Update:<BR>When I got home from work I pulled off the airfilter with the engine off and looked down the carb throats and the secondary was closed (should be), then turned the throttle mechanism to full open, and saw a good squirt of gas in the primary. At this point I could manually push the secondary open with a long screwdriver (good, it's not stuck). Put a longer vacuum hose on the depression chamber so that I could use my mouth to pull a vacuum on it, and the secondary opened and held vacuum reasonably well. Looks like the secondary's ok. Tried pulling a vacuum on the distributer, and it advanced and held very well. So that's OK. Ran the engine with one vacuum hose off, and the idle was rough. Replaced the vacuum line and the idle went smooth again. Looks like the vacuum's OK. Checked for suction at the reed valve, that was OK.<BR>Sprayed some more carb cleaner down the throat just to feel like I'm doing something. Then started the engine, and looked down the throats, manually pulled the throttle to wide open, and used my mouth to pull a vacuum on the secondary. Wow, did THAT ever rev that engine! OK, so it wasn't a smart thing to do...<BR>Put it all back together, and took it for a run. It seemed better. Perhaps the secondary valve is just sticking, even though the vacuum's good, and the action's good.<BR>So I put another bottle of carb cleaner in, and went out on the hiway. Noticably better with the carb cleaner. It still gets bouts of power loss, but infrequently, and usually after extended driving with the throttle in the same position.<P>I doubt the cat's blocked, or oil fouled. It's only ~8 months old, and tested excellent on the shop's analyser on Tuesday, and the goverment's smog test on Wednesday. The entire exhaust is a custom job, tucked outa sight beside the frame. There are NO marks on it from the Rubicon, and it's quite open.<BR>The fuel pump tested OK, and it's an electric pump at the tank. The Oxygen sensor tested OK, I watched it's signal on the FLUKE meter, and it cycles nicely.<P>When i tested the dist vacuum advance, I noticed that the dist cap and rotor aren't too clean looking inside anymore, so I should probably replace them.<BR>I'm asuming backfiring in my situation is from too much fuel (or more specifically not enough air). But the valve chatter? I wonder if my oil pump is not working properly, and that the hydraulic lifters are suffering?<P>Questions, questions, questions.<P>Answers?<P>(Mitch, I went through a list similar to this about 18 months back, and it turned out to be an inadequate aftermarket mechanical fuel pump that had been installed recently. The mechanic tested cat and O2 sensor as well by simply unscrewing the O2 sensor and going fo a test drive. Apparently the O2 sensor's hole is big enough to act as an exhaust system bypass.)<p>[ 22 November 2001: Message edited by: don ]