Need 383 help please!!

[Hemi Rebel]

I agree with what everyone is suggesting. One thing you could try if you have the place to do it is put in a set of fresh plugs, run the car down the road and get it to misfire, then shut it down and pull over right there and pull a plug or 2 and see what they say. Reading the plug right after the event would give you a good indicator of what is going wrong. Worth a shot at least

 

[Gary_gtx67]

Here are my cam specs. Comp.
Lift is .525 on both intake and exhaust.
Duration @ .006. Tappet lift 275 intake 287 exhaust


Part # 21-227-4

The cam, pushrods and rockers are stock. The springs were not required by comp though reccomended. I also have the proper stall to match this cam. Now though I do apriciate everything that is being told I would like to say that I am doing what I can the beat I can with a father that doesn't spend the time to help me but yet his 67 charger with a 440 seems to runs just fine with the same fuel pump and fuel line as I am running. The only thing that I figured I screwd up was by putting these dang 906 heads on that I have no idea what the spring rates are and if they have been milled or not. I am going to do a leak Down and compression an a spring test. Idk why I am having such an issue with this that is why I turned to the group again for advice. Thank you all again and please keep it coming.

 

[Gary_gtx67]

Requote the cam of course is not stock lol

 

[IQ52]

Not knowing what springs are on the heads and the engine missing on the top end sounds like lifter pump up and the valves are being held open. Without enough spring pressure the lifters will fly over the top and back side of the lobe at high RPM (valve float). While they are not being held against the cam the engine oil pressure will fill them up the amount of their preload (lifter pump up) and hold the valves open briefly until the spring pressure can squeeze the excess oil out of the lifter. The design of the lifter allows it to pump up faster than it bleeds off so it takes a brief time after the RPMS drop when the transmission shifts for the lifter to return to normal and let the valve close when the lifter is on the cam lobe base circle.

This is why when you set the lifter preload to zero using adjustable rocker arms, you will get more rpm capability. Because with no preload the lifter cavity is already full of oil and the lifter cannot pump up any more even though it unloads off the cam. Then you are just fighting valve float and not the compound problem of valve float and lifter pump up.

The Comp Cams 924 dual valve spring recommend for that cam is much stronger than the stock valve spring. I'm guessing you have stock 2bbl valve springs on that head and they will always give up by 5000 rpm or so.

Another possible problem may be that the retainers are hitting the valve seals. Generally the stock big block heads aren't much good for lift over .500" unless you cut the valve guides down for more retainer to seal clearance.

 

[Gary_gtx67]

Iq52!!!! Very good info as well. Thank you!! That's also why I am going back to the 516 heads because I already have spring work done to those and milled. I was on the fence with the adjustable rollers until now. It will be a mandatory item most def!

 

[Budnicks]

Not knowing what springs are on the heads and the engine missing on the top end sounds like lifter pump up and the valves are being held open. Without enough spring pressure the lifters will fly over the top and back side of the lobe at high RPM (valve float). While they are not being held against the cam the engine oil pressure will fill them up the amount of their preload (lifter pump up) and hold the valves open briefly until the spring pressure can squeeze the excess oil out of the lifter. The design of the lifter allows it to pump up faster than it bleeds off so it takes a brief time after the RPMS drop when the transmission shifts for the lifter to return to normal and let the valve close when the lifter is on the cam lobe base circle.

This is why when you set the lifter preload to zero using adjustable rocker arms, you will get more rpm capability. Because with no preload the lifter cavity is already full of oil and the lifter cannot pump up any more even though it unloads off the cam. Then you are just fighting valve float and not the compound problem of valve float and lifter pump up.
The Comp Cams 924 dual valve spring recommend for that cam is much stronger than the stock valve spring. I'm guessing you have stock 2bbl valve springs on that head and they will always give up by 5000 rpm or so.

Another possible problem may be that the retainers are hitting the valve seals. Generally the stock big block heads aren't much good for lift over .500" unless you cut the valve guides down for more retainer to seal clearance.

It's nice to have a professional "head" guy, no punn intended...LOL.... & a professional engine builder, weigh in on this stuff, thanks IQ52, great info...

 

[Burnsey]

had same problem with 440 weak valve springs

 

[mopar 3 B]

Running 292° duration in one of my 383 and not a problem so I am not buying the to much duration bit. What is your tach telling you when it goes into its fit. Rpm,s all over the place ( bad coil) Just will not climb (more than likely ECU) do not buy the mechanical fuel pump thing either 3/8" lines and running to 6800. What are you using for plug wires and spark plugs? Have you checked the plugs? Also the 383 will run just fine with the 906 heads just look at the 383 RR engine for 68. Right 906 heads.

 

[texas69bee]

I am not sure why this thread is still alive cause it is 2 yrs. old but my guess is Burnsey got it right. In the thread it mentioned he had springs with a load of 200lbs. With that cam it calls for 347lbs. I am not sure what heads you have them on but check your springs!!!