strange idle

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duster340

street racer
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Mar 13, 2005
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here is a problem i have been working on for a long time now and still havent found what is wrong, my 340 idles pretty rough at 800rpm wish is not a strange thing because of the big rollercam but if i rev it up some to impress my neighbours or something like that when i let of the thotle it wont go down to that low idle again it levels at 1400rpm wish is way to high in my opinon, i have tried to turn it down by using the idlespeed adjustment down but that does not work if i shut the engine of and restart it will idle at the normal idle speed, this is not a big problem during normal driving since the whole drive train will pull the rpm down to that low idle when i stop at a traficlight but it is verry anoying, it also has a small problem with of idle hesitaion but that has is not a primary problem.
engine has a hooley strip dominator intake manifold and an edelbrock 800cfm thunder series avs carb (had the same problem with the holley 650 dublepumper that was ther earlier) it is equiped with an msd pro billet distributor and an MSD 6A ignition box engine has 11:1 compresion a big solid roller cam ported J heads and runs like hell its just this small problem!

thanks in advance!
 
Could it be the advance weights are right on the border of moving.
Had a similar problem on mine, found the advance was coming in right where I wanted it to idle, I pulled the heavy sprin and all is well now.
 
Check to make sure the advance weights aren't hanging up as mopower suggests also check the throttle shaft for excessive play (one sign of worn throttle shafts is a varied idle )
Also I have seen where big cams pull such a demend on incoming air requirements that they don't let the thropttle pull down closed after being reved up (you may need to drill the throttle plates to let it close again. )
Also check any vacuum hoses you have on this carb to make sure they arent holding open any choke pull offs or egr valves etc
 
have to check the ignition and the vacum hoses again then the throtle shafts feels ok and i have dubble return springs that is pretty stiff...
thanks!
 
play with the timing, i bet its distributor related.
my car did the same thing, except it would die back down to a crummy idle after a couple (10-15) seconds. i have been playing with my timing and reading my plugs, adjusting the carb. i tuned 2 mpg out of it, much better idle, i have almost eliminated a higher rpm skip, and it has more power.
 
Sounds like to light of a advance spring.

You need a bigger carb to. that 650 is kinda small.
 
yup, read the MSD book witht he distributor. You can swap springs and bushings in the distributor to properly set up your engine's ignition curve. One of the springs is too light. What you prob need is one steel(silver), one blue. 2 silvers are too light.
 
I believe its a carb problem.I think you could have a sticking secondary shaft just keeping the secondarys open enough to cause the problem,Mrmopartech
 
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