Reversion?

[pmangelos]

Hello all,
I'm a long time lurker, love Cobra's but right now I have a Mustang I'm in the process of building and have a few Weber specific questions and you guys seem to have the best knowledge base anywhere on the net, so hopefully you won't mind a few questions. The car is a 65 Mustang with a 427W running 48 IDA's and am having what I perceive to be a problem. The engine runs very strong up to about 5500 rpm but then it falls on its face. I have the hood off the car right now and can see white puffs coming out the top of the carbs at the same time that this happens, the engine sort of coughs and doesn't want to keep revving. I have a feeling that I might be having valve float, but am not certain. I know Webers can be prone to reversion, could this be reversion by itself, and if so does reversion cause a significant decrease in the engines performance? Or is this likely valve float that is causing this?

Here are the specifics on the engine and cam:
427 Windsor, Canfield High Port heads, Full Length headers, MSD Ignition, Comp Cams Magnum roller rockers, Ford Racing roller lifters.
Custom Cam ordered specifically for use with Webers:
Hydraulic Roller
Lift: .603 Intake, .584 Exhaust
Duration 280 Intake, 291 Exhaust
Duration @ .050 227 Intake, 241 Exhaust
Lobe Separation: 115

The Webers are currently set up as follow:
40mm chokes
170 Main Jets
F-7 Emulsion tubes
140 Air
65 Idle
60 Idle Air
50 Pump Jets
60 Bleed Back
Electric pump with return line, set at 3 psi

It seems to me that this engine should pull to at least 6000-6200 rpm, am I wrong? It certainly wouldn't be the first time. Any advice is greatly appreciated, if you need any more info, please just ask.

Thank You in advance for your time,
Sincerely,
Paul

[Caprimaniac]

Sounds to me one of two.... Valvesprings????
Or maybe timing check what's happening up there with a light on the balancer.

If your carbs are synced I see no reason to any reversion happening. Even with bad sync, the problem should be less up at the rpms and worse down low.

Very long ex duration and little overlap on that cam....

[pmangelos]

Thank you for your time in answering my question. Valvesprings were my first thought as well for the possible valve float. I purchased the springs from the cam maker, unfortunately I do not know the specs of the springs, and has since retired. As far as the timing, I do not have it very far advanced, about 13 degrees initial and about 33 at full advance. I can certainly advance it more, I will go ahead and try that. It does seem to be stable at higher rpm's as far as a consistent amount of advance.

Thanks again
Paul

[Gaz64]

Paul,

I think you need a larger venturi for that engine, like 42-43mm.

Personally I think your carbs are undersized, 58 IDAs would suit better.

Your air corrector and idle air holder seem wrong as well.

Your main circuit appears too rich, - rich tube, small choke, small air.

Have you run a wideband O2 for A/F ratio as yet?

[Tom Howland]

You should check the Cam degrees as set to the crank. I had a similar problem on a different stile motor and changed the cam 3 degrees to the correct spec. and can now rev to 6500. Use a degree wheel not the dots on the gears.

[Rick Parker]

I think the cam needs more duration. Closer to 240 on the intake and 250 on exhaust. With that displacement you're moving a lot of air.

[pmangelos]

Thanks again to all for your information. The engine builder installed the cam, I was told it was installed using a degree wheel and not just dot to dot. I think that I'll pull a set of valve springs and have them measured as far as seat and opening pressure, perhaps I will try something a bit stiffer otherwise I may check into a new cam.
Thanks,
Paul

[Roush #1]

Checking the valve springs to see if they meet the cam shaft manufacturer's recommendation would be on the top of my list.
If the springs are ok, I would change the cam timing to see how it responds before I changed the cam shaft.
Good luck!

[Gaz64]

Personally I don't like hydraulics.

For me, I'd have the lightest best quality solid lifter with a pressure fed roller, a rev kit, springs to suit the cam, and the lightest valvetrain.

Here is a thread on Berg 58s:

58 mm Berg IDA Carbs--Here They Are

[olddog]

If it is valve float, the springs are not closing the valves, and the lifters will pump up to take out the slack. Lifters do not bleed back down quickly. So if you are making good power at 5400 rpm and it falls on its face at 5500, you need to back down to 5400 and see if it is still making good power. If it was float, the lifters will still be pumped up and it will still be flat. Typically you have to back off the rpm quite a bit for a while to get the lifters to bleed back down. Try dropping 200, 300, 400,,,,, and see how far you have to drop to get the power back. If backing the rpm off a 100 instantly restores power, I doubt the problem is valve float.

I think this simple experiment will shed some light.

I agree that a wide band O2 would be very helpful.