Please help! I have a 305 TBI. It seems to idle smoothly, although the tach twitches every few seconds (usually sits rock steady at idle). But under acceleration the car stalls and backfires! All 8 plugs are getting spark (checked with timing light). Attached a vacuum gauge to the TB and it didn't fluctuate at all (read 21 Hg, whatever that means). I can rev it past 5000rpm in park with no problem. I replaced the cap and plug wires within the last two months. The only things I can think of are fouled plugs, clogged fuel filter, or damaged rotor (I know I should have replaced that when I did the cap ). Any ideas?

 

i would say start with the rotor cuz there cheap then replace the filter and run some fuel injector cleaner through the system to clean the injectors. or it also could be a week ignition module or coil, sounds like its not getting enough spark at higher rpm when under load. and if all else fails just move the timing, cuz my 350 showed the timming correct with a timming light but actualy the ruber in the harmonic balancer was sliping giving me the incorrect reading.

 

Check the fuel filter before you do anything else. You should be able to blow thru it easily.

 

If it backfire here's a generic rule of thumb:
If it backfires through the carb/TB it's the timing
If it backfires through the exhaust there's a valve sticking
Simple enough right?

 

Yep, but also exhaust leaks will cause a backfire too. Oops that's deacceleration.

 

Contrary to the rule stated above, backfiring / surging / chugging under load but idling smoothly often indicates a lean condition.

replace the fuel filter first. If it still does it, check your fuel pressure and injector spray pattern.

You're right, you should have replaceed the rotor instead of the cap. Rotors fail abotu 10 times as often as caps do. Put a new one in it even if that's not the problem (probably isn't actually).

 

and i'd run a compression test

 

OK, this might narrow it down a little. I found while driving today that it only has problems when the engine is warm (or hot perhaps). Runs great when cold. Although my temp gauge never exceeded 220*. Any more thoughts?

 

what he said....emphasis on the module and rotor