[backn65]

.030 over 400, #15 heads, speed pro #1022 cam, quadrijet#7042262, 3.55 posi, th350, engine builder spec sheet indicated hd springs set at 105 lbs, i cannot get it over 4500 rpms. i have conflicting responses from 2 different engine guys, 1) not enough fuel, 2) valve springs not stiff enough, both knowledgable with plenty of pontiac experience.
the engine has about 5k miles on it. pulls very strong until mid 4k's in rpms, then the bottom falls out of it and it just hesitates and pops.
any suggestions?

[ta man]

Check your WOT fuel pressure.

[AG]

105# seat pressure for a hydraulic flat tappet cam is enough, I doubt it is spring pressure related.

[Murf]

Will it pull past 4500 at part throttle?

If yes, I would think fuel delivery.

If no, I'd look at ignition then maybe the valve springs.

Good luck
Murf

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[backn65]

murf, it has a rebuilt GM HEI with new accel parts and wires. i am thinking if i go to 1/2 throttle it will just stop pulling.
thanks, paul

[steve25]

105 is a bit low , but check to see how high the motor is willing to revv out of gear to confirm a spring pressure issue and or fuel / spark issue.

The fact is that if your springs where set to 105 before you fired the motor you likely down to around 95 or less after break in and some driving and that's the limit in my book depending on the Cam lobe to get the motor even up to only 5400 rpm cleanly!

Pontiac 2.11" Intake valves are big and heavy so I like to set them up for 125 to 130 psi before Cam break in , and oil extream break in additive is the best you will ever find out there!

[Cliff R]

Most likely fuel delivery or the fuel inlet seat is too small in the carburetor or a little of both.

Seat pressure alone doesn't tell the whole story with valve springs, would be nice to know which springs were used, single, dual, installed height, spring rate and open pressures, etc. I'm still betting on a fuel problem vs inadequate spring pressures......Cliff

[Murf]

Can you not try it? Sometimes it's easy for a person to get fixated on one thing & lose sight of others.

Good luck
Murf

Originally Posted by backn65

murf, it has a rebuilt GM HEI with new accel parts and wires. i am thinking if i go to 1/2 throttle it will just stop pulling.
thanks, paul

Sent from my Z936L using Tapatalk

[gtofreek]

105#'s is nowhere enough pressure for Pontiac valves with 30° seats. Needs more like 120-130#'s. The 30° will bounce off the seat without enough pressure. I would never install springs on a Pontiac that only made 105#'s.

[steve25]

If I may Paul allow me to detail your correct comments more.

105 psi will work with 30 degree seat 2.11" valve But only with stock ramp Cams up to the 067 Cam .
Any thing faster / more aggressive like the 744 Cam, the 041 Cam and ton's of aftermarket Cams need more pressure.

If you need to prove this to yourself folks just go look at the factory parts manuals and the springs that where called for in regards to different coded motors .

To fully detail this out even more in fact there was even a difference in the valve springs speced out between any ram air motor between the manual trans cars and the auto Trans cars!

[421mike]

So what should the minimum seat pressure be with 041 and 068 cams after break in?

[Sprocket]

This is like a classic law school question: your problem has all the hallmarks of common q-jet fuel starvation at high RPM with a red herring about spring seat pressure thrown in.

[KEN CROCIE]

sounds like lifter pump up to me. But do check the fuel pressure first. If OK on FP, try "zero lash" on rocker adj.

[Bruce Meyer]

Originally Posted by Sprocket

This is like a classic law school question: your problem has all the hallmarks of common q-jet fuel starvation at high RPM with a red herring about spring seat pressure thrown in.

I agree but I doubt its spring pressure. That cam he is using doesn't need any more than 105 lbs.on the seat.
Unless he broke a valve spring or two I would bet good money that its fuel delivery.

[gtofreek]

Originally Posted by 421mike

So what should the minimum seat pressure be with 041 and 068 cams after break in?

Factory used 115-120 on the seat. They knew that 30° seats need more pressure than 45° seats.

[steve25]

Curiosity over took me last night so I looked up some factory valve spring info .

Some interesting things to note here is that on some heads of the same casting number, but used in B body cars as opposed to A body there where,
1) different springs used with the same Cam .
2) different springs used between manual Trans and auto Trans motors with the same Cam .
3) different springs used on the same heads but with a different pressure used between intake and Exh.
4) different installed heights used with the same springs .

1965/1966 389/421 motors.

STD 389 / 325 hp 92lb seat on both valves.
389 with RA package 99lb on seat , both valves.
421 356 hp or 376 hp with auto Trans 99lb on seat , both valves.
421 376 hp with manual Trans 118lb on seat , both valves.

1967 motors. 400 428.

STD 400 350 hp with 067 Cam, 99lb on seat ,both valve with auto or man Trans.
360 hp HO with 068 Cam, 118lb on seat , both valves.
I guess the above means both 400 cid and 428 ?

400 360 hp RA, 744 Cam, 670 heads , 107lb on seat , single spring both valves.
400 360hp RA with 997 heads, 127lb on seat, both valves with a 1.712" installed height, or plus .126" more due to taller valves.
428 376hp, 068 Cam, manual Trans , 118lb on seat, both valves.
428 376hp, 068 Cam, auto Trans , 99lb on seat , both valves.

1968 motors 400 428.

STD 400 350hp in GTO, 129lb,both valves.
400 350hp in B body cars, 129lb seat Intake, 110lb seat Exh.
428 390 hp, 129lb on seat, both valves.
400 360hp, RAI, 068 Cam,auto Trans , 129lb seat, both valves.
400 360hp, RAI, 744 Cam,Manual Trans ,127lb , both valves with 1.712" install height.
400 366hp, RAII, round port, 041 Cam,127lb on seat , both valves , 1.712" install height.

1969 400

400 350hp, 067 Cam, auto Trans, 110lb both valves.
400 350hp, 068 Cam, manual Trans, 136lb on seat both valves.
400 366hp RAIII, 068 Cam,manual Trans, 129lb on seat Intake ,1.591" installed height, 110lb Exh, 1.561" installed height.
400 370hp RA4, 123lb on seat 343lb open , both valves, 1.820" installed height.

1970 400

400 350hp, 067 Cam, auto Trans, 110lb on seat both valves, 1.591" installed height.
400 350hp, 068 Cam, manual Trans, 129lb on seat both valves.
400 366hp RAIII, same as 69 motor.
400 370hp RA4, 041 Cam, 124lb on seat both valves, 1.818" installed height.

1971 400

400 4bbl, 134lb on seat, both valves , 1.568" installed height.
455 325hp, same as above.
455HO, 126lb on seat both valves, 1.560" installed height.

1973 400 455

400 200hp 4bbl, 066or 067 Cam , manual or auto Trans, 129lb on seat both valves.
455 SD 290hp, manual or auto Trans , 117lbon seat , 292 open.

[Steve C.]

"Always run enough seat pressure to control the valve action as it returns to the seat. Heavier valves require more seat pressure. Strong, lightweight valves require less seat pressure. When in doubt, run slightly more seat pressure . . . not less."
Crane Cams



.

[backn65]

problem solved, i noticed the fuel pump had a drip or two when i was about to put in storage. i replaced it with an edelbrock higher volume and took the lemans for a test run today. got to 5000 rpms in no time, th350 shifted and it continued to pull, at that point i was going a little faster than i should have for where i was. looks like the spring pressure for my set-up is fine @ 105#. the engine builder is a long time pontiac guy and i figued that he would not have put in springs that would not perform.
the car seemed to be more responsive while at normal driving situations.
thanks guys for you suggestions and help. i look forward to spring when i can enjoy the new found power.

[steve25]

It sounds like your pump was sucking air and not fuel which is very easy to do with a leak situation since air is much light then fuel!

How old was the pump you replaced? Was it installed before our fuel got cut with Alky?
If so then you did yourself twice the favor by replacing the pump!

[Cliff R]

I see this sort of thing a LOT with fuel delivery systems.

Recently we had a customer we sent a carb to call up at least a half dozen times complaining of all sorts of issues with the carb we just sent him. Poor starting, didn't run well, running out of fuel on hard runs, difficult restarts, "vapor lock", empty for cold restarts after sitting a while, etc, etc. Before sending it back I had him check a few things. He claimed that the entire fuel system was "new" and not part of the issues.

I still recommended that he go over EVERYTHING before the carb and even check or replace the fuel pump before sending the carb back. He did as we requested and discovered a leaking rubber fuel line on the suction side of the mechanical pump. Tightened up a clamp and ALL his issues disappeared. It appears that despite the seemingly good fit of the rubber hose on the pump that it was sucking some air in with the fuel, but not loose enough to leak fuel when the engine wasn't running.

That's not the first time we've seen that sort of thing with fuel systems components and it also appears that this new fuel is extremely hard on rubber fuel hose. I know this is true because we frequently have to replace the rubber hose from the fuel pressure regulator to the carburetor metal fuel inverted flat line on our test engine. Even though it's removed many times per year the rubber hose still swells up some, cracks and has a shorter service life than one would expect..........Cliff