My car is doing the strangest thing.

At constant throttle, let's say 20%, or cruising downhill at idle, when I go over bumps, it goes rich for a moment. This causes a stumbling and also often an exhaust pop (cool flames but annoying).

On perfectly smooth pavement the car runs perfect!

This must be electrical right? I sanded and tightened all grounds recently.

 

Sounds like you have a problem with the carburetors. Either the oil in the dashpots has leaked out (quite common on the older SU carbs), or the spring in the dashpot has been left out the last time you tried to fix the idle (or was it the hesitation on acceleration?) This allows the dashpot too much freedom of movement, resulting in alternately over-rich or over-lean conditions and the engine stalling.

Oh, wait, this is a Mx5?! Let's start again then ...

Could be. Start it up and let it idle (if it will cooperate). Start moving electrical wiring (you may have dislodged a connector), trying to move only one wire/bunch at a time, maybe start around the earths you fixed. Keep an eye out for one that hasn't been secured properly, or at all.

It might also be a hose on the inlet side that is leaking or come adrift, do the same with the hoses if the electrical side checks out.

 

Thanks, Gee Emm !

I wish I had a carbureted car. Wish I learned that way before getting into this EFI world.

Those are some good diagnostic protocols. Will go do that and be meticulous about it.

 

If you're on a standalone ECU, take a log and check your TPS signal and acceleration enrichment trigger. I have this at low speed on my bumpy street. I thought I was holding the throttle steady but the TPS log showed short sharp fluctuations as I hit the bumps which was enough to trigger enrichment. AFR would shoot down to stupid rich for a split second and caused an engine stumble. Seems my TPS rate to trigger is too sensitive at low throttle inputs.