Am restoring 74 Dt 125 and it has a compression release---the valve at the head with cable and little lever. Looks aftermarket to me. I have Yamaha factory manual and the Clymer manual and neither mentions this as a part or evenan accessory. I understand what compresion release is--or thought I did--and can't see why a dinky 125 two stroke would have it. It occured to me maybe for the electric start. I have the incorrect battery--very weak power, but good enough to check my so far electrical connections--it will almost turn the motor over but just one or two chugs and that's it. But with the little lever engaged it turns over great--but no spark --cus no compression. I guess I don't understand compression release--my old XR200 has it engage as the kick starter is cranked--and I remember a cousin's big XR 600 had a manual lever that was stock and he had to use it to kick over ole thumper. I can't find anything in my manuals to explain exactly how it works. I guess I really want to know if it was an option or accessory for the old 74 DT or was it added on?? And Why?? I have an identical parts bike with the plug in the cylinder head--as the mamual shows. Would LOVE some enlightenment on this. I have learned way more on this web site than any manual I ever had. Am hoping for some more luck. Thanks a lot!

back then they had a two plug head, comp release was aftermarket back in the day the compp release screwed in to second plug hole. , sort of a jake brake for bikes in theory.