A few people are saying fix the carb first. They are 100% wrong. Timing at idle [ & more if IT BIS needed ] IS NOT A BANDAID. What it shows is that folks do not understand what is going on in the cylinders...at idle!!!
At idle, the air entering the engine is throttled [ almost closed t/blades have 'stretched' the air ] so that the air below the blades is less dense than the air above the blades. There is now a greater distance between air molecules.....so the flame front needs more time to complete the burn for MAXIMUM EFFICIENCY AT IDLE, ie, more timing is needed at idle. Then you have residual exh gas left inside the cyl, caused by reversion at the end of the exh stroke. Mixed with fresh fuel, it burns more slowly & needs more idle timing. Increase cam duration/overlap, & it gets worse, even more timing is needed to get the optimum burn TIME at idle.
Sure, you can use less timing than OPTIMUM at idle & use more throttle opening, but you will never have:
- the best idle quality that is possible
- the best fuel efficiency
- the highest vacuum
- the coolest running engine at idle/off idle
- because the engine is most efficient when it has the CORRECT timing, uses the least amount of fuel & produces the highest amount of HP.
So get the idle timing correct FIRST before you touch the carb, NOT THE OTHER WAY AROUND.