I have almost finished a t-plant for a 454 1 ton truck the 1990 454 ate a valve and I swapped a 1989 454 with less miles. It started today but I don't know what GM's base timing is and I have a backfire if I try to rev it fast off idle. It will rev slowly without a problem and idles smoothly and rev's smoothly if you rev slowly. I have tried 0* to the full tab on the initial advance timing. Any Ideas? It is all completely stock. Thanks for any ideas.

Lock it out at 42*;)

The computer would have a conniption fitl:)l:)...

Sounds like what Chads car was doing with a bad distributor cap...

Want us to look for another clear one like he got from Darryl?:D

Something to check. Not obviously broken when I put it back...

Mine looked fine until I took it off. The center rotor post was burnt and a few of the plug posts had melted plastic on them. Not a good environment to provide healthy spark in :noway:

I'd just pop it off and clean it up with some sand paper. That should help if you don't have any vacuum leaks or the like hiding somewhere.

I am sure you looked at the distributor's guts, right?

I had one at the store a few weeks back where the MA springs had rusted apart!
The advance weights were rusty AND stuck. Also, the VA diaphram was broken!

Vacuum leak and or idle mix...
Did you try another carb?

Just got back from the base no 'puter access. The cap and rotor, and wires were new. I'll be looking into it further today and may get an obd reader to check for codes. The TBI seems to have a good pattern when I look at the injectors while running.

I think the factory initial timing was around 6-8* with the computer sensor wire harness unplugged? Another thing that these engines are known for is a bad knock sensor, if it fails it will assume detonation is happening and pull timing advance down. Most times it dose not result in the backfire issues your seeing, it sounds like timing or a vacuum leak.

Thanks Mike I'll need to figure out what wire to pull.

I was told there was a brown and tan sensor plug in but all I see is one plug to the dist. with black, white, purple, and tan. the second plug has white and red to the coil.

Everything I can find on this says it's a tan wire with a white stripe located under the glove box. I called a friend to ask and he said some model years where different and it should tell you what the factory advance setting and what wire to pull on the emissions sticker on the core support.

Thanks for all of the effort Mike I appreciate it:thumbsup:

Got it, the plug in was on the fire wall by the junction box after I reset the timing I reset the ecm and no backfire:hurray::hurray::hurray:. Thanks again Mike. Aloha

That's why Mike's "Da Chief".:yes:

No problem Mike! I'm glad you got it running well:thumbsup:

The BIG KAHUNA:yes: