I'm pulling my hair out with my 1972 GMC Jimmy 5

Your timing should be around 8 degrees advanced. Without vacuum line connected. You might try adjusting your accelerator pump as it may not be giving enough fuel but first check to see how much fuel is filling the carb and what pressure you should have at least 3.5 psi and about a 1/2 pint of fuel in 30 seconds. If you hear a slight audible miss check for a vacuum leak with choke cleaner. Is this electronic or point style dist? If point style replace points and set DWELL at 30 degrees not by feeler gauge also lube the point rubbing block as these wear very quickly when not lubed and will close the point gap thus screwing everything up like timing also what shape is the rest of the engine in? Has the vacuum advance been checked for proper operation is the pin worn inside the dist? All these can do this.

You should be able to check the vacuum advance like this, hook up timing light and watch mark, rev engine it should advance quickly. Also check to see if vacuum is going to unit on dist finally with an HEI unit the wires leading to the module wear from the pickup coil as they are very fine, to fix it the distributor has to come out and the pickup coil will have to be replaced. Make sure there is dielectric compound under the module in dist as well. And make sure your plug wires are fully inserted into the cap also you need a 12 volt feed going to the HEI unit not the pink wire from old dist that is only 9 volts and you need like a 12 gauge wire from a good source to feed it 12 volts.

SORTA NOT BEING THERE WITH YOU

SOME PEOPLE'S TUNING, JUST AIN'T LIKE OTHERS!

I'M NOT DOUBTING YOUR ABILITIES..........I'M NOT MUCH OF A WRITER.....I'M NOT SCREAMIN' EITHER----MY PUNCTUATION IS BETTER USING CAPS, I TEND TO FORGET TO CAPITALIZE, WHEN USING LOWER CASE............I MUST ADMIT, THE CAPS DO GET YOUR ATTENTION!............I DO DOUBLE SPACE, I HATE SEEIN' ONE BIG JUMBLED PARAGRAPH

THINGS PEOPLE IGNORE......

VACUUM LEAKS....HOSES.....UN-CAPPED VACUUM PORTS

ATTEMPTING TO SET THE TIMING....... OR ADJUST THE MIXTURE SCREWS WITH THE RPMs TOO HIGH! ..........THESE ARE DONE AT IDLE SPEED ONLY (EVEN IF THIS MEANS RESETTING THE IDLE SPEED EVERY TIME IT CHANGES DURING A PROCEDURE

FAILURE TO DISCONNECT AND "PLUG" THE VACUUM LINE AT THE DISTRIBUTOR DURING TIMING ADJUSTMENTS.......FAILING TO HOOK IT BACK UP WHEN DONE!

WHICH BRINGS ME TO VACUUM ADVANCE ITSELF MY JEEP CJ 5 RUNS BEST ON "MANIFOLD VACUUM" TO THE DISTRIBUTOR, WHEN I HOOK MY VACUUM LINE BACK ON AFTER TIMING...IT ADVANCES, MY RPMs GO UP I RE-ADJUST IDLE SPEED TO 700 RPM.....THEN ON TO THE MIX SCREW ADJUSTING!

SOME RIGS RUN BETTER ON "PORTED VACUUM" TO THE DISTRIBUTORUSUALLY THERE IS NO VACUUM ON THAT LINE, TILL THE THROTTLE IS CRACKED OPEN, SO WHEN IT'S HOOKED BACK ON, IT DOES NOT ADVANCE

IF YOUR RPMs ARE HIGH DURING ADJUSTMENTS---THE MECHANICAL ADVANCE.....IS ADVANCED, THROWING EVERYTHING OUT OF KELTER!

THIS LINK MAY OR MAY NOT HELP YOUBUT I'M SLINGING IT OUT THERE, SO YOU DO HAVE MORE INFO AT YOUR DISPOSAL......I DO GET CARRIED AWAY AND MAY SEEM TO SPEAK IN TONGUES, MAYBE YOU CAN FILTER SOMETHING OUT OF IT ALL!

https://www.2carpros.com/questions/1980-jeep-cj7-timingtuning-problem-with-a/page/7

THE FUF-FUF.....FUF-FUF AT THE EXHAUST PIPE MAY BE SIMPLE MIXTURE SCREW ADJUSTMENTS

WORST CASE----A VALVE BEGINNING TO BURN

I'M PULLIN' FOR THE MIX SCREWS!

IF I CAN EXPLAIN SOMETHING BETTER....PLEASE ASK!

RETURN WITH A BIG OLE GRIN!

THE MEDIC