaust. That is often overlooked and the cause of poor running issues.

When you are checking the valves at TDC, turn the motor over ONE turn and let it stop so that both valves are just slightly open. If you are careful you can "feel" when they are both open the same amount (equal pressure on the springs, camshaft,etc.). Check the position of the crankshaft at that point. Should be at the T mark. If it is over at the F or an equal distance the other way, then the cam timing may be off a tooth. If there is a lot of slack when you rock the crankshaft back and forth with a wrench then the timing chain could be all stretched out.

Be sure that it is getting a good spark. Check for defective spark plug cap.. over 7k ohms resistance. A loose wire to the coil or bad connection at the generator wiring plug, can cause some grief, along with a bad kill switch/wiring.

Bill Silver