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Old 12-07-2012, 07:05 PM
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Very Involved Problem 400sbc

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.

Thanks!

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Old 12-07-2012, 07:19 PM
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I'm not an engine guru like many guys here on the site but could you have a vacuum leak? Lack of power under load, back fire, do you have a vacuum advance on the distributor? That would effect your timing.

Ray
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Old 12-07-2012, 07:19 PM
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plug wires crossed . hydraulic lifter collapsed first two I thought of .
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Old 12-07-2012, 08:08 PM
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Vaccume is in the green on the guage. Wires are good/no miss at idle and goes good in first and second. starts backfiring when it has to work a little harder because of the taller gears in third.

not hearing any lifter noise and I think tht would show up on the vaccume gage(?) and cause a miss.

Could be timing related possibly... Not sure where it should be set at with this set up.
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Old 12-07-2012, 08:17 PM
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Review cam chain/sproket install.

Maybe a few lifters preloaded too much.

??
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Old 12-07-2012, 08:19 PM
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You could try advancing the timing, with the modifications I wouldn't know what to set it at, but, you could try this, put it in gear with your foot an the brake...get it to rev at at about 2000 RPM and advance the timing until it starts to ping, then back the timing just a little...this is by no means the proper way to set your timing for your engine but, if it works than you know that timing was an issue. Questions like degreeing your cam and things like that I'm sure will come into play when the guys that know try and help.

As I mentioned earlier, if you have a vacuum advance distributor and have a vacuum leak it could cause this effect...even though your vacuum is in the green, you would need to check the vacuum going to your advance and if it is , is your advance working as it seems to give you the problem under load
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Old 12-07-2012, 08:26 PM
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Classic symptoms pf fuel pump failure. Wil rev and run fine but under heavy load the engine fail down and will respond only for a short busrt pf power.

If it always runs like crap in third most likely jetted way to lean. Also possible vacum advance is way off as well. But not as lkely as fuel.

Is the edelbrock electric choke from the factory or manual choke. Does this motor have eletric choke and the old motor had manual. Electric choke carbs from edelbrock are fuel milaged tuned manual chokes are performance tuned.

Do you still have the 02 sensor bungs installed. You may want to get a ,ean rich guage on the engine to see whats wrong when it breaks up. If it stay the same it may be vacum advance or leaks.

Hope this helps.
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Old 12-07-2012, 08:34 PM
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Do you have the vacum advance hooked up and does it have a lock out installed so it only deliver total of 11 degrees or so. Also mechanical locked out as well.

If nto this could be an issue if its knocking sometimes detonation just sounds like the engine shaking badly and bouncing off a wall. Stumbles and back fires can be caused by massive detonation like having the stock dizzy in a performance engine.
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Old 12-07-2012, 08:55 PM
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Advance is hooked up, and the line going to it is hole free. We have messed with the springs in the distriburator a little trying to get better results. not much luck. There is no lock out installed... might sound dumb but is that something you buy or something you modify?

The carb is a manual choke and we have tried to richen the mixture... no wideband to test the air/fuel though.

The fuel pump was kickin out about 8 or 9 psi at idle so we put a regulator on it... not sure if that disproves anything or not, but it is a stock mecanical pump, not sure why it is putting out so many psi.I may hook up the eletrical pump again to make sure its not fuel pump related.


Thanks for all the replys! Hopefully we can get this thing figured out before we get too much snow!
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Old 12-07-2012, 09:03 PM
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If you slowly accelerate and shift into 3rd gear,coast for awhile the hammer it,does it instantly fall down or is it ok for a few seconds?
I think hcompton is on the right track
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Old 12-07-2012, 09:18 PM
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Are you using the stock dist or a aftermarket? With all that you have done to this truck I would use a good aftermarket dist. the stock ones do tend to give you the problem that you have.
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Old 12-07-2012, 09:45 PM
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Sure does sounds like ignition/advance issues.
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Old 12-07-2012, 10:31 PM
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Quote:
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.

Thanks!
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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Old 12-08-2012, 12:49 AM
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Very Involved Problem 400sbc

Read the article Cobalt327 provided a link for. To me it sounds like it is advance related. You say you changed springs in the distributor they might be too weak. Most of the time you can use 1 light spring and a medium spring. Get a Crane PN #99600-1 vacuum advance kit. Start with two blue springs. Unplug the vacuum line from the distributor and plug it. When checking, have someone hold the rpms at 3000-3500. Set the advance on the timing light to 36* and the mark on the balancer should align to zero. Then zero the timing light advance. Check the timing at idle. You will want your advance curve to start at about 800 rpms and to be all in by 3000 rpms. Make sure that the weights are not sticking, clean and polish them if necessary.
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Old 12-08-2012, 03:52 AM
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I would check/ Replace the fuel filter.
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