Change in settings needed without the cat?

[sparkydave]

Well, I finally got rid of the under-hood furnace (a.k.a. the catalytic converter) from my car, and things are much cooler under the hood already. Without the back pressure of having to push the exhaust through the converter matrix, would there be a need to readjust the timing or the carburetor? It seems to run about the same, although it backfires when shifting gears. Any thoughts?

-Dave

 

[Matthew E. Herd]

That may be due to the air pump/egr valve, if I'm not mistaken. My brother's MGB did that when the air pump was connected, but no longer does it after the egr allowed hot exhaust gases into the air pump and seized it (shredding the belt). Needless to say, he left it that way, and it runs fine. His cat was still there, but when we later pulled the engine to replace the clutch, we discovered there was no matrix left in it, except for a few wispy shreds.

 

[sparkydave]

Hi Matt, thanks for the info. My air pump is still connected. I do have to correct one thing, though. The EGR valve can't leak exhaust into the air pump; the check valve in the air pump line can. My EGR valve is still present too and works fine. I might try unhooking the air pump belt and see if that helps the backfiring. Thanks for the tip!

-Dave

 

[Dave Russell]

The original controls were intended to shut off the air pump flow when decelerating, which of course reduces the backfiring. The air pump originally had, & may still have, a one way valve to prevent exhaust back flow into the pump. Removing the air pump belt should cure the problem. You might gain a little more by removing the EGR valve & blocking off it's ports, or simply disable the EGR valve.
D

 

[sparkydave]

Thanks Matt and Dave, the air pump was the trick. The air pump's check valve was fine last I checked, but I'm sure the engine can do without the extra load. No more backfires though, thanks!
-Dave