Thread: BBC 454 choking up and full throttle

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I am having a problem now with my 454 engine that I took from a '79 suburban and put into my '69 nova.

The engine runs fine except for when you hit fill throttle (at any speed) it chokes and starts bogging down. Someone told me it might be the fact that I am running dual 2.5 inch exhausts and that they are too small for it, does this sound right?

If not, I cant imagine what is wrong since i didnt change a thing other than re-set the timing and distributor when i put it in the nova. I do have it hooked up to vac advance and i tried both ported and manifold vacuum with the same result. I also played with the timing a lot and got the same result every time.

I never got to try full throttle before I put the exhaust on because of the noise issue in my neighborhood.

Anyone have any suggestions??

Thank you

I doubt your exhaust is too restrictive as that suburban had much smaller pipes. Sounds to me like your manifold vac is dropping, causing the timing to be retarded. Run it up to full throttle with a timing light hooked up.

At this point do not play w/the timing - put it at factory specs. As far as the stumpling that sounds like a fuel problem. Time to rebuild the carb........Don

A while back a a friend of mine put a BB in a 68 Camaro. It had about the same problem (he was also running a 2 1/2 in exhaust which is plenty). We went through a bunch of trouble shooting and finally one night took it out to a deserted strech of road and unhooked the exhaust from the headers. The car ran great. Turned out he had put on a cheap set of turbo mufflers that just wouldn't flow enough to let the engine run.

Eric,

What happens when you ease into full throttle... at a constant slow acceleration do you reach a point where the engine stutters or backfires, or does it sound like the engine is starting to starve for fuel? You might get a better diagnosis with a more gentle acceleration to full throttle.

Dan J

Hi, thanks a ton for all your replies!

I may have to look into the fuel issues if I find the exhaust to not be a problem, i know the carb does look pretty old & dirty.

Mike P - There may be some validity to that... It looks like the shop that did my exhaust put some old used magnaflow exhausts on there (I asked for something cheap!), Once I fix the alternator I will find a place to take it to test that definately!

Dan J - The problem occurs no matter how fast I step on the accelerator. I could be on the freeway cruising with almost full throttle for 5 minutes, then the second the pedal hits the floor at any rate, it studders and starts backfiring etc.

Keep in mind the only thing I did to the engine when I transfered it was I put an electric fuel pump to replace the mechanical, took off all of the emissions systems (plugged all the old manifold holes), and put a new exhaust on.

Hope I can figure this out soon, so far only thing I can think to try specificly is the exhaust change!

thanks!
Eric

What's the fuel pressure regulator set at?

Abe

Originally posted by ericingram
the second the pedal hits the floor at any rate, it studders and starts backfiring etc.

Eric,
When I hear you say "backfiring", I have to think that you've got some sort of timing issue. Set the timing to factory spec as Don suggested, and then check out the advance. I'd leave your exhaust change out until last... sounds like that would be money wasted at this time.

Dan J

Hmm thats all the relevant changes I made, I did put on a new flex fan and oil pan, but i don't see how that could affect anything.

The fuel pump is mounted on the firewall, and I think the regulator is set at about 10psi.

I'll try the timing thing soon. Honestly im not sure what factory spec timing is for it, where can I find that out? Sorry if I don't know that much, this is my first engine endeavor

Wow ok, never thought of that! I'll give that a shot! Thanks!