Fired up the 383 the other night, all set up just right, timed at 6 degrees, TPS set, checked all plug wires - - - but runs like it has plug wires mixed up. Barely runs, bucks and snorts and backfires etc, running super rich. 280 XFI cam, custom tune by a guy that knows more than I.

Thoughts?

 

With all due respect, maybe he needs to know a bit more. If all the basics are covered, (wiring, vac/fuel lines, fuel pressure, correctly installed cam, etc.), I'd guess the tune is off.

 

If your heads are different,,, make sure you have the right spark plugs for them. Vortec and most aluminum heads take a different plug than the stock cast iron heads.

 

Tried a new distributor - - no change.

Tried a Hypertech chip - - no change.

How will the stock chip run with a 383, 280 XFI cam, & 30 lb injectors?

 

You could try and see. Did you set your base timing with the all-important spark control wire unplugged?

 

I would imagine super pig rich if not properly tuned. The 30# injectors alone would be dumping tons of fuel if the injector constant hasn't properly been set and the computer still things that you are running 22# injectors.
Have the guy that tuned it data log how it runs.

 

So I got it running better tonight with the new distributor, properly installed (old one had green corroded stuff inside.) Not well, but better, will actually idle on its own but only once it warms up to closed loop. Surprisingly it runs best on the stock chip; it was the only one that would allow it to idle long enough for me to walk around the front of the car to time it (6 deg BTDC, wire unplugged.) Was late at night so I didn't have a chance to rev it up much, my neighbors still like me so far . . . Still running very rich (stock chip is for 22 lb injectors, I guess.) Idle picks up a little with a vacuum leak. Fuel pressure is about 40 psi. No CE light at this time (also surprising.)

The part that worries me a little is: 7 inches of vacuum, 140 PSI cranking compression. 140 is a little low for an 11:1 engine, not? And 7 inches is DEFINITELY low vacuum, cam shouldn't be that radical for 383 ci, should it?

Obviously the tune needs work, but I think it's something else to start with. Any thoughts?

 

...

 

What I was thinking, left a message for my builder . . .

 

Had the chip/memcal installed wrong, got it all corrected and now runs well enough on the custom chip to drive it around the block, but still a little rough and surges/bucks somewhat. Vacuum steady at 9 in hg. Will not run on its own until it hits closed loop; and if it dies needs a shot of the gas pedal to restart. Should be able to get a scan tool on it this weekend to find out more. Ran it carefully through first and second gears, so far so good . . .

Will check for vac leaks now that it runs on its own, that does keep coming up as a suggestion.

 

140 PSI does sound low for a fresh build with 11:1 compression ratio. Mine was upwards of 160 PSI after the rebuild with the stock 9.3:1 compression ratio.
Has the cam been properly installed, degreed? If you are off on the cam timing by a tooth or so, that could produce the lower compression you are seeing.

Generally cams with a good bit of overlap will produce lower vacuum. I think comp cams have a chart on their website with the amount of vacuum their different cams produce at idle. You may want to check and see if that agrees with what you are seeing.