402 Big block problem

[Jcentsr6]

I was speaking with one of my techs at work....he is trying to tune my new carb on the truck today. ( I have never had it running great after doing the cam and lifters and all) Well...we hooked up a vac guage and the engine is only pushing out 4-5 psi. I am told that it should be like 13+....Is this correct? The tech thinks we went to big of a cam for the truck. He says inside the carb that the vac is too low to keep some "block" from working properly (sorry I dont know alot about carbs) so it will not run right. QUESTON: What can I do....
Do I have to replace cam again? Thank for your help

[CharlieBrown]

what intake/carb combo you running

[Jcentsr6]

Elelbrock Pro performer intake...and Holley 670 street avenger.

[CharlieBrown]

well

have had better luck with the elelbrock carbs

they need a divider type intake..other words..not one big hole under the carb for the idle circut to work

on my 383..tried running a holley intake..single hole...no idle vacume...

switched to a the..elelbrock intake and carb..dual plane ..picked up idle vacume..works very well

[corn]

Which cam?

[Jcentsr6]

lunati voodoo 262/268 i think...or the 268/276

[slammed1]

Are you running any type of torque/stall convertor with your combo?

[Jcentsr6]

Originally Posted by slammed1

Are you running any type of torque/stall convertor with your combo?

NONE. I have not changed anything in that area just yet

[69halfton]

i JUST read about this thing in hotrod.

you can buy more or less an electirc vacuum pump.

it has a switch on it, and when idleing and such it comes on to allow your power breaks to work etc. its deisgned specifically for big cams on the street.

im not sure as to weather it would work in this sitution or not..

[My72BB]

Can you give us more details on the cam? Need to know the intake/exhaust lift...The duration is 268? or what you had listed...the lift is what really effects the vacuum.

I am going to go out on a limb here...but to have vacuum in the 4 to 5 range??? That is way low. Even a solid lift street cam will normally pull more than that. I would investigate your manifold carb set up...sounds like something was left out or installed wrong?

Pulling the cam is a PIA...hopefully it is something simple like a carb gasket/intake gasket.

[Jcentsr6]

Originally Posted by 69halfton

i JUST read about this thing in hotrod.

you can buy more or less an electirc vacuum pump.

it has a switch on it, and when idleing and such it comes on to allow your power breaks to work etc. its deisgned specifically for big cams on the street.

im not sure as to weather it would work in this sitution or not..

Thanks..but I dont think that it works for Carb vac pressure....its designed more for power brakes and such.

[Jcentsr6]

Originally Posted by My72BB

Can you give us more details on the cam? Need to know the intake/exhaust lift...The duration is 268? or what you had listed...the lift is what really effects the vacuum.

I am going to go out on a limb here...but to have vacuum in the 4 to 5 range??? That is way low. Even a solid lift street cam will normally pull more than that. I would investigate your manifold carb set up...sounds like something was left out or installed wrong?

Pulling the cam is a PIA...hopefully it is something simple like a carb gasket/intake gasket.

HERE IS WHAT I GOTS!

Lunati Voodoo Cam and Lifter Kits
Cam and Lifters, Hydraulic Flat Tappet, Advertised Duration 268/ 276, Lift .542/ .554, Chevy, Big Block, Kit

[slammed1]

The cam is rather small in bigblock terms but big enough to make issues if other things arent addressed. An RV type convertor or a 500-1000 more stall than factory may be needed for this cam. The reason I say this is that for your truck to idle in gear plus run great on the road without detonation,I bet your timing is off. If your timing is dead on (specs) you will probably have to adjust it to a setting that makes the all around idle and WOT performance work together. big Lumpy cams need some type of stall convertor or RV style to help with idle lugging when the timing is set.

Let me see if I can describe this in an example.
I had a 408 small block built with 11.0-1 compression,.488-.288 cam and it worked great with my 67 Impala with 3500 stall and 4.11 gears. I took same engine out and placed it in my 72 truck with stock convertor and 3.08 gears and I had to tailor the timing and carb setting to where it was on the verge of pinging and still idling good enough. Im not saying you need a really loose convertor but a looser than a stock convertor with cams is even recommended with alot of cams on the market......its the cam manufacturers recommendation on most cams " HYDRAULIC: Great for street machines. Use headers and 9:1 compression. In 396-402 use 2500 stall, low gears. Rough idle." (example).

Here is a write up on ignition timng and vacuum settings.
"IGNITION AND VALVE TIMING: A low, 10 to 14 inch, but steady reading indicates faulty ignition or valve timing. Set the ignition timing to the correct recommended spec and everything will probably be OK. You can even set the timing very close to specifications with the vacuum gauge. Adjust the distributor to the highest steady vacuum reading at idle and you'll be close enough to know if that or valve timing is the problem. If you can't get the reading into the "normal" zone by adjusting the distributor then valve timing is the problem. Check and set the ignition timing with a timing light before driving the car, or at least back the timing off a hair and be on the alert for any pre-ignition pinging in the engine. Timing with a vacuum gauge will normally result in timing that is more advanced than what specifications call for. Faulty valve timing is pretty rare in a properly assembled engine, it doesn't change on it's own."

Hope some of this helps.

[bryanw1968]

You shouldn't be running into any problems with that setup. I have a very similar cam in my SBC and pull 12-14 inches of vacuum at idle. It sounds like you don't have enough timing and or the carbs idle circuit isn't tuned right. Even with an 8:5:1 motor if the rockers are adjusted right and the timing and idle circuit is remotely close you should be at 10 inches of vacuum. You might check to see if your powervalve is good.
Also, I kind of scanned through this post so I hope I'm not repeating anyone but......
I'd pull the powervalve anyway and look at what size it is. A factory powervalve should be 6.5 inches of vacuum. If you are not pulling that much vacuum the powervalve could be fluttering and you may need to go down to a 5.5 or a 4.5. But with your setup I really don't think you should be having any issues. If not any of the above maybe a vacuum leak?

How's the idle/part throttle? Where is your timing? Are you running a vacuum advance distributor and if so how is it hooked up ported vacuum or direct?

[slammed1]

I was kind of wondering myself what the exact problems were.

I found the Lunati site states this about the 268 Cams
"Hydraulic;The most awesome "268" cam ever produced out powers all others!
This High Performance Street cam likes 2400 RPM stall,800 cfm carb,dual
plane intake and headers. Makes unequaled power to 6200 rpm with proper
valve springs. If your looking for a very strong cam with great street manners
for your crate motor this is it."

[bryanw1968]

Originally Posted by slammed1

I was kind of wondering myself what the exact problems were.

I found the Lunati site states this about the 268 Cams
"Hydraulic;The most awesome "268" cam ever produced out powers all others!
This High Performance Street cam likes 2400 RPM stall,800 cfm carb,dual
plane intake and headers. Makes unequaled power to 6200 rpm with proper
valve springs. If your looking for a very strong cam with great street manners
for your crate motor this is it."

You just made me think of something VERY important. Even though that isn't that big of a cam it is considered and "aggressive" profile. AND, if you don't have probably at least 120lbs of installed seat pressure you will bind those springs big time! That happened to me when I had first put my motor together and the machine shop sold me crappy springs. I had really low vacuum until I changed the springs out.

I think this happens more than people realize. They upgrade the cam and have stock heads and just assume that the spring will be fine.

[franko72]

You might have already did this but if not,Try checking your vaccum with a different gauge. Just to rule out a possible faulty test gauge.

[Jcentsr6]

Okay....lets say the timing is out? When installing a gear drive...how hard is it to install wrong? The gear drive was installed at the same time as the cam and lifters....Is it easy to get the timing off with a gear drive? I thought it went on only one way? (One of my techs installed this by the way....not me, as you cal tell) Sould I start there?

[Jcentsr6]

UPDATE: I just checked compression...front two cylinders are at 125psi. Give me some more ideas please. By the way..thanks for all the imput so far

[slammed1]

Please let us help you by telling what exactly your having problems with and what all other mods you have done. The distributor is where your secondary timing is set once the timing is set with the timing set.
Do you have a aftermarket convertor?
Did you replace the valve springs?
Do you have headers?
What exact problems are you having except low vacuum?

[Jcentsr6]

Originally Posted by slammed1

Please let us help you by telling what exactly your having problems with and what all other mods you have done. The distributor is where your secondary timing is set once the timing is set with the timing set.
Do you have a aftermarket convertor?
Did you replace the valve springs?
Do you have headers?
What exact problems are you having except low vacuum?

Thanks. Sorry. I have
Lunati Cam
Full rollers
Edelbrock pro performer intake
pete jackson gear drive
(org springs)
Holley Street avenger carg 670cfm
Hooker comp headers, Flow master super 44's, Full custom mandrel bent side exit exhaust.
MSD HEI dist...MSD wires, K&N air filter...8an FUEL stainless lines.

Problems. Running rich (plugs look black after 1 day of starting engine to test it)
No POWER...this truck will not spin the rear wheels.
MAIN PROBLEM...NO POWER!...hense I installed all power making parts.
Intermitt back fire and stall.

[1968SWBBigBlock]

Did you degree in the cam when you put it in?
Lunati Voodoo cams are ground with 4 degrees advance in them.
You also need to make sure you vac gague is hooked to the correct outlet. Manifold vac is what you need to look.
125 psi is on the low side for pressure.
A gear drive is easier to install than a timing chain.
Also do a soap test and look for a leaking manifold.
As was said make sure you do not have coil bind and that you are not hitting the valve seals at TDC.

[clay68c10]

Sounds like a lot of valve lift for stock springs. I'd run whatever springs Lunati recommends for that cam.

[bryanw1968]

Originally Posted by clay68c10

Sounds like a lot of valve lift for stock springs. I'd run whatever springs Lunati recommends for that cam.

Like I mentioned earlier, there is a good possibility that those springs are not gonna work with that much lift. Your lack of compression could be that two of the valve springs have already broken. I'd take a look at the top end before I tried to drive it anymore. You could end up dropping a valve ontop of a piston. Just my two cents.
Gooo luck!

[kwhite4]

I had a simular problem with my old 468, cam was in the 574 lift range Crane flat tappet.If your running factory style springs then more than likley thats the problem , I went with Comp honey comb style and Man what a change.