MFI Backfire

[50/50]

Plenty of power, pulls strongly to 7K plus. Off throttle backfires, deminishes after getting warmed up but never entirely goes away.
Car is set up with headers and no warm up equipment.
Any thoughts on cause? Minor exhaust leak?

Thanks!

[setho]

I'd definitely start by a thorough check of the exhaust. Even small leaks can cause off throttle backfiring. It's seemingly worse with headers. I recently had something similar. It was a tiny crack right at the seam of the collector. A quick weld fixed it right up.

[Flieger]

Is your speed switch working?

[bliebler]

You may have a backfire because your MFI is set a little rich to compensate for the lack of the warm up thermostat. It also could be a lean pop during warm up because MFI needs a little enrichment during warm up.

[boxster03]

most likely as bliebler said - mfi running rich - dial it in - also check for throttlebody wear

[50/50]

Is your speed switch working?

Yes just installed a NOS unit sourced by the Legend- Jim B of Easy

[50/50]

You may have a backfire because your MFI is set a little rich to compensate for the lack of the warm up thermostat. It also could be a lean pop during warm up because MFI needs a little enrichment during warm up.

It was leaned out slightly. Same result.

[boxster03]

timing advance and valve adjustment

[mobius911]

Yeah, basically you need to make the sure the so-called basic adjustment is right and then proceed to diagnosis. The basic adjustment is points/dwell, plugs good and gapped correctly, timing belt set correctly, distributor timing correct, basic correlation correct (length of that one rod has to be 21Xmm), etc. Then you can start to tune. There is a whole list of steps in the "Check, Measure, Adjust" book that should be available as a pdf. That is a really good reference. I have it but the file exceeds the limit. Maybe it's in an archive on the site somewhere. Worst case I can email it.

When I went down this path I ended up getting the basic MFI toolset of 3 long-handled drivers, as well as a Gunson CO tester.

[RSTarga]

usually it's that little switch being misadjusted as mentioned above. just because you changed it doesn't mean it's dialed in. another common fault is leaving ones foot on the accelerator, take it completely off as just the weight of your foot can crack the throttle plates. points closing up can also cause this. as jeff says above, follow CMA procedure.