[red78ta]

Around 5200 it starts to break up & backfire through the carb. My combo should be good to 6500 but will be happy with 6000. Have tried numerous timming setting & advance curves including locking out the advance totally. Only slight changes in reaction but still present. Doesn't matter hot or cold. Seems like a timming issue but am running out of ideas on where to look. Getting VERY frustrated ......... could this be a valve float issue even though the cam spec says valve float should not occur till 7200? fresh built 400 (about 2500mi. now) stock crank & rods, cast pistons, bored 30 over, 6x-4 heads shaved for 9.5:1 comp.(this is a street/strip car), Crane cam (P/N 284281) adv duration 296/304 (234/242 @ .050 tappet) with 473/488 lift, stock 1.5 rockers, Valve adj. to 0 lash w/Polly locks, Tomahawk intake, holly 4150 750 cfm carb. & Pertronix HEI.

[67cruiser]

78ta, are you running the springs on your heads designed exactly for that cam?? sounds like they are not, alaso what gear are you running in your car?? Not much cam with a 1.5 rocker but if i was to take bets and still think its in the springs on the heads or a timing issue.

[red78ta]

Not sure what springs were installed by the builder, was actually wondering about that also. Think that will be where I turn my attention next. Was just looking for another opinion, thanx

[7D9TA]

I have this exact same cam in my 400 in my 79 TA. I also have a very similar issue. Motor just does to pull past 5400. The motor goes flat and get "jerky". Very unsmooth power. I am running an LM-1 and AFR is 12.5 to 11.5. I have tried 3 different carbs off buddy’s cars. Several timing curves. Total advance has been moved from 30 to 36 and still no difference in max RPM. I am also running the Pertronix HEI. I have tried there module and my MSD 6AL. I have swapped everything but the plug wires. I do not remember the part number on the springs but they were ordered per spec for the cam. I have also tried the MSD, Petronix flame thrower and Hypertech coils. No change. I have run out of parts to borrow off buddies cars. I am going to rewire everything to make sure I have adequate voltage and current to the system to see if it helps.

I have run a 12.20 @ 110 with 5200 RPM pill in the shift light. LM-1 shows shift was at 5400. Anything higher than this and the car slows down. If I try to stay in it to 6000 the motor sounds so bad breaking up that it sounds like it is going to come apart. Let me know if you come up with anything other than the cam. I am not apposed to going to a little more durration but I really dont want to do that on a whime.

[red78ta]

Sounds like very similar condition, I am hesitant to change springs now if they do not seem to be your problem. Hate to have to change this cam as I love the lope of it, there has to be a solution we are overlooking. I'm sure someone out there has some more insight, come on guys help us out ............


P.S. I am kinda glad to hear I'm not alone on this one.

[Pete D]

Does the stumble happen under load or not? If under load it could be the fuel supply leaning out from lack of sufficient fuel pump pressure. Not sure if different carbs would change this or not.

Hydraulic lifters? Is the oil pressure ok? If the pressure is high and the ramps real aggressive on the cam the lifters might be pumping up if the springs aren't correct or weak.

On the first GTO I ever had it would start to stumble at 4500RPM. It was valve springs.

[prostreet64]

red78ta, are the heads ported? It sounds like you may be running out of breath with the relatively low lift 1.5 rockers and if the heads are stock. Springs could also be an issue; especially if they are not set up with correct seat and open pressure.

[red78ta]

Happens under load at WOT, never tried to go part throttle at that speed. Am running Hydraulic lifter's & a high volume oil pump. The heads are not ported but do flow well with 2.11 intake & 1.66 exhaust vales from factory. They are the 6x-4 heads from the w-72 package. The header's are Dynomax ceramic coat with 1 7/8" (I think) primaries

[AllRise]

Ran into this very sam problem when I installed the Pertroni unit on my Points Distributor. Switched back to points, and she ran like a raped ape. It really sounds like an ignition issue to me. I assume you have checked for crossfire and all as well, but I would slip a different distributor in there first, and see what happens, if you have one readily available. Good luck! -Travis

[red78ta]

Had the same result with my original HEI & MSD module,coil, cap & rotor. Could the HEI be the culprit? do I need to switch to another type like maybe an electronic non-HEI?

[goathead455]

I have that camshaft in a 455 and there is no problem revving to 6000.
The best ET's are shifting at 5200 but it will rev easily until it hits the limiter.
I would check the springs as well.

[lost nuts]

The problem sounds like your saturation coil(over looked alot when adding power to engines) and could also be your cap. Best set up I've found for higher rpm's without breaking up is proform's HEI distributers.... of course I'm only talking about HEI set-ups.

[BlueMoon]

Loss of power in the upper RPMs is generally caused by leaning out or weak ignition. Loss of power in the upper RPMs with backfiring through the carb is generally caused by valve float(weak springs). To rule out not enough fuel, do a 1/4 run with a fuel gauge attached to the carb in visible view. A wideband O2 is ideal. To rule out ignition, close your plug gap to around .035 - .030 and note any change. If the ignition is weak, closing the gap will cause some improvement.

My best guess is valve springs. A new cam requires springs that meet the requirements of it's profile. If you don't have a valve spring pressure gauge there really isn't any way for you to check them. There are minimum seat and open pressures that you probably don't meet. If you're on a tight budget and don't wanna disassemble the motor to take the heads in to have the springs changed, you could just change the springs on the car yourself. All you need is a valve spring compressor bar and an air fitting for the spark plug hole(both found at most auto parts stores). All that would need to come off would be the valve covers and spark plugs. Not a hard job at all.

I can give you more details if you get to that point.

Good luck.

[red78ta]

I am leaning toward springs as well but wanted some more input before going that route. I believe my Ignition is strong enough & my fuel flow adaquate. Thanx for all the input, gonna work changing those springs & will try to give an update afterwards.

[llwta76]

Had a prob very similar in my combo. Check for cracked shell on a spark plug or two. That's what I found and my full throttle break up in upper rpm range went away. Good luck

[red78ta]

Update ........ tried closing plug gaps from .55 to .35 to test ignition strength & ran to 6200 with no noticible miss or break-up. Down on power though because of smaller gaps. So I am looking at ignition issue. I guess the next step will be to verify voltage to the distrubutor, everything else is new & high perf.

[7D9TA]

Glad to hear that it is turning out to be something external vs. internal motor issue. That puts you and me in the same boat. Only thing I have left to look at that I have not tested is proper voltage/current to the ignition under load. I had already tried .32 gaps and it still did not help. Let us know what you find after checking wiring.