I have no idea where the vacuum advance is in the engine. Where is it usually
located?

I can't explain the gas smell but I can suggest that you check the vacuum
advance. This is an "Old Tech" problem but that is what I found wrong with
my carborated '85 CRX CVCC. ]

Just to look and all those hundreds of vacuum hoses and different components
is intimidating, however, from years of old, low tech, cars I started with
the obvious and after finding one broken vacuum hose and then both diaphrams
in the vacuum advance leaking, the engine hesitation went away.

On my 1998 Honda Accord 4-Cyl, my engine starts to shake/stutter when I'm
in 4th gear (automatic) usually between 1600-1900 RPM's. If I force the engine
to downshift, it goes away. I also started to smell a little gas vapor coming
from my vent right around the same time this started happening. It's faint
and I can only smell it when the car is not moving. I don't smell it with
the A/C on.

So far, I've replaced the spark plugs and wires and distributor cap and rotor.
I've also gotten the timing belt package replaced. Again, this started before
I did any of this stuff. I looked into replacing the fuel filter but the
1998 Accord has it built into the fuel pump so I don't think that's it.

Other than this annoying things, the car runs smooth. I have no idea what's
wrong. Can anyone suggest something? Thanks!

Erick