This is on my GS400. I changed out the old exhaust, which was 2 1/4 to the muffler and then 2 from there back, to a Summit 2 1/2 with turbo mufflers. Now it runs rich and bogs, when it didn't do that before. I didn't do any other work to it. It doesn't make sense but I thought I'd rule it out for sure.

It would actually make it run leaner. You have something else going on here.

advance the timing some and check your float level.
The exhaust upgrade really shouldn't have changed anything that radically.

I suppose it's leaning out? I was going by the smell, but would you have incomplete combustion if it were too lean, making the same smell?

If it's lean you'll get an off-idle surge. does it run smooth at part throttle cruising, and no hesitation when you are cruising and crack the throttle?? Any black smoke??

If it's lean you'll get an off-idle surge. does it run smooth at part throttle cruising, and no hesitation when you are cruising and crack the throttle?? Any black smoke??

It runs smooth cruising, idles a little rougher, and stinks terribly at idle. No hesitation when I crack the throttle. Under acceleration it has more power in the upper RPM but less below 2500 or so. Stomping the gas makes it bog, less so at cruise than at a stop. It ran perfectly before changing the exhaust. It will still smoke the tires though :)

Could be lean misfire. A too lean mixture won't fire and that unburned fuel gets pumped into the exhaust. That could account for the stinky exhaust, rougher idle, and the bog.

Have you tried adjusting the idle mixture screws out??

Have you tried adjusting the idle mixture screws out??

I just reset the timing, I had it at 4° BTDC (Buick spec is 0°), set it a little earlier, seems to idle smoother. Don't want to push the timing to far past 4-6°, the 400 doesn't like too much initial timing. The idle screws were turned in pretty far, only about 1 turn out, I set them 2 1/2 turns out, exhaust seems a little better but still stinks. Idle slightly better but still rougher than before. I didn't test drive it, it was pretty late and the wife was giving me the eye for making too much noise in the driveway :)