tuning a magnum

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cammacgt50

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I'm new to a-bodys only. I've searched high and low for good tuning info on magnums with little results. first off my car is a 67 barracuda fastback with 5.9 and 18 spline 4 speed. 391 gear. stock bottom end, r/t heads with 2.02/1.65 valves and hughes stage 2 porting. m1 intake, speed demon 650 vac secondary carb. hughescam .228/.236 duration @.050 .544/.549 lift with 1.6 roller rockers. fuel supplied by a malory comp110 efi pump, 1/2" supply and return lines, fuel pressure set to 6.5. Msd 6digital plus box, pro billit dist and blaster2 coil... with carb at factory specs i need 20* initial timing before i really stop gang vacuum(15"at idle) had to turn a bushing to limit the mech advance to 14 which gives me 34 total and its all in at 2300. Engine staeasy and idles great. Normal driving its suprisingly well manneed. but under full throttle it stumbles at higher rpms. off the line its fine or from a rolling start. it will pull hard up to maybe 4000 then acts like i hit a rev limter for a second then comes out of it and pulls again but as as hard as it should. I've never ussd a vac secondary carb before and I'm tempted to switch to a double pumper but I'm not convinced that my problem lies only in the carb. the secondary spring is gold colored which i believe is medium tension. Float levels are set halfway front and back. anybody out there running a similar combo with any advice would be greatly appreciated. i really wanted to run this year at the track in terre haute but now thier done for the season. i still have hopes on burning some tires before it gets to cold up here! Thanks in advance. cam
 
Unless you have something mechanically wrong with the distributor (spark advance / scatter problem, etc) or something electrically wrong with the ignition

I bet this is carb. Power valve? Consider getting yourself a wideband O2 setup
 
I have a very similar combo and had a very similar problem. Mine is stock 5.9 mag bottom end. I have a custom ground comp cam with very similar specs to yours, stock heads with some touch up work, edelbrock air gap, Holley 750 DP, cheap headers. I did use a mechanical fuel pump though.

I had the same issue, it would pull hard out of the hole but once I hit about 4,000 it wanted to surge. Mine ended up being a bad fuel pump. New pump and it runs great. I would definitely guess it's fuel related in some way or another.
 
Put a temp fuel pressure gauge that you can read thru the windshield. Place a paperclip on the secondary vacuum rod pushed up against the housing. Run the car WOT and watch the fuel pressure gauge when it stumbles. If pressure drops there's your problem. Check the paperclip and see if it is push down on the rod indicating the secondary opened. Due yourself a favor and buy a wideband setup, there cheap and you can dial a carb in easily.
That carb is plenty for your motor. Vac Sec on street and DP for the track is what I prefer to run.
 
I was under the car today and noticed the fuel pump motor is not tight against the pump body. I'm gonna order a reseal kit from mallory and a couple new filter elements. Then ill mount a fuel psi gauge on the cowl and see what's what. Think ill check the float drop as well, maybe the needle is in the way and I'm not able to fill the bowls fast enough.... what do you guys set your total advance at on a magnum?
 
I was told by a wise fuel guy that one out of every 4 needle and seats is worth a dam. I went through 12 to find 3 good ones. They will hold back 11 psi!

Stick to the basics first.
Do a compression test. Spray the intake to check for vacuum leaks etc, etc.

Check your distributor for proper indexing, most Mopar Dizzy's are off. Period! Drill a 1/2 " hole in an old cap and watch what your rotor is doing. You might be amazed.

Is the motor properly grounded, each head, the block etc?

Send your carb out to a reputable carb expert along with your car's specs. Have them go through it.

That's about the only thing that will cost you a little money and time while they have it. I've used Ben at Promax and recommend his work.

How is your fuel system plumbed? is your pump below the level of fuel when the car is sitting? Did you put the pressure regulator in front of the carb and leave it soft mounted?

Where is your fuel pick-up and is it 1/2"?
 
I just replaced the intake gaskets and no leaks, dizzy is a msd pro bullet without vac advance and I'm using a 6 digi plus box and blaster 2 coil. Good grounds, just rewired the whole car. fuel system is a 16gal cell 2 -10 lines feeding the filter then -8 supply and return lines. pump is below the cell.
 
Update.... i switched from ngk to autolite copper core plugs, changed the powere valve from a 6.5 to a 10.5, switched my mechanical advance springs from 2 blues to 1blue 1light silver, set the timing at 18 initial 33 total and checked my fuel pressure at wot. took it on a test run and smoked the tires in 3 gears. I pulled the plugs right after and the front half still look brand new while the rears are a light tan. the car still had a very slight off idle hesitation and a slight high rpm stumble. but much better than before. i think I'm on the right track here. going to up my jetting from 70/78 to maybe 72/80 and put a lighter spring in the secondary pod. hopefully the weather holds out another day. thanks to all that replied.
 
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