I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter. Dont know how else to describe it. No backfire or big drop in acceleration, just a feeling that the power curve had hit the wall. The car has a stock LS-5 and the Q-jet was replaced with a 650 Holley spread bore.
I bring this up in the 1st gen forum because I have a similar motor in my 69 Camaro that I'm just putting together. The engine builder says that 650 CFM is enough for the 454 but the majority of guys here say thats not enough.
Is the LS-5 sucking wind with too little CFM or is it another problem?

how old is the fuel ?

You are under carburated!....That big bad boy should have at least a 750-800 cfm carb and preferably a double pumper!

Could be valve springs or lifters giving up... was the LS-5 a hydraulic cam?
But I also agree that 650 is pretty small to feed 454 cubes, stock or not, especially a spread bore design. Wasn't the old quad an 800, even being spread bore it would prob suit the application better than a 650 spread bore.

At 5500 rpms using 83% Volumetric Efficiency a 454 only needs 600 cfms.Check for bad gas or clogged fuel filter.

I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter. Dont know how else to describe it. No backfire or big drop in acceleration, just a feeling that the power curve had hit the wall. The car has a stock LS-5 and the Q-jet was replaced with a 650 Holley spread bore.
I bring this up in the 1st gen forum because I have a similar motor in my 69 Camaro that I'm just putting together. The engine builder says that 650 CFM is enough for the 454 but the majority of guys here say thats not enough.
Is the LS-5 sucking wind with too little CFM or is it another problem?

Its another problem.You can still point the finger at the carb..but it is not a cfm issue if it goes to wot.
Fuel delivery/secondary ignition voltage/weak valve springs/air in,air out/bad gas and the list goes on and on.
I would focus on the miss/stutter under load.Lightly mist the cap/coil/wires and boots with water while a buddy brake torques her in low light levels while viewing underhood.Look for spark energy jumping to ground.
My initial thought was just to go out again and blow the snot out of her after sitting..your call on that one.

I put new fuel into her today, the good stuff. Supreme non ethanol.
The engine only has about 35k on it and has an aftermarket mild/stock hydraulic cam.
As Chad wrote, the engine should need about 6-650 CFM. But...that may be appropriate for an engine that GM wanted to fit multiple apps and needed to be more reliable than muscular.
The guru's usually say that engine problems tend to be more on the electrical side than on the carb side. My plugs look very good. A mild brownish tinge with no oily deposits. Breakerless ignition is new as is the ignition wires. New lifters and springs were installed at the time of the rebuild. I guess there is alway the possibility of a weak spring that made it past the QA process. Thats why I am narrowing it down to the carb.
Does anyone know if there is a carb, regardless of brand , that is a spreadbore, but over 650CFM?

Most stock or all stock Q-jets ARE 750-800cfm, even on the small 305's they were 750's...
35k, I wouldn't doubt the springs are bad, remember these are performance vehicles and depending how you drive it and the car is set up, many of us lose clutches as little as 8,000miles

I am voting on the carb. is too small, how can 600 -650 cfm be ok for a 350 and a 454, doesn't make sense to me. I did the Holley carb. calculator for my application and it said I needed 815 cfm for optimal performance, so I went with 825 cfm demon, for what it is worth it picked up a full 1/2 second in the quarter mile over a 780 cfm Holley. I'm sure you can run the 454 on a 650 if you are looking for fuel economy, but come on its a 454 not a Honda Civic. The question was high rpm not cruising at 2000 rpm.

Dan E.
69 SS396 4spd. 4.10 posi. x66 coupe

they stuck 780s on the 302 small block i would have at least a 750 on a big block

A 6212 Holley is a spreadbore double pumper 800 cfm.

I took the "other" car out for it's first ride after the long winter. I merged onto the freeway and nailed it. The engine ran smooth up to 38-4k when I noticed it starting to lightly miss or stutter.

3800 to 4000 is not high RPMs. He is not running out of CFMs at that RPM.I have a 6210 Holley on my 408 and it will turn 6K all day long.It's not the carb. CFMs.Too small of a carb. will cause it to fall on it's face not sputter and miss.Not enough fuel (lean sputter and miss) caused by old gas or varnished jets?Is then a new install or have you ran it before?You may just need larger jets the vac.sec. spreadbore Hollleys are set up lean.

My first inclination is ignition as it has been gathering cob webs for a while, second thing that came to mind would be valve springs and lifters sitting for a while (lifters bleed down and springs take a set until they have been heat cycled a few times). But a 650 is way to small a carb for a 454. Factory used a 800 cfm Rochester QuadraJet or a780 cfm Holley carb on the 454 with hydraulic cams and a Holley 850 double pumper with a solid cam. I don't think you have run out of carb yet (your vacuum gauge would tell you if you have one hooked up), but most likely ignition wires creating a ground path.

Larger Dave

[QUOTE=Dan E.;1245461] The question was high rpm not cruising at 2000 rpm.



[QUOTE=67 Plum;1245523]3800 to 4000 is not high RPMs. He is not running out of CFMs at that RPM.

Right on 67 Plum.:yes::thumbsup:.

I replaced the Q-Jet with the Holley last fall. It sat all winter, so I'll check the carb for problems. If I dont find any problems, I'm going to check ebay for a holey 8212. I may be able to find a good used one, put in a rebuild kit, and swap it. I'll check the fuel pump pressure at that time also.
Thx for the input everyone. I'll update this when I resolve it.

they stuck 780s on the 302 small block i would have at least a 750 on a big block

The only reason the Z/28 had a 780 was so it would be legal for Trans-Am racing, where it spent most of its time between 4500-8000 rpm at wide-open throttle; for normal street operation, it's WAY over-carbureted.

:beers:

The only reason the Z/28 had a 780 was so it would be legal for Trans-Am racing, where it spent most of its time between 4500-8000 rpm at wide-open throttle; for normal street operation, it's WAY over-carbureted.

:beers:
Thank you for that^.:yes:
Most folks have a tendency to think their VE is higher than it is..operate 90% of the time at low rpm..but carb for max hp with no thought of midrange torque and driveability.Hp sells cars..torque wins races.

Can you borrow a carb from a friend? I agree with folks above, you will not run out of cfm at 4000 rpm - trying a borrowed carb will let you know if this is source of issue or not. My guess is valve springs - if power is falling off you could easily be floating valves. The LS-5 should be able to turn around 5-5500 rpm - something here is definately not right.

you are under carburated!....that big bad boy should have at least a 750-800 cfm carb and preferably a double pumper!
x2
Could have a bad coil

Just a quick update and an answer to the reason why I was losing power in the upper rpm range.
As it turns out, just about everyone that replied to this thread was right. Ken and Fatblock hit it with the gas being old (sitting all winter) and taking the car out and running it hard with new fuel in it.
I ran the car in a local parade yesterday and as you all know, it's pretty slow going. The new 4 core did a good job of keeping her under 200 degree's but the 1 1/2 hours of idling it along the parade route loaded it up pretty good.
The next day I took the car and ran it hard down one of the back country roads we have here, and it slowly came together. Starting off from 30 mph I stood on it hard and it pulled like you would imagine, very torquey. Not breaking the tires lose, the engine is winding up now with very little hesitation. Took it to 4800 rpm. Now it feels good and smooth. I let the rpms drop a little then shifted into 4th gear and pressed it as far as the road would allow. It continued to accelerate smoothly to about 4800 again and it was clear that the lag or hesitation was gone.
Lesson learned. Use new fuel at the start of the season, dont be afraid to wind it up and clear the cob webs out of the exhaust.
I believe everyone was right about the cfm's being too small. The 650 works well, but I think a larger carb would allow it to breathe much better, thus getting to redline quicker.
I do plan on getting a Holley 6212 spreadbore as soon as I can find one.
This is a picture of the car.

Glad to hear you got it figured out.JMO on a stock LS5 I would put the Qjet back on it they are hard to beat on a stock or mildy moded engine.I had a 73 coupe LS4 4speed 3.31 gears with a Qjet and it ran great.