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Old 12-07-2012, 10:31 PM
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cobalt327 cobalt327 is offline
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Originally Posted by 89k1500 View Post
First I'd like to apoligize for making my first post a "Help Me" post. Kinda rude but I to just get to the point.

I have a 89 chevy 4x4 with a drivibability issue, which is: The truck (5 speed manual) will pull good in first and second, but when I shift to 3rd and give it any amount of gas it falls on its face/backfires and has no power. It will do this in any gear above second, and even the first two gears seem lacking from what it should be. What I need to know is if my combination just doesn't mesh or if I have a timing issue, or anything else you guys can think of. The build is as follows:

400sbc .040over, speed pro 4-valve releif flat top pistons, moly rings, Comp Extreme energy 4x4 cam cl12-243-3 270deg/278deg @ .480/.498 lift, pro comp aluminum heads 2.05/1.60 valve 215cc runner cnc ported 64cc chamber strait plug, comp +.100 push rods .080 thickness, comp 1.5 pro magnum roller rockers, weiand action plus intake, edelbrock 750cfm performer carb, stock fuel pump(running around 5psi), shorty headers, jegs off road y-pipe into 3inch with a thrush vortex muffler, stock HEI

Trans is a NV4500 5-speed and has 3.73 gears with 33x10.5r15 tires.

We had the same setup minus the aluminum heads and pushrods(preveiously it had a set of 92 5.7 heads with 220k miles on them) and the truck ran fine as far as no backfiring it just blew smoke because of the crappy heads. My brother bought the heads with plans of changing to a more agressive cam but then gave me the whole set up for my truck.

I think we have a little over 12deg timing... but this is the first we have messed with aluminum heads and I think I read somewhere that that changes where you would want your timing?

I also know that the carb is probably a bit big for what we have but it was on the setup before with the crap heads and it would at least function without backfiring.

so with that wall of text i'm hoping someone can give me some insight on where to start to get rid of the backfiring and lack of power.

The carb has to have the correct step up springs to match the amount of vacuum the engine makes when it is under a load. Refer to the Edelbrock Owner’s Manual for that. Edelbrock sells a tuning kit w/the parts needed to adjust your carb to most any application.

The initial timing may need to be advanced from where it's at, but if you add initial timing you must be sure the total timing (initial plus mechanical) doesn't go too high or it'll detonate (ping) under a load.

More on timing can be read here.

I would try the vacuum advance on manifold vacuum (full time vacuum in other words). It may give too much advance. The use of a limiter device is in the link above.

I tend to think it's not the fuel pump, but it is possible. You can show good pressure on a fuel pressure gauge but still have a lack of volume. If you haven't tried this set up without the pressure regulator, try it- some regulators are very restrictive. If you have an electric pump that worked before, by all means try that.

Last edited by cobalt327; 12-07-2012 at 10:39 PM. Reason: Fix link.
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