# Am I over, or under-carbureted?



## Tony Kellogg (Jun 4, 2013)

Hi all - just a general query about my 66 Tempest, which has a serious stumble on transition to WOT....

I have timed, tuned, tested, and cannot rid this engine of its hesitation. I admit that I do not have the experience behind me that most of you do, but I wonder if this engine is over-carbureted?

My specs:

Chevy 350 engine - brand new block, heads - everything

TrickFlow 23 degree 72cc heads- Intake runner 195cc, Exhaust runner 75cc
2.02 Intake valve, 1.60 exhaust valve
.30 over 9:1 compression
Trick flow cam 292/296 duration, 532/540 lift
Eagle H Beam rods and Eagle crank
MSD billet distributor – no vacuum advance – initial timing at 12
750 Holley double pumper mechanical secondaries
Jets = 67 front, 75 rear – squirter = 31 – orange pump cam
3:83 Posi, Tremec 5 speed

I will do what it takes to get this right (buy carbs - whatever) - anyone have any words of wisdom?

Thanks - Happy Fourth Of July!

Tony


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## Rukee (Feb 8, 2007)

Hey Tony, what's your total advance at say 2500-3000rpm?


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## Chris-Austria (Dec 23, 2010)

What's the max rpm with this cam?! With 350cui and 9:1 CR this cam looks pretty big but I don't know much about Chevy engines.


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## DanT (Jun 29, 2013)

I used the formulas below to check the CFM of my carb to make sure that I have the correct one on my 67 Tempest. All you have to do is plug in the values.

From HP Books "Auto Math Handbook", a great purchase from Amazon. It breaks down all the math formulas that go into an engine.

Theoretical CFM = (RPM x DISPLACEMENT) / 3456
Volumetric Efficiency = (ACTUAL CFM / THEORETICAL CFM) X 100
Street CARB CFM = [(RPM x DISPLACEMENT)/3456] x 0.85
Racing BARB CFM = [(RPM x DISPLACEMENT)/3456] X 1.1


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## chuckha62 (Apr 5, 2010)

More than likely it's not the size of the carburetor, it's the timing of which everything is coming into play. On my small block with a Holley 3310 (780 Vacuum secondary) carb, it needed 16 degrees initial in order to overcome that stumble. THAT was with vacuum secondaries which are more forgiving in general. With mechanicals, you're dealing with ignition timing and the timing and volume of your accelerator circuit. I'd try more initial timing, first and see what you get. Ideally you should have somewhere around 38 degrees total and have it all in somewhere south of 3000 RPMs.


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## Tony Kellogg (Jun 4, 2013)

Hey guys, thanks for the replies

My total advance is 31
RPM - Ive had it up to 7500RPM and get to 5800-6000 often
I plugged in the numbers on the calculations for carburetor and came up with 787
I will try advancing the timing
It runs at 160 degrees (thermostat) - do you suppose that a 140 degree stat would be better?


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## chui1980 (Jun 5, 2013)

I am going to put my two sense on this if you dont mind. You have a 750 cfm carburetor in you r SBC 350 however if you not pushing over 450-500 HP it is overkilled. Cam and other things do not matter unless you generating lots of vacuum in your engine. I can bet you that you are not generating anymore than 25psi Vacuum. That is not enough for a small engine such as a 350 with a 750 CFM carb. I will suggest you put a 600-650 CFM in there and reply back.
Give it a shot.


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## chuckha62 (Apr 5, 2010)

chui1980 said:


> I am going to put my two sense on this if you dont mind. You have a 750 cfm carburetor in you r SBC 350 however if you not pushing over 450-500 HP it is overkilled. Cam and other things do not matter unless you generating lots of vacuum in your engine. I can bet you that you are not generating anymore than *25psi Vacuum*. That is not enough for a small engine such as a 350 with a 750 CFM carb. I will suggest you put a 600-650 CFM in there and reply back.
> Give it a shot.


Huh? No way, are you going to generate 25 psi vacuum! Vacuum is measured in HG (inches of mercury) and generally you're good if you're into the 13-17 hg range. 1hg is roughly equal to 1/2 psi (.49109778), so at 17hg, you're only generating -8.5 psi.

Again, a V8 engine needs somewhere in the vicinity of 36 to 38 degrees of total timing. 31 is really not enough. Try crowding the initial up a few degrees and see what happens. Also, the proper thermostat for your engine is 180 degrees. It's a fallacy that a cooler t-stat will make your engine run cooler. All the number is, is the opening temp of the stat, that's it!


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## chui1980 (Jun 5, 2013)

well my 70 nova is at 18 psi Vacuum. It is possible but not necessary recommended but I still believe that 750 is too much. When I went to a near Carb/speed shop near me the old men could have sold me a 750 but instead he indicated the folllowing: "if you just going to install this in a SBC with street/strip components at around 350-375 HP then 650CFM is more than enough. Dont buy the 750. I said thanks for the info something I am trying to share with you guys here.


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## chuckha62 (Apr 5, 2010)

chui1980 said:


> well my 70 nova is at 18 psi Vacuum. It is possible but not necessary recommended but I still believe that 750 is too much. When I went to a near Carb/speed shop near me the old men could have sold me a 750 but instead he indicated the folllowing: "if you just going to install this in a SBC with street/strip components at around 350-375 HP then 650CFM is more than enough. Dont buy the 750. I said thanks for the info something I am trying to share with you guys here.


Ahhhh... Check your vacuum gauge. It's not in PSI, it's in HG.


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## Tony Kellogg (Jun 4, 2013)

I have wondered about too large with regard to this carb. I am having it chassis dyno-ed this week and will keep y'all posted.


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## BearGFR (Aug 25, 2008)

You've got lots of things working against you on this thing.
1) Mechanical secondaries. As soon as they open you're dumping a huge amount of air into the engine with no flow velocity through the carb to make it feed fuel, so you're going to need huge accelerator pump shots to help cover that, and it might not be possible to make them "big enougn" to do the job especially on a little 350.
2) Realm of operation. Yes, I remember you've said you've spun it to 7500 rpm before but that has absolutely nothing to do with your transition problems which are happening at a relatively low rpm (when air-flow is very low, especially when it's a little 350 that's doing the 'sucking'). Building a race engine is a whole 'nuther animal from building a streetable engine. A race engine is going to get launched probably at a higher rpm than your street engine is going to see 90-95% of the time. High rpm translates to air flow velocity, and that's what makes a carb feed fuel. Engines aren't computers - meaning that you don't plan capacity for peak-load conditions. You plan capacity for the realm of operation where the engine is going to "live" most of the time. With that in mind, for a street driven 350, yes your engine is -vastly- over-carbureted (and made worse by the mechanical secondaries). 

This is not a problem you're going to fix with ignition timing tweaks or any other trick.

If this was a race-only engine, then sure - you might be able to make it work and if the cam, valve train and rest of the intake/exhaust system is designed/tuned to be efficient at high rpm, a 750 with mechanical secondaries might be just the ticket.

If it were me, and I wanted the best chance at building an engine that would be reasonably successful in both regimes (street and race) then I'd ditch the mechanical Holley and go with a 750 cfm QJet. There's a reason GM put those on everything from OHC 6's to big 455's - they're infinitely more adaptable and tunable over a very wide set of conditions than that big Holley is, especially one with mechanical secondaries.

Bear


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## chuckha62 (Apr 5, 2010)

...but yes, there is a VERY good chance you're over carbureted. As stated, the double pumper is not your friend right now. You could possibly tolerate this size of Holley, with vacuum secondaries but you've got a lot working against you. I still would never discount the effects of lazy timing. I have fixed MANY carburetor issue by correctly setting timing.

As Bear said, the BEST carburetor for your application may very well be the Quadrajet. The small primaries will give you much better low end response due to the increase in intake velocity. It will also render better fuel mileage AND better top end performance than most Holleys. For performance, CFM for CFM, I'd take a properly set up Q-Jet than any other carburetor.


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## chui1980 (Jun 5, 2013)

BearGFR said:


> You've got lots of things working against you on this thing.
> 1) Mechanical secondaries. As soon as they open you're dumping a huge amount of air into the engine with no flow velocity through the carb to make it feed fuel, so you're going to need huge accelerator pump shots to help cover that, and it might not be possible to make them "big enougn" to do the job especially on a little 350.
> 2) Realm of operation. Yes, I remember you've said you've spun it to 7500 rpm before but that has absolutely nothing to do with your transition problems which are happening at a relatively low rpm (when air-flow is very low, especially when it's a little 350 that's doing the 'sucking'). Building a race engine is a whole 'nuther animal from building a streetable engine. A race engine is going to get launched probably at a higher rpm than your street engine is going to see 90-95% of the time. High rpm translates to air flow velocity, and that's what makes a carb feed fuel. Engines aren't computers - meaning that you don't plan capacity for peak-load conditions. You plan capacity for the realm of operation where the engine is going to "live" most of the time. With that in mind, for a street driven 350, yes your engine is -vastly- over-carbureted (and made worse by the mechanical secondaries).
> 
> ...


I hope everyone gets the point I was trying to make with this quote. Just take it off and a put a 650CFM and respond with a very nice smile. By the way a Qjet may not be the answer for what he is looking for. Qjet is only a strictly street carb. Forget about performance. Is gonna work better than what you have for sure


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## Tony Kellogg (Jun 4, 2013)

Hey y'all, thanks for the excellent input. I actually have been doing a lot of reading and discussing and was leaning toward a 650 street avenger or Edelbrock 650 AVS. As far as a Quadrajet - I have a square bore manifold that I like. anyone have opinions on either model? I like to tune on it myself, but need to get a pro to at least get it in the neighborhood. 

The dyno says Max torque 355.92, Max HP 339.31 - does that sound adequate?


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## BearGFR (Aug 25, 2008)

chui1980 said:


> .... By the way a Qjet may not be the answer for what he is looking for. Qjet is only a strictly street carb. Forget about performance....


Really! Please don't tell my car that, it doesn't know any better. It runs high 11-second quarter mile times at over 110 mph --- with a QJet. 

Bear


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## gjones (Oct 8, 2012)

*quadrophonic jettison*

^^^^^ :lol: I've been saving my pennies for a real, honest to goodness q-jet. I'm gritting my teeth and chomping at the bit! Haven't even laid eyes on one since the late 80's, early 90's maybe! :willy:


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## Rukee (Feb 8, 2007)

Tony Kellogg said:


> Hey y'all, thanks for the excellent input. I actually have been doing a lot of reading and discussing and was leaning toward a 650 street avenger or Edelbrock 650 AVS. As far as a Quadrajet - I have a square bore manifold that I like. anyone have opinions on either model? I like to tune on it myself, but need to get a pro to at least get it in the neighborhood.
> 
> The dyno says Max torque 355.92, Max HP 339.31 - does that sound adequate?


Did you ever bump your timing up to 38* total advance @ 2500-3000rpm?


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## Tony Kellogg (Jun 4, 2013)

Hey y'all - update on stuff. Still a pretty good bog on transition. Timing is now 15 BTDC with 34 deg total advance by 3000. Seems ok except for that stumble. 

What is everyone's opinion (other than Q-jet cause I don't want to swap manifolds) - Holley or Edelbrock 650 with vacuum secondaries? Any experiences good or bad? Anyone want to buy a barely used 750 DP?


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