New 408 stroker runs rough at idle

Fuel and Air Systems

  1. Moparmadness408

    Moparmadness408 Active Member

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    I have a 408 magnum stroker from blueprint engines and it seems to run rough at idle. It’s got edelbrock performer rpm cylinder heads, edelbrock 7577 intake, edelbrock performer series 1411 750 cfm carb, msd 8388 distributer, with a blaster 2 coil. It also has a hydraulic roller cam with a .580 inch lift on the intake valve and a .577 inch lift on the exhaust valve. It’s the bpe4085ct long block off of the website. So basically, I set the ignition timing at 10 degrees btdc at idle, and with the engine warmed up at 700 rpm, attempted to adjust the idle mixture screws. With both turned all the way in the rpm only drops very slightly and runs rough the whole time. Some say I have a vacuum leak or a bad carb, some say it’s normal for an engine with a big cam to run a little rough, and they make so little vacuum that the idle circuits can’t work properly in the carburetor. I just want to know what everyone’s experiences are with either this or similar set ups and what you guys think about this.
     
  2. 340demon

    340demon old man

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    I similar set up but to get to run smooth had to set timming at 12 d and idle at 950 to 1000 and it smothed out you might give that a try
     
  3. yellow rose

    yellow rose Walking the crooked mile

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    You’re going to need 16-18 initial and then you need to shorten up the curve so you don’t end up with more than 34-35 total.

    You gave the cam lift but not the seat to seat and at .050 timing. What are those??
     
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    • mbaird

      mbaird mbaird

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      Make sure the throttle plates arent open too far... primary and secondary
       
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      • thesiren74

        thesiren74 Well-Known Member

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        Not nearly enough initial timing!
         
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        • Phreakish

          Phreakish Well-Known Member

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          What balancer and flywheel or flex plate?

          Define "rough". As in shake, misfire, what?
           
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          • moparmandan

            moparmandan FABO Gold Member FABO Gold Member

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            I'd check in with their main man @Johnny Mac first. He's a member here, you know?
             
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            • Johnny Mac

              Johnny Mac www.blueprintengines.com FABO Vendor

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              Glad to help you buddy. Those make around 11-12 inches idle vac. Not sure if you have checked that.

              You mentioned it was a "CT". so longblock w/o intake when it left.

              You should be able to crank in the idle screws on your carb and kill the engine. Adjusting those with no change says floats or float levels are worth checking.

              These are hydraulic rollers, they sound good. But they are not what i would call radical.

              Gimme a call tomorrow. I can help in here too, but alot of bases to cover.

              It ran here with the dyno carb/intake/distributor...So I tend to start with the equip that went on after and work backwards to start.
               
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              • Johnny Mac

                Johnny Mac www.blueprintengines.com FABO Vendor

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                They also don't "make too little vac" for a 4160.
                700 idle is low.
                What is total timing at?
                 
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                • 70aarcuda

                  70aarcuda Master Hoader of SBM FABO Gold Member

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                  I set the ignition timing at 10 degrees btdc at idle, and with the engine warmed up at 700 rpm,
                   
                • Johnny Mac

                  Johnny Mac www.blueprintengines.com FABO Vendor

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                  What is total timing at, and why trying to idle at 700 vs 800, even 850.

                  10 initial is low, but need to know what it has total first and foremost. That distributor may only have 10-15 degrees of sweep, pending what has been done to it. Hard telling if you don't verify total .
                   
                • 70aarcuda

                  70aarcuda Master Hoader of SBM FABO Gold Member

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                  dont know...that is what he posted at the beginning...i just copied and pasted
                   
                • Moparmadness408

                  Moparmadness408 Active Member

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                  ok so I killed the spark to each cylinder 1 at a time, and the idle drops noticeably for cylinders 1 and 3. The rest make literally zero difference. I pulled a plug on the number 7 cylinder and it’s black. And so is the number 1 cylinder. The number 7 plug wire has a very hot spark. Anyways, I put a vacuum gauge on it, and I’m pulling 8 inches idling at 850 rpm. The vacuum climbs up to 18 inches at 1500-2000 rpm. I live 5000 ft above sea level. Then I noticed the vacuum plug for the pcv port was slightly loose. I messed with it, took it off and plugged my finger in there, and put it back on, and now it idles a lot better. I then happened to notice the fuel filter which is clear plastic was empty, with just short spurts of fuel coming from the fuel pump side. But the engine stays running. Is this normal?
                   
                • rumblefish360

                  rumblefish360 So close, yet so far away FABO Gold Member

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                  Weak fuel pump, time for a tune up and carb jetting.
                   
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                  • Moparmadness408

                    Moparmadness408 Active Member

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                    it has a new fuel pump its the Holley 110 gph fuel pump.
                     
                  • Moparmadness408

                    Moparmadness408 Active Member

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                    Just an update, so I found that the intake gasket is leaking a lot of oil out the back side. Went to redo it, and pulled off the carb and looked inside the barrels and found the holes where the idle mixture screws are have big cracks around them, and even some of the casting has actually lifted up making the holes bigger. The guy that my dad lended it to must have over tightened those screws at one point.
                     
                  • Johnny Mac

                    Johnny Mac www.blueprintengines.com FABO Vendor

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                    You can lead a horse to water :p
                     
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                    • moparmandan

                      moparmandan FABO Gold Member FABO Gold Member

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                      LOL. You tried...........
                       
                    • SLOPAR72

                      SLOPAR72 Well-Known Member

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                      Never loan out your woman or your carburetor..... You never know what might happen.... been there, done that on a carburetor atleast lol. Sounds like it's headed in a direction.

                      JW
                       
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                      • K.O. SWINGER

                        K.O. SWINGER Well-Known Member

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                        That right there is some great advice! I for one don't want anyone screwing with either one of them
                         
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                        • Moparmadness408

                          Moparmadness408 Active Member

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                          Ok so I bought a new carburetor and started and it still seems to run rough, even when warm, and I turned both idle mixture screws in and it stays running. I bumped the distributer to 16 degrees initial timing and it doesn’t change anything. And now it doesn’t seem to want to run and will bog down and die if you give it gas.
                           
                        • 70aarcuda

                          70aarcuda Master Hoader of SBM FABO Gold Member

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                          can you move the carb to the left a little I can not see what you have,,,,,,,,,,,,,,
                           
                        • Moparmadness408

                          Moparmadness408 Active Member

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                          It’s an Edelbrock 1411, I’m running the same one I did before, just a new one.
                           
                        • BluePrint Engines

                          BluePrint Engines www.blueprintengines.com FABO Vendor

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                          please call us, or speak with @Johnny Mac 308-236-1072
                           
                        • Moparmadness408

                          Moparmadness408 Active Member

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                          is the vacuum readings you mention at sea level? I’m getting 7-8 inches at 5000 feet above sea level.
                           
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