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Discussion Starter #1
For my .060 over 396, LS6 heads, Air gap intake,9.5 comp.

I have a 850 cfm vacuum secondary and want to go with a 750 mechanical do to the 4 spd and the cam doesn't like the the vacuum carb. So my question is, what do I need to do to make my 750 cfm work with this set up? I'm not overly familiar with tuning Holley's.

Here's the cam specs
 

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is your

850 vacume carb one of those with a power valve in the front and back ? If it is , i think i would block the back for this set up and adjust the back jets. It prolly isn't pulling the secondaries in as quick as the back power valve is opening and causing a rich condition here

That looks like a pretty healthy cam for just 9.5 comp. in a 396. I think the cam isn't liking the compression more than the carb.

hmm ,, wonder if a set of Rhoades lifters might help you out here too

I like vacume carbs myself , yeah sure put a DP carb on it and its a no-brainer , all 4 barrels will be open , but i like to tinker with vacume carbs. A spring kit can be alot of fun to play with on a lonely back road. Since you do have a 4 speed , you don't have the luxury of a stall converter to help get through low speed response situations on the street so i think if you got the vacume carb working better , you might be better off. just my .02

expand on whats going on with the car more.
 

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Discussion Starter #3
It's lazy to pull away and has a slight flat feel right off the start under normal street driving. It has no trouble coming to life at 3000rpm when rolling. If I launch it hard it goes fine, I would just like to tune the low speed drive a bit more. It's not real bad, but I'm new to carb tuning and don't want to chase my tail.

Thanks for the input.:yes:

The carb is from a crate 502, this one here.
http://www.holley.com/applications/CarburetorSelector/0-80531
 

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Sounds like you need a touch more initial timing.

Once you get the ignition system worked out, then you can start leaning out the primaries and idle circuit to get your part throttle cruise where it needs to be.

For part throttle operation a double pumper is not gonna make any difference.:no: and might even be worse since they typically have a little fatter baseline.:yes:
 

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That cam isn't ging to produce any stella off idle performance in your engine. It would be much better at more like 11:1 compression. You're probably not putting good power out untill 3500rpm or so. I ran that cam in a 400cid engine at 10.75 compression with a 3.70 rear end. It really liked the 4.56's when I swapped them. It was just more of a high rpm engine. I think a 750 will give you a little throttle response back, but I think you just need more tuning with the carb you have now. If you go with a new 750, I'd suggest one with the Proform main body, or a HP so you can tune the carb with the air bleeds as well. Something like this:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=QFT-Q-750&N=700+4294919007+1051506943+115&autoview=sku
 

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it does have

a secondary power valve. There is a conversion factor to help adjust the jets to compensate for blocking the power valve. If it wuz me , i would block it and adjust the jets for sure. The Holley tech line would be a good source for the equation or they might give you a " ball park " jet size themselves. I've saw it on the net before but for some reason can't find it today , i'll keep looking.

Myself i'd try a good ol' tried and true 750Vs 3310 Holley and keep the 850 if you ever venture to a 454 or bigger street motor.

I'm guessing you have closed chambered pistons and open chamber heads ? Gotta wonder if this is the case , what swapping a set of closed chambered heads on it would do.

That cam isn't going to have the greatest low speed manners i'd say no matter what carb is on there. Ya gotta remember a hydraulic cam acts like a solid with anywhere from 8 degrees to 12 more duration at .050. Your cam is pushing the " street " in street/strip to the edge of the envelope while your compression is not near the edge.

I think i'd look at this cam ,

http://www.lunatipower.com/Product.aspx?id=2110&gid=277
 

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You need to step up 8-to-10 jet sizes for blocking the power valve. :yes:
 

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Discussion Starter #8
I can't find any Lunati dealers up here!:( Pistons are open chamber, trouble up here is race fuel isn't all that readily available so compression has to be conservative with steel heads. It's not out to lunch, but it is off a bit and I still need to get used to driving it. I'll tweak this and keep my eyes open for a 750 in the mean time.

Oh ya, at 3500 rpm this thing wakes up!:yes: I'm not sure if I want to run a 456 gear though, I was thinking 411's with the M20.:yes:

Thanks for the help this is a new area for me, so appreciate the help!
 

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If you go with a new 750, I'd suggest one with the Proform main body, or a HP so you can tune the carb with the air bleeds as well. Something like this:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=QFT-Q-750&N=700+4294919007+1051506943+115&autoview=sku

Why do that when he can get a custom built for his application ready to bolt on and fly carb from prosystems for the same amount of money? He would be looking at $590 for the prosystems.


http://www.prosys12.macwebsitebuilder.com/proseries4150.html


 

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It's lazy to pull away and has a slight flat feel right off the start under normal street driving. It has no trouble coming to life at 3000rpm when rolling. If I launch it hard it goes fine, I would just like to tune the low speed drive a bit more. It's not real bad, but I'm new to carb tuning and don't want to chase my tail.

Thanks for the input.:yes:

The carb is from a crate 502, this one here.
http://www.holley.com/applications/CarburetorSelector/0-80531
What makes you think it's the carb?

Your timing and dist curve will play the most important part.


My Factory 1970 Chevelle LS6 had the same issue, the close ratio M-22, 3.73 rear gear

Al
 

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very true

What makes you think it's the carb?

Your timing and dist curve will play the most important part.


My Factory 1970 Chevelle LS6 had the same issue, the close ratio M-22, 3.73 rear gear

Al
have you tried doing anything with your centrifigal advance ??

I think some of this is a mis match though of some critical parts. Great big hydro cam , not so much compression , Rhoades lifters and a steep timing curve might help some here ,,,
 

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have you tried doing anything with your centrifigal advance ??

I think some of this is a mis match though of some critical parts. Great big hydro cam , not so much compression , Rhoades lifters and a steep timing curve might help some here ,,,


Back then I installed a Accel dual point and yes, it helped!

25 years ago, a Speed Shop and his Dist machine was the deal!


Al
 

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Discussion Starter #15
What makes you think it's the carb?

Your timing and dist curve will play the most important part.


My Factory 1970 Chevelle LS6 had the same issue, the close ratio M-22, 3.73 rear gear

Al
It's not fully broke in yet, so the timing is not 100%, it's @ 25º adv @ 3000 rpm. The engine builder said it would work better with a Mechanical carb, because of the cam. Even when I go to the Holley site a 750 mechanical carb comes up for choice. It just has a slight flat spot from the get go, so I posted here, for all the good knowledge members like yourself.
 

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It's not fully broke in yet, so the timing is not 100%, it's @ 25º adv @ 3000 rpm. The engine builder said it would work better with a Mechanical carb, because of the cam. Even when I go to the Holley site a 750 mechanical carb comes up for choice. It just has a slight flat spot from the get go, so I posted here, for all the good knowledge members like yourself.

Its at 25 total?
 

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Sounds like you need a touch more initial timing.

Once you get the ignition system worked out, then you can start leaning out the primaries and idle circuit to get your part throttle cruise where it needs to be.:yes:
What makes you think it's the carb?

Your timing and dist curve will play the most important part.
:yes:
Al
have you tried doing anything with your centrifigal advance ??
Sounds like the first place to start.:yes:
With a cam that big and low compression, it's gonna want as much initial timing as you can give it!! Probably upwards of 20 initial and 36-to-40 total.

Once the timing is somewhat dialed in, THEN you can start tuning on the carb. I'd Start with the accelerator pump discharge nozzle size if it's just got an initial hesitation on "Tip-in".
 

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Discussion Starter #18
Yes, it's total.:yes:
 

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Sounds like the first place to start.:yes:
With a cam that big and low compression, it's gonna want as much initial timing as you can give it!! Probably upwards of 20 initial and 36-to-40 total.

Than you can start with the accelerator pump discharge nozzle size if it's just an initial hesitation on "Tip-in".
I went from a 40 to a 45 and it did help. I'll get some miles on it and then dial in the timing and then go from there.

The engine was built with street tolerances as apposed to race tolerances, so it will need a bit more to seat everything.
 

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I'd be careful putting too many miles on it with the timing too retarded. That will lead to higher exhaust temps, cylinder temps, exhaust valve temps, etc....

Not to mention a generally LAZY running engine.:yes:
 
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