Hi all, I had some problems a while back that caused me to have to do a rebuild. After the rebuild and upgrade of several components I seem to be a little light on the top end now. My new combo is as follows:

454 (454 .060 = 468)
10.2:1 - hypereutectic pistons
VooDoo 60204 Hyd.
781 Oval port heads with 911 Comp springs
Weiand 8005 Intake
Holley 750 CFM vacuum secondaries

The engine revs good up to 6300rpm's, but it seems to not pull that well after about 5000rpms. I have the timing set at 20 degrees and everything responds perfectly with the exception of the drop off after 5000rpms. My question is whether I should up my carburetor to a 850 CFM Holley or keep the 750? According to the carb size calculators it seems like an 850 might be a good idea, but I thought I'd ask everyone's opinion before I started carb shopping. Any suggestions?

When you say timing set to 20 degrees.......... I'm hoping you have about 34 degrees, all in by 2500.

I think I'd put a fuel pressure gauge on it first you maybe running out of fuel
line size or fuel pump could be needed more then a bigger carb

I think I'd put a fuel pressure gauge on it first you maybe running out of fuel
line size or fuel pump could be needed more then a bigger carb

I agree, the 750 should pull plenty strong to 6300 rpm.

James, what are you running for an ignition? Also, what components did you upgrade and what did they replace? We need just a little additional info to help you solve this.

Thanks for the reply's guys.

When I checked the advance it was all in at 2300. 35 degrees total.

I installed a Holley red (7PSI according to Holley) about a year ago and also upgraded to a new Aluminum hard line, 3/8 inch the whole way from tank to carb. Also installed a new 3/8 inch pickup with a new sock to keep everything clean. I checked the fuel line and filters to make sure there were no flow issues. It all checks out okay. everything seems to be getting fed just fine until 5000 then it seems to get a little more hungry. The engine doesn't seem to cut out, it just seems to not pull as hard.

As for the ignition, I'm running a Mad Dog performance HEI with an upgraded MSD module.

As for the reason for the upgrade..... here is the sad story....

I purchased a used Comp Cams XE 270h Marine cam, (I should have known better, but it was from the Team Chevelle classified...) installed it with a new set of lifters and did the break in procedure according to Comp cams. Well I lost a lobe anyhow. The lobe took out a couple lifters and they in turn filled the block with little metal shavings that looked so pretty in the oil. They in turn, messed up my bearings so I knew it was all over. At that point I pulled the engine and did a complete rebuild. I had the block bored from .040 to .060 to clean up the cylinders. Purchased a new bearings, nodular iron crank, new Legend rods with ARP bolts, new Speed Pro Hypereutectic pistons and a new Voodoo 60204 cam with lifters. After assembling all of the new components, I broke in the ending according to Lunati's instructions and boy does this combo sound good.

It idles like the beast it is, pulls hard from 2000-5000, and then starts to feel kinda like it's leaning out. I figured it may be carburetion..... but maybe something else.....

By the way, TW what are you running on your 502?

Hi James, That bites, I feel for you on having to do the rebuild! Looking at the cam specs, it should easily pull past 6K. Timing sounds good enough. Does the car pull hard to higher revs in 1st but not in 2nd, 3rd, etc, or does it not reach over 5K in any gear? do you know if your fuel pressure drops at the higher revs? Have you checked to see if the vacuum secondaries are opening? Have you checked the accuracy of your timing marks on your damper? Did you degree/check the cam when you put it in? Hopefully we can narrow this down for you!

I put a Lunati solid roller (251/262, 0.660 lift) in my 502 last winter. It's quite a beast! I have to do something about the brakes. It's only pulling 5-7" Hg at idle.

this is a little hard on the fingers...... but you need to get a plug reading ... right after running it to your problem area & shutting it down coasting to side & pulling plugs to see if your lean on top .... fuel pressure gauge should tell if your mods have you covered on the supply end of it ...... I'm betting its lean on top did you rejet after cam & rebuild ?? or is where you are at now ??
unless of course you know someone with a dyno & air fuel sniffer :D

Hi James, That bites, I feel for you on having to do the rebuild! Looking at the cam specs, it should easily pull past 6K. Timing sounds good enough. Does the car pull hard to higher revs in 1st but not in 2nd, 3rd, etc, or does it not reach over 5K in any gear? do you know if your fuel pressure drops at the higher revs? Have you checked to see if the vacuum secondaries are opening? Have you checked the accuracy of your timing marks on your damper? Did you degree/check the cam when you put it in? Hopefully we can narrow this down for you!

I put a Lunati solid roller (251/262, 0.660 lift) in my 502 last winter. It's quite a beast! I have to do something about the brakes. It's only pulling 5-7" Hg at idle.

Well TW the car pulls hard in all 4 gears to about 5k and then starts to lean over a little. I don't have a fuel gauge on the engine that I can see at speed. The vacuum secondaries seem to be opening by the sound that I hear. As for the cam, yep, I degree'd it when I installed it. Installed it straight up. I did the old trick of placing a Permanent mark on the damper Before I ran the enging to make sure I had no drift.


this is a little hard on the fingers...... but you need to get a plug reading ... right after running it to your problem area & shutting it down coasting to side & pulling plugs to see if your lean on top .... fuel pressure gauge should tell if your mods have you covered on the supply end of it ...... I'm betting its lean on top did you rejet after cam & rebuild ?? or is where you are at now ??
unless of course you know someone with a dyno & air fuel sniffer :D

This sounds like an interesting thing to try. I will pull the plugs next Saturday when I take it out again. As for the rejet, yep. I'm now running 72's and it seems to tip in and WOT just fine until about 5k. Think I need to jet up?

The reason I asked if it pulls hard in 1st gear and falls off in others or if it does not pull to 6K in any gear is because if the float levels are set correctly, there is easily enough fuel to pull through 1st gear. It will usually have issues after that if it's a fuel delivery problem. This doesn't sound like a fuel delivery issue (unless floats set wrong). As 68KMENO mentioned, lean on top is a good possibility, but it can be difficult to read plugs on a car that runs unleaded. What you are seeing will have to be extremely lean! Are your secondary jets replaceable? You might try jetting up a bunch to see how it runs (try 4 jet sizes all around). Does your HEI ignition have a MSD box or a rev limiter on it? Also, what are you running for valve springs, are they the ones Lunita recommends? Was the installed height for the springs checked when they were installed?

Well TW, the car pulls to 6k in all four gears, but it seems to lean over a bit after about 5k. I would think it would continue to pull equally hard pat 5 up to 6k. Unfortunatly the 3310 that I have has a metering plate on the secondary side so I cant up the jetting there. I can up the primary side a couple of sizes though. The HEI does not have a limiter on it, and the MSD module is internal to the distributor. I talked with the Lunati tech when I was purchasing the 911 springs and he said they would work great with the cam I chose. Installed height and spring rate are right on.

I'm still wondering if an 850 may not be a good choice. According to the Holley site, and the CFM carburetor calculators I should be running a 825 - 850 CFM carb if I have 82-88% effeciency. But I don't want to over-carburate..... ???

you can install a different metering plate ... or drill out the .072 holes with a .076 drill ..... or buy the kit to install removable jets into the secodnary side now that you've said its a 3310 I'm surprised you haven't done this a long time ago ... :D
P.S. I'm 99.9% sure its LEAN !!!!

I'm leaning towards 68KMENO's conclusion also (no pun intended). As a quick check, you could go way up on the primary jetting to compensate for the lack of adjustability in the secondaries. If it improves, you know you are on the right track. You could switch to the 850, if that's what you are planning anyway. I would just make sure to get one with mainjets at all 4 barrels.

Tw..... I knew all you big block guys didn't care for S.B.s but wishing my Conclusion .... :eek:
I was only trying to help .... :thumbsup:

I will try bumping the jets up this weekend and see how it reacts to that. If that doesn't do it then I'll look into drilling the metering block.

If all else fails then I'll move up to an 850.

Tw..... I knew all you big block guys didn't care for S.B.s but wishing my Conclusion .... :eek:
I was only trying to help .... :thumbsup:


OK, I think I missed something :confused: , did I say something wrong?

Hey, I love small blocks! My Wife's 72 Monte is a 350 and I have a SB400 block just waiting for a project......maybe a Chevy II, or something small and unusual (240/260/280Z??)

Its ok TW .... my post was a friendly joke :D you didn't say anything :beers:

no 850
get a metering block for the sec, of that carb..
and tune it..
but also make sure the sec.. are indeed opening and opening ALL THE WAY
that sec. metering plate... aint helping matters..
and on the hei.. was this car points at one time???
is the risistance wire being used to power the hei??
is the hei getting a full 13 volts???

if you think you're pushing the limits of your 750 you don't need to worry about being lean unless your fuel system can't keep up and the fuel pressure is dropping. Holley carbs traditionally get rich as the rated CFM is approached. AFAIK the only way to fix this to change out the booster venturiis.

What is the fuel pressure at 6000RPM? Is it maintaining 6-7PSI?

What bowtie said: hook a voltmeter to the BATT terminal of the HEI, check the voltage with the engine running. It should be very close to the charging voltage, 13.8 - 14.2.

no 850
get a metering block for the sec, of that carb..
and tune it..
but also make sure the sec.. are indeed opening and opening ALL THE WAY
that sec. metering plate... aint helping matters..
and on the hei.. was this car points at one time???
is the risistance wire being used to power the hei??
is the hei getting a full 13 volts???

The secondaries are opening up from what I hear. I changed the springs back and forth and I think I have a purple or white one in the carb now. (I can't remember at this moment) The car was originally points I believe but I rewired the HEI to get 12v straight from the ignition through a standard stranded core wire, not fusable link or resister wire.

if you think you're pushing the limits of your 750 you don't need to worry about being lean unless your fuel system can't keep up and the fuel pressure is dropping. Holley carbs traditionally get rich as the rated CFM is approached. AFAIK the only way to fix this to change out the booster venturiis.

What is the fuel pressure at 6000RPM? Is it maintaining 6-7PSI?

What bowtie said: hook a voltmeter to the BATT terminal of the HEI, check the voltage with the engine running. It should be very close to the charging voltage, 13.8 - 14.2.

I don't know the fuel pressure at 6k because I can't see my gauge at that time. I'll have to do some creative plumbing to see it while I test this weekend. As for the charge, I'm getting 14v as soon as the alternator energizes. It holds solid through the entire RPM range, only drifting a little as the car vibrates at higher rpm's.

Its ok TW .... my post was a friendly joke :D you didn't say anything :beers:

Not to worry, no thin skin here! I just didn't get the joke and didn't want to be left out :)


What is the fuel pressure at 6000RPM? Is it maintaining 6-7PSI?
Tom, Even if the fuel pressure was low at high rpm, I would think he would be able to get through 1st gear before it leaned out -- what do you think?

but also make sure the sec.. are indeed opening and opening ALL THE WAY

I agree. I would add a loose zip tie or something on the arm going to the vac diaphram and see how much it moves after a hard run.