69 BB won't rev past 4,200 in 3rd gear
 

Car has an ATK 468BB crate engine (Edelbrock e street heads, hyd. cam with .540 IN X .540 EX, 10:1 CR). I have QJet that was rebuilt by Lars about six years ago (when I was running the stock 427/390 engine with a comp xe268H cam) and a pro comp tach drive HEI distributor and MSD wires. I am running the stock 427/390 intake and headers. When I changed the engine, I also installed a Hughes TH400 and 2,500 converter and changed the governor weights to enable full throttle shifts at 5,200 RPM. Car has stock 3.08 rear.

When I first realized that it wouldn't rev past 4,200 RPM in third gear, I changed the module, rotor, cap and coil in the HEI to MSD stuff. Car is timed with 15' initial, 36' with mechanical advance and 50' with the vacuum advance. All mechanical advance is in by 3,000 RPM. I'm using the VC 1838 vacuum can as per Lars' tech sheet. Car pulls strong vacuum and will rev past 4,200 RPM in lower gears. What else could cause it to stop revving at 4,200 in third gear?

As always, any help is much appreciated.

a friend of mine has a 454 with edelbrock heads an hyd. roller cam that it will spin past 4,800 rpm but the dyno graph shows ridiculously rich. it only spins that hard because the springs were changed to as the ones that came with the heads weren't for hyd. roller. May want to call edelbrock and confirm they will work correctly.

the cam is a regular hydraulic cam (not roller) as installed by ATK so I expect the springs in the heads are spec'd to match the cam installed. In addition, it will rev past 4,200 in the two lower gears (and runs very strong)

Are you running out of fuel? Restriction in the system somewhere or are you using an anti drain back inlet filter in the Q-Jet? They jamb and restrict fuel flow.
:cheers:

Thought I saw roller cam my mistake. Does it do this when making 1/4 passes or does it do it from an internet speed when you floor it? Do you have a fuel pressure gauge you can check when it does this?

those are good questions. I am running the stock fuel lines, stock fuel filters and an AC delco fuel pump. What confuses me is that the car will rev to 5,300 (that's the highest i've taken it) in both 1st and 2nd...?????

It happens when I am on the highway, pedal to the metal, secondaries are open, it's gaining speed in 3rd then just flattens out and won't rev past 4,200 RPM...and I don't have a fuel pressure gauge...

Any idea of hp estimates?
could be through first and second the fuel pump has the fuel bowls full at the start and your engine consumes more than the pump can make up for slowly draining the fuel bowls by the time you hit 4200. Off the wall thought, is it is creating a vacuum in the tank that over powers the the suction of the fuel pump by the time it makes it to 4200.

After it does it remove the gas cap and see if you hear vacuum.

Thanks - It's rated 500HP. I've never heard of the condition you describe with respect to the gas tank vacuum.

I suspect this could be a fuel issue but would want Lars or another carb guru to weigh in on that...Again, I am confused by how strong it runs in the lower gears and that it will rev well over 5,000 in 1st and 2nd and even in third under full throttle until it reaches about 4,200 RPM. I don't know if the load is different at that point because it's in 3rd so I'm looking for answers

It's a wild card idea, thinking maybe the vent system in you car has become clogged, sealed or a bad gas cap.

I would think that the previous tuning would still provide enough fuel to allow the new engine to rev past 4200 even if its not making full power. Borrow a fuel pressure tester from a parts store see if you can make it work. Trying the cheapest tests first will keep you from spending money (except for speeding tickets) and may help figure out what the problem is.

I agree, check fuel pressure under WOT. What do the plugs look like?

edit: are you running manifold vacuum to the vacuum advance?

x2 on a fuel demand its high in 3rd gear anyone you know have a better pump you can try out temporarily? That 468 is probably wanting more fuel than your old 427 did is the carb still calibrated for the stocker?

Not saying this is your problem but ran into identical problem with an sbc yrs ago problem was the pump.
Carter makes pretty reliable ones

Just a thought: Are you certain that the secondaries are opening fully? I had a similar RPM drop-off in third gear a couple years ago. The carb opened fully when I checked it under the hood, but when looking at the throttle cable at the accelerator pedal arm I could see that the cable had stretched. This caused the pedal to hit the floor (and stop) before the secondaries were all the way open. I got rid of the slack in the line (which raised the pedal about 3/4 inch), and the engine pulls like a BB again.

You could have a plugged fuel filter; the little pleated paper filter in the inlet of the QuadraJet. Or, a failing fuel pump.

:iagree: check the secondary lockout tang.

Get rid of that little inlet filter and get a good inline filter instead.

As mentioned, make sure the secondary butterfly is opening fully. I had a similar problem and had to "bubba bend" the throttle cable bracket to have enough cable movement to open the secondaries.

Secondaries are not opening...at all. Probably due to the lockout lever which is activated by the choke. If choke is ON, secondaries won't open. You could also have some other linkage or throttle cable problem or some mechanical interferrence issue. Lockout lever issue is my guess.

Plugs look good and I am running full manifold vacuum to the advance cannister

Manifold vacuum is not an issue at WOT. The vacuum drops to near zero, and vacuum advance is nil. It's your secondary butterflies.