So are you saying it might be a 413 rather than a 410 and what about the embossing on the engine where it says 1298 CM3 instead of 1296 CC?

Just for the heck of it I went out and took a look at the displacement markings on my truck.

I should have said CM3 (CentiMeters Cubed) instead of CC (Cubic Centimeters). Oh and I had a rare "off-by-two" error in the displacement value.     1298 CM3 is correct.

Visit my website, Ack's FAQ (the link is in my signature), for all kinds of helpful stuff about Suzukis in general and Samurais specifically.  Try a search for fsm (Factory Service Manual). The first two resulting links listed will be VERY useful to you - especially the 1990-1992 Samurai Supplemental Service Manual which contains specific information about your 1991 2WD Samurai.

You can buy a cheap tachometer for under 30 bucks and temporarily wire it into the system for diagnostic purposes.  The instructions that come with the tach along with the wiring diagram in the last section of the FSM will help you get it hooked up correctly!

You did say it has a electric fuel pump.  I would Check to make sure you have good pressure at the motor.  I have worked in the auto parts business for about 6 years and i have seen new fuel pumps go bad due to a tech who did not know his a$$ from a hole in the ground.

i have worked on a couple of vehicles that if the ecu is going bad you can sometimes tap on them while the vehicle is running if it runs worse while you are tapping on it it is a good sign of a faulty computer (relays sticking). just out of curiosity could it be a vacuum problem he said after he slammed on the brakes it started having the flatspot again. i had a 98 sunfire that when you hit the brakes it would stall out and die . it turned out to be a leaking brake booster so the motor was loosing all vacuum. just an idea:-)