[CROS34]

Recently purchased a 78 TA with standard 400. Trying to get on the road. Had a bad hesitation so rebuild carb without issue. Car ran rather smooth after rebuild with good throttle response but not quit right. I could not get the air mixture screws to change engine performance that much and the motor is running rich. So I started looking at the ignition system. Not knowing history of anything on the car, I replaced the plugs, cap and rotor, with little to no change. Than pulled the new plugs to examine. Found most plugs to be copper to tannish color, except 5 and 6. They were a dark black as if they were not firing. At this point I replaced coil, ignition module and wires, again not knowing history of the motor. Again with no change, on what I think are dead cylinders. I think if I get all cylinders firing correctly my carb tuning issue will go away. Any incite on where to go from here would be greatly appreciated. I am out of ideas. Thanks

[78w72]

Compression check, possible those 2 cylinders have blow by from bad rings or a head valve seal issue causing the black plugs? You can verify spark a couple ways to rule out dead spark.

Also if you replaced the module or coil with a chinese parts store type, youre probably better off swapping back the original GM parts, if they are GM.

[CROS34]

I did a compression check this evening and found most cylinders at 105 and a couple down to 95. Not sure if this is good or not. Thought it should be 130 - 150. As for module and coil, neither were original GM parts. Will check fire on each of the plugs later this week.

[nUcLeArEnVoY]

Originally Posted by CROS34

I did a compression check this evening and found most cylinders at 105 and a couple down to 95. Not sure if this is good or not. Thought it should be 130 - 150. As for module and coil, neither were original GM parts. Will check fire on each of the plugs later this week.

An engine will run with those compression levels, but it's pretty tired. What is good is that the compression is relatively even and barely makes the within-10-percent deifference rule, so that's a plus.

Did you make sure to open the primary's and secondary's on the carb while doing the compression check? Also, were all spark plugs removed and was the crank fast and strong? Just need to make sure you're testing under the best conditions to ensure an accurate result.

[78w72]

Good points on doing the comp test right... you can also put a tablespoon of oil in the cylinders then do the check again to show if its rings or a valve issue. Should be details on that in a manual or search online.

[CROS34]

Yes, had WOT when testing, along with all plugs put. It did crank well with no hesitation. I don't recall the milage but guessing 130K. Would have been happier with 120 to 130.