My long term project 72 340 Duster is acting strange. When I try to get it to idle and punch and sound normal, it isn't responding normally to minor tuning - idles mixture, turning the distributor for timing changes, trying different vacuum advance ports. I decided to get out the Innova digital advance timing light that I bought but haven't used til now. I had a lot of initial timing dialed in - 33 degrees . I had the vacuum advance disconnected and the port plugged at the carb. When I rev the engine up to 3000 - 3500 the timing mark goes the wrong way ( retards 20 degrees or so) and returns to the initial timing spot even though the engine is still revved up. I'm new to the advance light and don't know what's up with the Duster readings. It has Mopar Performance electronics conversion distributor, a chrome box, a pertronix coil. I tried it on another project - 71 Challenger 440 engine msd 6 box, proform ? procomp? something like that for a distributor, Mallory street strip coil - the engine runs pretty good and the timing acts like I expect. It advances some and holds when I rev it up. So I don't think it is a timing light problem. I hooked it up the same. What's wrong with the Duster 340 ? Distributor all messed up? Just not getting a good reading?