My manifold & fuel pressure split gauge on my 0-360 M4 packed up due to a backfire in starting I suspect, the MAP needle is frozen. Anybody know the make and model or where to buy them, there is no manufacturer name on the front of it and I’d like to get a replacement before I pull the old one out.

What we're using in the 0-360 is either the Edo-Aire/Sigma Tek IU028-005-38 or the United Instruments UI6333-H.170

Try smacking it first.

Deep inside the fitting to the MP line on the back of the instrument is a small screw (valve) that gets gummed up. Carefully screw it out and clean it. Put it back in and adjust it till the needle returns to field pressure smoothly and not too fast. You might have to file down a screw driver used for glasses. Its pretty small.

This is the one

The teeny screw jet is sometimes straight , sometimes philips, sometimes allen.
The jet is split and tapered so as you tighten it back into its threaded hole the jet closes down.
Its tiny so be guarded with a catch towel if removing from under the panel with instrument in place.

This one may have me beat. The MAP/Fuel pressure gauge is impossible to come off working from underneath, i tried then had a young skinny mechanic contort himself but could only get one pipe of the rear of the gauge, the other 2 he couldn't get off as no room to work the spanner, if gauges 4 front screws are removed to get a better angle we cant hold the gauge tight enough by hand to slacken. Even if we got it off from bottom we certainly couldn't get it back on tight enough to ensure the fuel pressure line wouldn't leak. So here are my choices:

1. Cut and remove the fuel and map lines and replace gauge with a modern gauge that has the sensor in the engine bay so no need to have fuel lines in cabin. The down side is they only make 2.5" gauge sizes and the old gauge is 3.25" so panel would look like shit.
2. Button it back up as is and forget about manifold pressure or get a seperate gauge somewhere else for MP leaving only the fuel pressure part of the gauge working.
3. Get at the gauge from the top. Cut a hole in the glare shield caperting to get in from the top or is there a way to remove the carpet with minimal damage ?

Any ideas?

Worked on my M5-235C last year. May be the same carpet. I pulled the corners of the carpet up and found 6 screws that held the glair shield I'm place Had good access from top. Good luck.

Those fittings are often tight but with perseverance will release. Remove the seat, fill the footwell and rear of landing gear truss to make a comfy level platform then with headlight on and patience and good wrenches, it will release.
Sometimes need a back wrench also so you can squeeze them together .
If the copper lines won't release from the fitting in gauge then cut them and remove the gauge for easy rear access. Then you can set the jet precisely by sucking very gently on a tube attached to fitting whilst looking at front of gauge.