1. Im having the same trouble on my new 408 than i had on my 351m, when i step half way on the gas pedal it runs amazing, but when I step on it the whole way 2 things may happen, 1 when its cold it simply dies, 2. when its hot it backfires thru the carb. I have used 3 carbs( the stock 2bbl, a quadrajet and an edelbrock 1407) and 2 oem replacement 80s 400 dizzies, all with which ive had the same problem...any ideas? and the timming seams fine. could weak wires cause this? or maybe the stock coil doesnt do it?

2. My scr is calculated at 10:1 but im using a long duration cam bringing the dcr to 7.4:1, i want to change my cam to a xe274h which will bring my dcr to 7.8:1. On open chambers and with a running temp of 180-190 I think I may run on 93 octane, any thoughts? mybe 91?

3. A friend of mine just gave me some stock but new adjustable chevy rocker arms that he had on his 350, can i adapt these just as any other adjustable rockers by just shimming the pedestals on the heads?

Thanx for reading and helping me...

 

Did you check push rod length ? No, the sb chevy rockers wont work.

 

1. It could be a weak spark.
What haven't you changed? Cap, Rotor, wires, fuel pump, fuel filter?
Check the coil wire for resistance. I have seen similar problems with a high resistance coil wire.

2. With 7.8:1 DCR you should be able to run on 91 or even 89 Octane fuel under certain conditions.

3. Check the rocker ratio on those chevy rockers. They should be around 1.73:1 ratio. I think that chevy rockers are all stud mount(7/16" stud?). You have to modify your heads to use stud mount rockers.

 

I know you said you changed out the carb, but have you checked to see what the accel pump size is or which hole the rod is connected to?

I would think the 1407 would work fine for a built 400.

Just a thought.

 

Thank Guys, I will be changing the wires since its the only thing i havent changed, hmmm good dcr then, i dont mind using 93 but i will try 91 when i change it, and yeah I thought so bout the rockers, i saw a thread around here where someone said to be running these chevy rockers on a 400, Thank you agian and i will let you know how it goes..

 

Sounds like its leaning out when you punch it I think the carb needs tuned not replaced they don't run perfect right out of the box. I haven't messed with edebrocks can you change accelerator pump cams on them? Do they even have them? If it was a holley I would try different power valves and accel pump cams. I'm not much help on tuning edelbrocks sorry
-Johnboy

 

The e-brock carbs have a rod that you can adjust the pump with. On a 600cfm it comes in the middle setting which is good for a built 302, or an average 351, and a little on the lean side for a 400. Since 80% of the motors the 600-700cfm carbs will be bolted onto are 302 Fords or 350 Chubby that is what they are tuned for out of the box. If you are running a built 400 you need to tune the carb and adjust the accellerator pump or everytime you punch the accellerator she will die.

If it was wires they wouldn't all go bad a once..... Wires usually only effect a single cylinder.... unless it is your coil wire. It could be your coil, it does not sound like it. If you have a super high voltage coil then? I have seen some MSD Blaster coils arc over, but the motor just runs rough. It did not die.

When you start it does it fire right up? Does the starter sound like it is struggeling?

 

A bad coil wire will cause a weak spark on all cylinders. It may run OK at light loads, where the cylinder pressure is low, but when you open the throttle and the cylinder pressure climbs, the spark is too weak to jump the plug gap. I have experienced this. It also seemed to run better when cold, possibly due to the resistance of the coil wire climbing as the motor got warm.

 

Thats exactly whats happening to my engine, So it may be the answer, also by coil wire you mean the wire that sends the spark from the coil to the center of the dizzy right ?(sorry for my stupidity, just to be sure)

 

The coil wire is the one from the top of the coil to the center of the distributor cap.

There could be a problem with the vacuum advance not advancing the timing enough for the engine to be getting spark at the correct time when you are decide to 'punch' the throttle. Testing the vacuum advance canister only takes a hand operated vacuum pump. You may also want to remove the distributor cap, and see if the rotor rotates slightly in one direction, and then springs back.. That will make sure your centrifugal advance is not stuck open or closed.

I am assuming you are using the same distributor you had in your old engine?


Another possibility, in addition to everything mentioned in this thread would be fuel pressure.

What kind of fuel system are you running? Stock diameter fuel line? Mechanical or electrical fuel pump? Do you have a fuel pressure regulator installed? Do you know how much fuel PSI your engine is getting at WOT?

At lower rpm's or at cruising speeds an engine can run okay off of lower fuel pressure, but bogging or stalling, or rough running conditions can happen if the fuel pressure is too low anywhere in the rpm range (most common at higher rpm or when suddenly WOT).

The back firing thru the carb sounds like an ignition problem, but then again the dying when it is cold sounds like an air / fuel mixture problem.

Something to keep in mind also is that there could be multiple problems you are having that are all contributing to one single symptom. There may not be just one thing that you fix or change that will make the problem go away. Sometimes it takes a combination of replacement parts and tuning to get problems ironed out.

When i ran an edelbrock on my truck, it idled great, but no matter what i did to it there was always a drastic stumble.

If you do not mind going out and spending a little bit of money, i would recommend buying either an HEI distributor, or an MSD ready to run distributor with a blaster coil (if installing one of these you should be able to remove the ballist resistor from the power wire to the coil, setting yourself up with a green label Carter electric fuel pump, with a fuel filter inbetween it and the tank, aswell as one in between the pump and the carb, and finding one carb that you like ( personally i believe Holley is the way to go, nothing bigger than a 650 for the 400) and then go to town researching tuning procedures for the new parts and giving that a shot.

I had a TON of problems with my truck, and did not always have money laying around to go out and stockpile the parts i wanted. Often i had to tune the truck just good enough to keep it running, and keep my foot out of situations i knew would cause problems until i was able to buy the parts i wanted.

Do you have ANY way of making a short video of the symptoms and posting a link? Sometimes having a big group of people see and listen to the symptoms can be VERY useful in figuring out the problem.

Sorry for the long winded response, im on my 5th rockstar energy drink of the day and my mind and hands are going crazy.

You may also wanna go by a "rule" i go by with all of my vehicles. Until the engine is up to full operating temp. , keep the RPM's below 2000. Parts that are not up to operating temperature usually have less tolerance for thrashing than parts which have reached operating temp.

 

Most likley the dying and backfireing are one issue (Fuel or ignition) not both.
I'd think ignition becasue of the different carb testing.

One thing I want to clarify. The "350" chebby rockers.
1 They could work on a windsor style head. If you converted to studs.

2 No way will they work on a clevland (351C/351M/400) style head no matter what the ratio, distance from stud to valve is just too different.

 

ok first, I also thought it was the fuel preasure so in adition to the mechanical pump i added an external electrical pump, and..... it wasnt the issue, no I changed the dizzy twice and tested the vaccums, yeah you are right a holley is a better carb than a edelbrock, but they are a nightmare to tune in comparison to the easy tuned edelbrock. My first carb was a holley 2bbl and had the same problem rulling out the carb. Yeah a 650cfm will sufice on a 5k mild 400, my peak power with the cam i have and the one i will use is estimated at 55k and running good at 6k so a 700cfm carb is needed by the holley formula, Yes you are right about it probably being several things at the same time, and yes i would love to have one of those all in one hei units but Im short on cash and im saving for other more crutial stuff.... Tomorrow im trying the wire thing as a start, then if there´s still trouble, im tunning my carb with lots of detail, or maybe going to an expert around here...

also the coil cable seams to be it, another sympthom i just remembered was that in park or in neutral i could step on the gas pedal at once with out having any trouble, the trouble seams to come on load...

 

Sounds like that should have been the first thing you did.