I built a 383 and at 5000 rpm the engine starts to miss and if I keep my foot in it it will start to back fire in the exaust. My fuel pressure is rock solid and it only does it in first and second gear and only when i put my foot in it. I have changed timing ,fuel pressure, different plugs and gaps. I even changed intakes thinking that I might be sucking some air. Also my a/f gauge starts to lean out. What am I missing ???? anybody have any ideas?????  

just curious...   did you rejet the carb for the extra cubic inches.???/
thinner tipped secondary needles.  one size at a time..  slightly taller rod hanger... that is if you still have the Q jet ....    does it maybe have the first generation feed back carb system on it... ??????  some cars back to 1978 here in california had them...  by 81 almost all of them had them...


also.. did you upgrade the HEI dizzy???   better coil, better module, power relay to supply the power from a closer place than through the ignition circuit..  

you wouldnt happen to be running a holley 600 cfm carb list #1850 would you? its what i have and ive heard they are notorious for back firing  

Ya I agree with what wayne said at the bottom of his reply if the electrical system is not strong enough to ignite the air/fuel in the combustion chamber there will be left over fuel shoved out in the exhaust stroke and because of the heat surrounding the fuel by the exhaust manifold or headers it will explode which is the backfire you could be hearing. Another thing you might want to check is the advancement springs in your HEI distributor.... they might not be advancing the timing far enough to keep up with the engine rpm. MSD sells a kit with three different springs with different tensions for different advancement curves.  

It's either that, or the davnce springs/weights aren't enough in the distributor to handle high RPM, so the advance is going out way too much.  

Yes when I built the engine I installed a holley the a demon Put it on a a dyno and they took care of the jets. then I changed cam and heads and this is when the problem started.I run a msd 6al ignition  

I have tried different springs, Tried setting my timing from 32 degrees to 42 degrees still the same thing. What is really bugging me is why just in first and second gear. 5000 rpm should be 5000  rpm in any gear, or is my thinking out to lunch ? Guys i really want to thank you for your replies. this thing is driving me nuts.  

No I am running a holley 750 . O also tried it with a demon 850 double pumper. still same thing  

ok... since you have msd...   can you call msd... and ask them how to diagnose a problem like you are having..

i still think that you may have a problem. with grounds...  and with power to the msd unit... it takes a lot of current to power it ...

instead of charging the coil 4 times per crank revolution  it charges it..12 times ,.. so the feed power to it is critical...   grounds the same...

you did catch that electrons flow from ground to positive...

if you are running tiny wires or have loose connections to the msd or coil...  it will not pass enough current to charge the primary windings at high speed...

a single loose crimp,  a dirty connector, an overheated quick connect at the bulkhead connector,,,  

just be careful ...   the msd units tend to have very high primary voltages to the coil....

one more thing..........     rotor phasing...    is there a chance that the rotor tip at full advance is going past the cap terminal..  

pull the cap and read the pattern on the inside of the cap terminals.. is it all too one end?????   does the rotor tip show an even pattern... or is it all on one side...

that would indicate the cap, rotor phasing is off.. or there is a problem with the pickup coil, reluctor alignment..  

When you are setting your timing, try this. With vaccum disconnected, set timing at 10 deg. BTDC. Do a full throttle pull and listen for pinging. If none, switch to lighter advance springs and test again. If no pinging is heard, go to the lightest springs and try again. If it pings go back to the middle springs and if not, stay with the light ones and lock the distributor. From that point, dial in the vaccum adv. for part throttle driving. Hei distributors with vaccum connected very often will run 55 deg total adv. My point is dont shortchange the mechanical base timing to try and lower the total timing.For that matter, where is the base timing currently without vaccum hooked? I have a 10:1 454 in my truck that runs happily on 10 deg. base timing (no vaccum) and around 50 deg. total. If you increase your total timing to my reccomendations and still are breaking up and backfiring, then look to these other possibilities. What valve springs are you useing? Seat pressure and open pressure? The engine will wind up quickly in first and second gear and when inadequate springs and pressures are used, the valves will float. How about fuel supply? There could be a lean condition when the engine reaches that rpm. Is the line from the tank to the pump 3/8? When into the throttle does it maintain at least 6 lbs pressure? Just a couple of things to think about besides the usefull suggestions of the other posters.  JW.  

Upon re reading your original post, you said the A/F is showing a lean condition. If the timing procedure doesnt bring you luck, look into that fuel line diameter.  JW.