I am at a loss and my hair is falling out as I write.

ENGINE; 1971 454 rebuilt (I didn't rebuild it so I haven't much knowledge what's inside except TRW pistons(suppose to be 10.5-1) and small cam(only lopes when cold) had Q-jet now has holley 4175/650cfm spread bore double pumper/Qjet replacement. Stock points distributor.

PROBLEM; With timing set at 35* total it starts to "break up" sputter at about 4300rpm. It only does this under WOT. It seems that as soon as I get into the secondaries it starts missing/sputtering. If back the timing off to about 24*total it does it much worse and at a lower rpm. If I floor it from a dead stop it takes a while to do it(until it reaches 4300rpm) If I am rolling in 2nd (auto) and downshift into first it immediatly does it which leads me away from fuel delivery problems.?

What I Have Done:
1 Changed carbs/ still does it
2 Tried jetting the holley way rich on the secondaries (94's) not much change
3 Tried jetting lean (62) severe popping/backfiring out the carb
4 Changed plugs (had 42's now has 43's)
5 Changed cap and rotor, points and condenser. Dwell okay
6 Changed coil
7 Checked fuel filter in carb
8 Replaced all rubber fuel lines thinking one was collapsing
9 Remove sock from fuel line pickup in tank
10 Drilled hole in dist. cap to verify that rotor was directly underneath cap/wire contact when firing(with timing light).
11. Checked balancer to see if ring slipped although it shouldn't make a difference as it pings at 35* with the gas I have in it now so it should be pretty close either way.
12. Remove air filter
13. plug wires are fairly new and don't have any burn marks and are the steel braided type(on the outside) if that makes any difference.

It is strange that it only seems to do it when I open the secondaries even though I can open them at a lower rpm for a long time and it runs great. When I retard it the same thing happens only at a lower rpm but only when I start getting into the throttle (secondaries again)?

thoughts I have right now are.
Could the timing chain stretch cause this(engine only has about 2000 miles on it so I was told)? again I can't see it losing more that a couple of degrees?

Could the cam be indexed/off due to machine work and would it cause this? every engine I put together I degreed. I couldn't see a couple of degrees retarded making this happen.

I thought maybe cam lobes but it runs great up to that rpm and it is definatly not coming out the intake. It is in the exhaust if anything

Any ideas? Hopefully not as severe as what I am thinking. I hope not to tear into the engine. If I have to do that I guess I will just take it out and start over. If who ever put the engine together can't install the cam right then who knows what else has/hasn't been done?

I have my fingers crossed

 

try another coil wire first and check fuel pressure & VOLUME, if that's ok I'd re-adjust the valves next..... sounds like your not getting enough fuel????

 

I will pickup a new set of wires and test.
I didn't think fuel pressure/volume because I can load it on the freeway/steep grade at WOT for ever without any missing. Also if in second and I down shift it immediatly does it. I would thing the fuel in the bowl would feed it for at least second.?
Cam I was told is hydraulic.

 

BINGO... we MAY have a winner????? Hydraulic lifters need loving too!!!! I think there is a proper procedure written up in the tech section..... SET YOUR VALVE LASH!!!

 

Seems more rpm related than load so I suspect an ignition or valve spring problem. Could be a lot of things, including wires as already suggested. Weak (or a broken) valve springs would cause break-up at a certain rpm so I'd look at them carefully. 43's are a pretty cold plug more suited to high compression, high rpm applications. For street driving, 44's or 45's would be a better choice. If the plugs look good (light to dark tan) you can rule them out as a problem.

 

plugs look good. BTW, what is up with the 71 taking different plugs that the earlier/later 454's? The 71 is the only one calling for extended tip? Book says 43TS is stock. 70 and 72 are just regular "T"

Will try adjusting valves after the plug wires.
Would changing the ignition timing effect this?

 

I had a similar problem and found that the Holley bronze carb filters will go in backwards and restrict the flow.

 

Try checking the wire that runs from the coil to the point set. Maybe it is damaged under the breaker plate and when timing advance comes in it seperates. Just a thought. Besides, it's easy
and cheap to replace.