[Vroom_vroom]

Looking for some advice on my 2nd gen bird. it’s been an ongoing issue for years and I have yet to get it nailed down.

At wot a over 4.5-5k I end up with a surging effect killing power. It’s like someone turning a light switch on and off. How the heck do I tell if it’s a fueling or ignition issue?

I’m sure it’s an overload of info but here is the general my current set up.

I have a rebuilt 400 ( no added cubes). ‘74 block, stock crank, i beams, hyper pistons, kre heads, xr276 hydro roller cam, scorpion rockers, rpm intake, tri y’s, smi q jet(no internal filter but an inline) edelbrock mech. Fuel pump, Holley dead head regulator @ 5.5 psi. Pertronix hei set to 32 total and 16 initial, blue max wires and More I can’t remember off the top of my head. Motor has 10.1 compression.

T10, with 3.23s

Car runs hard but when you get above the 4.5 to 5k it’s like someone turns the fuel on and off. Put it on the car on the dyno a few years back (with bad lifters another story that has been corrected) and it put down 323/403 @ the wheels.

Thanks for helping out a new guy!


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[JSchmitz]

I had an identical situation with my car once. Turned out to be a bad wiring connection to the HEI. Have 12.5 volts without running and about 6 volts running. Fuel and ignition problems present themselves in very similar ways. Can be hard to differentiate. A fuel pressure gauge, that you can see while driving, is a huge troubleshooting aid. Check the usual stuff, like fuel pickup, etc. Could be a clogged sock in the tank, etc.

[25stevem]

Showing fuel pressure on a gauge does not guarantee you have the needed fuel volume for a given level of power at a given rpm in terms of WOT.
Try to take the car on a test drive where when it starts to level off on rpm gain/ power you can kill the motor , pull off the road and check a plug.

If it shows any sign of wetness then you have a ignition issue, if it does not then it's fuel related.

With your aftermarket heads and a hot Cam you may be exceeding what the stock 5/16" fuel line can provide, or it may be that the pickup sock / filter in the tank as seen its better days and is a restriction.

If the motor will rev up cleanly out of gear to 5000 or 5500 rpm and you can hold it there for 3 to 4 seconds then there's a good chance that the ignition system is ok.

One thing is for sure one you put on aftermarket high flowing heads the whole fuel system from the tank up to the motor should have been upgraded to work solid at a 500 hp level.

I have in the passed seen where the stock steel line along the frame rail in a guy's Buick was pinched in from a misplaced jack and with his built up 455 the motor coukd not get passed 4000 rpm at WOT due to the restriction.

And yes electrical connections can be suspect when at high rev's a poor connection can't pass the needed current even though you will still be reading 12 volts with a meter.

Any slip on connections like male or female spade connectors should have a tight enough grip on each other for them to be a bit of a pia to pull apart!

[Vroom_vroom]

Will have to check a plug this afternoon like mentioned and double check my hei connector.

The tank has less than 3k miles on it along with the sock. The thought had crossed my mind on the fuel line not being enough. I’m running the stock line from the fender back. No obvious damage to it, thought of using it as a return line and changing to a return regulator with a bigger feed line if need be.


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[400 Lemans]

Check the rev limiter on the hei module. Some of those Pertronix distributors have them.

[Vroom_vroom]

Originally Posted by 400 Lemans

Check the rev limiter on the hei module. Some of those Pertronix distributors have them.

Mine is not one of them, just the basic hei. Another fun fact, during the winter (sub 70 degree temps) the problem is almost non existent.


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[ta man]

Sounds like fuel supply related..stock lines stock pickup and mechanical pump will not keep up with that engine. Like mentioned before hook a gauge that you can see at WOT to confirm.

[78w72]

hard to diagnose these things over the internet, do some more trouble shooting as mentioned above, a fuel gauge is easy to run temporarily taped to the windshield to see what the psi is when the problem happens. also verify the volts to the HEI.

couple suggestions: the stock hei is as good or better than the pertronix stuff, a good GM module & coil will usually be more reliable & provide just as good of performance, might try switching back to the stock stuff or locate a good used hei for the parts.

second: the edelbrock pump does not need a regulator for a mid 70's q-jet carb, they are set at about 6psi, the regulator is just another restriction, no need for it in this situation. also dont need a full return line/regulator set up for a mech pump, especially one that may barely be keeping up as is, adding a return will reduce the psi even more. i ran that edelbrock pump on a 500+hp 467 stroker with E-heads & big roller cam, it supported that Hp on a engine dyno & let the car run low 12-high 11 second quarter mile times, but it did have starvation issues on runs with good traction, no issue on the street, but my car does have bigger 1/2" fuel lines & pick up. have since switched to a robbmc 1100 pump that works much better & have ran low 11's with it & no other changes.

they make 1/2" lines that fit factory routing, i got mine from right stuff detailing, or run your own or use -8 braided line.

[400 Lemans]

Originally Posted by Vroom_vroom

Mine is not one of them, just the basic hei. Another fun fact, during the winter (sub 70 degree temps) the problem is almost non existent.


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Definitely change that module out to another one. A GM if you have it.

[Vroom_vroom]

Man I wish I would have signed up here years ago. My hei power connector was less than stellar so I picked up a new pigtail and started playing with my fuel reg. Apparently the gauge is off by a good bit. Have been running low fuel psi for a loooooong time. So much so I had to back off the idle almost a full turn.

Ditched the reg and it turned what i thought was a good running motor into a brutal one! Thanks for the guidance!


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[JSchmitz]

Originally Posted by Vroom_vroom

Man I wish I would have signed up here years ago. My hei power connector was less than stellar so I picked up a new pigtail and started playing with my fuel reg. Apparently the gauge is off by a good bit. Have been running low fuel psi for a loooooong time. So much so I had to back off the idle almost a full turn.

Ditched the reg and it turned what i thought was a good running motor into a brutal one! Thanks for the guidance!


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Good to hear! Enjoy! A bad gauge can throw you off for sure. Garbage in = garbage out (information).

[johnta1]

One good reason to do a fuel flow test by flowing a gallon of gas should be less than 30 seconds using the pump.

[78w72]

Originally Posted by Vroom_vroom

Man I wish I would have signed up here years ago. My hei power connector was less than stellar so I picked up a new pigtail and started playing with my fuel reg. Apparently the gauge is off by a good bit. Have been running low fuel psi for a loooooong time. So much so I had to back off the idle almost a full turn.

Ditched the reg and it turned what i thought was a good running motor into a brutal one! Thanks for the guidance!


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glad the suggestion to remove the regulator helped out... the edelbrock pump is set at 6psi which is right at what most later 70's quadrajets like. i ran mine deadheaded with no reg for a couple years, it ran great on the street & the dyno, hard runs at the track showed problems though.