1972 Demon 340 engine mods
I heard explosions in the exhaust at crusing speed also.
Late timing for the conditions.
Ideal timing is based on rpm, and load. (However if the fuel mix is too far off, the burn rate will change - so its hand in glove relationship.)
This made me think as I was able to advance it to 30 degrees at idle and engine speed was getting higher even past this value.
In neutral, the engine will take a lean mix - which burns slower and likes more advance. But its inefficient and makes less power when the drivetrain is engaged than if a richer mix and less advance was used.
Also 30 degrees made it react to gas better(?!)... It started revving to 6000+...Before that 4000 rpm felt like a struggle. I am suspecting some electrical issue, weak spark due to bad coil or something.
Upper end under load should need more energy to initiate and burn.
Sounds like its the advance may be used up. If running an advance like used in the MP race distributors, set timing at 2000 - 3000 rpm. If running a normal advance mechanism, the advance needs to be measured at every couple degrees from 600 rpm until it stops, and then a few high rpm. If its the original distributor, it may have a long but fast primary advance like was used on the early smogged engines. If that's what you find, set the timing at 2800 to 3400 rpm and live with the idle timing until you have a chance to shorten the slots on the inside.
See the explanations posted here:
CAP vs Non- CAP High Perf Timing Curves
Timing with a MP "Tach Drive" Race distributor
I did not check the vacuum but I managed to set it to 850rpm idle and 20 degrees advance. It ran the best this way, also gas pedal reaction became much better without hesitation that it had before.
Obviously getting closer - its time to measure the curve.
Vacuum is a useful indicator of efficiency under a given load - if you can hook up a gage or MAP sensor
Carb tuning will be done with o2 sensor from my car.
Tune for performance. The WBO2 is just a way to keep track of how things change. At low rpm, with plugs that fouled, I wouldn't fully trust the WBO2 to be interpreting AFR correctly. Bottom half of
post #81 is super good suggestion.