Motorcycle Repair: 1971 Honda C50 idle, ngk spark plug, needle nose pliers

Question
Hi
I was hoping you may be able to help with a problem my 1971 Honda C50 has. It performs well above idle but the idle is either fast, fine or slowly decreasing until it stalls. I have checked the points gap, the valve clearances and installed a new NGK spark plug however the idle still seems to hunt. How should I use the two screws on the side of the carburettor? Both the brass spring loaded screw and the recessed flat bladed screw change the idle speed however I do not know how these two should be tweaked in relation to each other. Thanks in advance for any help offered.

Steve W

Answer
Steve, if the engine hangs up at 2-3000 rpm and then slowly drops down and stalls, it's probably worn out timing advance springs. You will have to pull the flywheel to get to them. I've used needle nose pliers to pinch the loops on the ends of the springs little to slightly increase the spring tension. Just don't close them too much or the bike won't run as well.  

There are two screws on the carb as you indicated.  The one in the center of the slide lifts the slide and controls the air going into the engine at idle so it controls the actual idle speed of the engine.  THe other screw is the idle air mixture.  That has to be adjusted to get the right air fuel ratio at idle.  Start with the screw at 2.5 turns out from the seat.  Then turn the screw in until the idle speed just starts to drop and then out until the idle speed starts to drop again and then place the screw half way between those two points.  

You will have to fix the timing issue first before you can adjust the idle mixture or just set it at 2.5 turns and see if that's good enough.

Regards
Rich