I am experiencing what I believe to be valve float at 4500 RPMs on my 66 GTO 389 with Tripower. I get stuttering and the engine seems to lose power under heavy acceleration at and above 4500 RPMs. I have done all I can to eliminate spark and fuel as the culprit. I didn't build the engine so compression, bore and stroke are bit of a mystery but I assume it's relatively stock based on its performance up to 4500 RPMs. Read below let me know your thoughts please.
Stuttering and losing power at 4500 RPMs under load (WOT)
Compression, bore and stroke unknown
1. Plugs replaced
2. Wires replaced
3. Coil replaced in
4. Pertronix ignition replaced with new
5. Distributor rebuilt
6. 12v to Pertronix coil from fuse box - resistance line still present but not connected
7. Installed New 068 cam and lifters installed with roller tip rockers Ė Cam advanced 4 degrees - 2019 (old cam lobes showed wear but problem persists after cam swapped)
8. Valve springs were not replaced. Reused old ones
9. Initial timing 5 degrees. Mechanical advance appears to be working.
I can ease it over 5000 RPMs with foot lightly into pedal but it stutters at 4500 RMPs under heavy load (at or near WOT)
Tried disconnected secondaries and ran on 2bbl. Same results at 4500 RPMs under heavy load (WOT)
It holds rpms okay without stuttering just below 4500 RPMs indicating it's not likely a fuel issue.
Overall it runs fine and for the most part I dont push it above 4500 RPMs. I would like to occasionally but well...
Appears you got it covered with all the parts you have already replaced. So here are a few suggestions to add to your quest.
Fuel delivery issue. Tri-Power when opened needs gas. Are the outboard carbs fully opening off the center carb linkage? What size gas line? Fuel pressure? Fuel filter? Air cleaners - heard some time ago there was a type of air filter that choked the air flow on the tri-power. Maybe try a blast without filter on.
I know the factory initial advance is 5 degrees, but just for fun, step it up to 9-10 degrees and see if any change. Just back out of the gas if you hear any detonation/pinging under load.
Most cams already have a built-in 4-degree advance - is this what you mean, built in advance of 4 degrees from the manufacturer? If you added another 4 degrees to the cam, you may now have 8 degrees which may be your problem right there.
What oil weight? Some lifters will not work with heavier oil per the manufacturer of the lifter, like 20W-50. The oil is not thin enough to drain back fast enough keeping the lifter pumped up and thus holding your valve open at the higher RPM's.
You did not mention exhaust. If still factory cast, one side of the exhaust manifold has a "butterfly valve." It uses a bi-metal spring that opens and closes the valve - which is used for faster warm ups in winter. These rust closed or partially shut.
You used the old valve springs and they could very easily be weak, allowing valve float (bounce). The "068" is not radical, and it has the same Pontiac lift most Pontiac cams have - .406", but your valve springs may not be strong enough. With some aftermarket cams, ie the Comp Cams Xtreme units, they have very fast opening and closing ramps. This requires their heavier and recommended valve springs to keep the valves seated or they can bounce on th seat. I would also assume other brands may also use the faster ramps to "enhance" the already good features of the "068" cam. Some barns cams are not exactly factory "068" and will provide all the opening/closing & duration numbers of the "068", but add a little more lift putting it around .447".
If the heads are still original, they will have bottleneck rocker arms studs. They simply get torqued down to 20-25 ft lbs. The problem here is that if any block milling, head milling, or thinner than factory head gaskets are used, it is possible that valvetrain geometry is off, ie your pushrods are now too long and could be holding the valve(s) slightly open at the higher RPM's. If aftermarket pushrods were used, often the heavier wall pushrods are selected for greater stiffness, but the trade off can be a heavier pushrod. It takes additional spring pressure to control as RPM's go up.
If you pull a valve cover, you should see the pushrods spinning because the lifter spins on the cam and that spins will be transferred to the pushrod. if you don't your rocker arms may be too tight or a cam lobe could be bad.
You might want to try to make your valvetrain adjustable by getting a set of 3/8" poly locks - not the 7/16" used for the Big Block 7/16" screw-in stud swap. By doing this, you can "zero lash" your lifters and perhaps compensate a little if the heads have been milled rather than get new pushrods. This will not fix your problem IF it is a valve spring issue.
So, one of these suggestions may or may not solve your problem.