454 timing and tune

[CJ_Chevy]

Well ive been spending a little time on tuning my truck now that all the vacuum lines are connected. its a factory lf8 with ac and th-400. the engine was completely rebuilt, put speed pro domed pistons, comp 260h cam. everything was redone, in the valve train as well. im running on 93 pump. emissions have all been deleted.

I was setting the timing and i set the intial close to factory spec, 8* but when i would go to wot, the carb would back fire and sometimes shoot a flame. i figured it was a timing issue so i advanced the timing up till it would wot without sputtering or backfiring. i checked the timing afterwards and its around 12*. that seems reasonable to me. now when i was using the factory port on the back of the carb for the timing advance, the idle would increase quite a bit. i could not get the idle to come down to a normal level. so i plugged that port and hooked the advanced up to the ported vacuum port on the front passanger side of the carb. this setup seems to work well, but i am not 100% sure because the truck isnt done and i cant take it for a test drive.

the only thing that seems a little off is that the idle still seems a bit high, and it kicks when putting it into gear. tranny was rebuilt and has a mild shift kit. i put new springs on the throttle lever and it doesnt seem to pull the carb shut all the way. i disassembled the carb and got a rebuild kit for the factory settings.

does this sound sort or normal? or do i need to keep the timing back more around 8*, or the factory rebuild kit came with too small of a accell pump? any thoughts would be appreciated.

[donut]

What exactly are you working on?
First thought is a vacuum leak somewhere, don't forget to check the carb base plate. (not idling down)
Another thing to check is the mechanical advance in the distributor, make sure the weights move freely. (backfiring, but goes away with more initial timing)
May be a combination of issues, but be methodical and troubleshoot one system at a time.

FWIW, my bone stock 454 is currently @16 btdc, full manifold vacuum.

[CJ_Chevy]

the truck is a 79 c20 with the factory 454. i am working on a lot of stuff on the truck right now. everything for the drive train and vacuum is all set. i was thinking that there is a vacuum leak as well. all the lines are new. the carb has been on and off a couple of times and i was worried about the baseplate gasket. i will get some carb cleaner tomorrow to check that out. and ill get some tighter fitting plugs for the ports that i am not using.

as far as the mechanical advance goes, i had the distributor apart before i installed it, i replaced the module in it. the springs and everything appeared to be fine.

when i advance the timing more my idle also increases.

[donut]

In my limited experience, the idle will increase, but you should still be able to dial it down with the idle screw.

Was the cam broke in with sufficient oil additives?
With a fresh engine, I'm assuming plugs, wires, timing chain are all new.

[CJ_Chevy]

everything is new. timing chain, gears, plugs, wires, cap, and rotor.
i have the idle screw completely backed off of the throttle mechanism.

i had my local mechanic who is heavily involved with race engines come over and fire it up for the first time. he never used any of the additives in the oil for any of his break ins and didnt think it would be needed. i was sort of surprised of this. but i trusted him and the engine fired right up and we ran it around 2000-2500 for around 20-30 minutes. i then changed the oil and filter.

i cant really imagine the cam lobes being wiped with less than 3 hours of run time total. im fairly new to rebuilt engines and cam swaps. I am not very familiar with how much the timing needs to be adjusted to accommodate a different cam.

i wonder if its a carb or distributor issue instead.
Could it be something as simple as crossed plug wires? they are accell 8.8 spiral core wires.