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Very Involved Problem 400sbc

4K views 27 replies 11 participants last post by  cdminter59 
#1 ·
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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#4 ·
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.
 
#6 ·
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
 
#7 ·
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!
 
#11 · (Edited)
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.
 
#12 ·
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.
 
#16 ·
Cam has 226 int./234 exh. @ 0.050" lift and needs no where near 26 degrees initial. Start around 16-18 degrees initial and work up from there.

Generally it is better to get the timing worked out, then go on to carburetion- as long as it's running well enough to set the mechanical (should be all in by about 3K rpm).
 
#19 ·
Depending on the distributor that was on it, that could have made all- or none- of the difference. That's why it's a better idea to make ONE change at a time, so you know what helps and what doesn't.

In any event if it's running better, something caused it and that's a step in the right direction.
 
#22 ·
Increasing the jets will not get rid of lean bog. Because not producing enough RPM to make the jets work. He is working mainly on idle circuit until he have enough RPM for the main jets to come into play. The smaller the primary the more responsive the primary jets come in.
 
#23 ·
I talked to my brother and he said he didn't change the jets. We want to get a 650 but can't afford it right now(already had the 750 on a shelf). We had a 650(?) Holley given to us so he got a rebuild kit and popped that on(before we got the fuel pump issue squared away) seemed to make it worse. Probably because it was a Holley. =) Haven't tried it since getting good and consistant fuel pressure. It started way harder than the eldelbrock so I'm not overly excited about trying it.

I did however drive the truck to work and I acualy paid close attention to what was happening. First and second go good with no issues than as soon as I shift to 3rd and the RPMs drop below 1500 it backfired under heavy throtle. as soon as I top 1500 it gets better and at 2000 it pulled very well but is still "missing". The same goes with the other gears. anything below 1500 and it falls flat if i give it too much gas. If I lightly roll into the gas it is not nearly as bad and the higher the RPMs the more gas it will take.

So should I get and try larger jets?

Also I want to say we did put heavier weights on the distriburator(before the fuel pump issue) and I hav't messed with timing since. Might be till after CHRISTmas that I can get the lock out plates and jets. I'm not too worried about squeezing out as much power as it can give. I'm just trying to make the thing drivable without falling on its face.

thanks again for the help!
 
#24 ·
Also I want to say we did put heavier weights on the distriburator(before the fuel pump issue) and I hav't messed with timing since.
Heavier weights, or springs?

The earlier you can bring in the mechanical advance, the better. The advance should be all in by no later than 3000 rpm in most cases. If you used heavier springs, that's going in the wrong direction unless you were having detonation.

Did you read the link I gave you on timing? ANY good timing info will do, it doesn't need to come from here.

Get the timing sorted out first, THEN the carb.
 
#25 ·
I want to say it was heavier weights and lighter springs.

as for the link on Hot rodding your HEI Distriburator, I went there, read it, printed it off, and read it a few more times. Its good info I just havn't had time to implement said knowlege. Not to mention its cold as heck outside so my ambition level is low. =)

So should I do the timing before messing with the jets? (EDIT: Just read the last line of the last post, please ignore this question....lol )
 
#26 ·
I get it- COLD! lol

The timing is easier to get in the ballpark. In a lot of cases when using a GM HEI distributor, two medium or one medium and one light spring w/the stock weights will be about right. Then it's a matter of finding the initial timing it likes (good idle, good vacuum, carb idling on the idle circuit and not into the transfer slot), followed by limiting the amount of mechanical advance to keep the total timing from being too high. The total timing being too high is always the case because the initial timing is always higher than what the factory setting called for.

The MSD distributor uses bushings to limit the mechanical timing.,With the stock HEI you have to use screws like you saw in the link. When you get to that stage, if you need help someone can give you a hand w/that.

Lastly, during the process of finding the right amount of initial timing, you will need to readjust the idle mixture screws as you change the timing. And you'll possibly need to lower the curb idle speed to keep the mechanical advance from creeping in, or raise the idle speed if it gets too low.
 
#27 ·
Just to update,

Pretty much we replaced everything! My brother bought a new ignition system for it ( with a digital controler box, new coil, and new distriburator) and I bought a new 650cfm edlebrock carb. After puting all that on the truck runs awsome! Before putting the stuff on, the problem got worse, and i'm pretty sure it was the distriburator. more specificaly the ignition module in the distriburator. By that point I was tired of screwing around and just wanted it fixed. =)
 
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