I had lost sight of the fact that it is only affecting the left side... that said, it makes the issue all the more perplexing. As I understand it, the problem is run-time-dependent, so what changes from idle to running within the first few miles /minutes?I'm all about checking the ignition first. One of my favorite sayings is never underestimate the ignitions ability to fool you into blaming the carbs.
Having said that.... we're talking a wasted spark system here. The list of things ignition related that can cause a one side only misfire are pretty small....
Plugs, caps and HT leads.... and the leads are a stretch. All that's been changed and some.
As long as Adam reports the problem only on the left side... well... these aren't the droids your looking for.
- does the behavior move from left to right if you swap the spark leads? To me, if it doesn't, that may eliminate the ignition system as the source of the issue. So:
- manifold vacuum leak on the left side?
- fuel supply issue on the left-side carb? Restricted petcock / filter etc? Something that causes the left-side carb to run dry or low?
- float bowl venting restricted on left-side carb?
- Other ideas from the collective?