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Old 04-10-2010, 10:13 AM
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What size carb? 350sbc zz430 crate engine

Wich carb do I buy? I was looking into few - Holley 670 street avenger vac secondary , Holley 750 dp manual sec, & Holley 650 dp. I have a 68 camaro with a 350 sbc zz4, aluminum gm fast burn heads, gm hot cam, 1.6 rockers, air gap, th350 automatic, approx 2800 stall converter, 4.11 posi. Right now I have a 4150hp 750 dp w manual secondary. It bogs bad when ever I nail the gas. I played with the carb and had many people try to fine tune it. It's still not right. I'm thinking maybe it's too big. Anyone have any carb recommendations? Also the engine has to be 100% completely warmed up to run any bit decent. When it's a liittle cold, the bog is extremely bad. It will stall and fall flat on its face. I found someone who has a 6 month old 670 street avenger or do I buy a new 650dp or do I work with what I have? Plaese help! Thanks

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Old 04-10-2010, 10:50 AM
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With everything working right it should run just fine with a 750 DP.
Keep looking there is a problem somewhere. How much ignition
advance are you running at idle?
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Old 04-10-2010, 10:50 AM
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if it is to be used street driven i would go vac secondary anything from 600cfm-750cfm should work fine depending how fast you plan on spinning it
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Old 04-10-2010, 02:25 PM
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Quote:
Originally Posted by automotive breath
With everything working right it should run just fine with a 750 DP.
Keep looking there is a problem somewhere. How much ignition
advance are you running at idle?
Agree. DP is recommended with stick shift or auto with 3K converter. 2800 is close enough to 3K to make it work. With a big gulp of air, you must provide a big gulf of fuel.....

Use this recommendation for dialing in ignition advance....
INITIAL TIMING
The chief function of initial timing is to provide a clean idle
and crisp throttle response. One of the best guides to
determine the initial ignition timing of V8 engines can be
found in the Barry Grant, Inc. catalog or at their website
under the Demon Carburetor Guide. Typically, they recommend 10- to 12-degrees of initial timing when
the duration of the camshaft is less than 220-degrees @ 0.050 of valve lift; 14- to 16-degrees of initial
timing with a camshaft duration of less than 240-degrees @ 0.050; and 18- to 20-degrees of initial timing
when the camshaft duration is less than 260-degrees @ 0.050 of valve lift.
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Old 04-10-2010, 04:12 PM
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techinspector1, I like the idea of using an ignition advance guide that uses
camshaft duration like the one you posted. It gives you a general idea
of how an engine with more duration may need more ignition advance at low
RPM, not just an increase in advance with an increase in RPM.

One thing I find many people overlook is the relationship of ignition advance
needs to throttle position. One of the best examples of this is vacuum
advance. With the canister connected to manifold vacuum the distributor
advances when the throttle is closed and retards when the throttle opens.

For a specific RPM, when the throttle is closed not much air is introduced
into the cylinder resulting in low cylinder pressure, slow burn and high
ignition advance needs. For the same RPM, when the throttle is opened
cylinder filling increases along with higher cylinder pressures, faster burn
and lower ignition advance needs.

Basic stuff thats often overlooked, although an engine may need 36 degrees
of advance at 3500 RPM with the throttle wide opened, it may need
much more at the same RPM with the engine throttled like at cruse RPM.
Makes me wonder why so many street rodders are so quick to eliminate
vacuum advance!
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Old 04-10-2010, 04:40 PM
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Well AB, you should be around when I`ve had countless arguements over the benefits of vac advance, especially on where it`s connected. It didn`t take me long to figure out the purpose they serve and it`s a good purpose. When the one on my car went out on the interstate one day I knew right away, the throttle response got sluggish and the temp ran 15 degrees hotter.
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Old 04-10-2010, 06:07 PM
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Quote:
Originally Posted by DoubleVision
... you should be around when I`ve had countless arguements over the
benefits of vac advance, especially on where it`s connected....
Hey DoubleVision, I'm a slow learner, it took me a long time to figure out
that you can't win an argument on the internet LOL.

Here's some food for thought for anyone that might be reading this. Let's
use a 1968 275 hp 327 as an example.
(why use this engine? because I have the factory specifications)

The base ignition advance setting is 4 degrees BTDC at idle with the
vacuum advance disconnected and plugged.

The centrifugal advance curve is like this:
0 degrees @ 900 RPM
17 degrees @ 1900 RPM
30 degrees @ 4100 RPM

The vacuum advance is:
0 degrees @ 10" HG
15 degrees @ 17" HG

So if my math is correct; with the vacuum canister connected to manifold
vacuum, the advance at idle would be 19 degrees BTDC.

and... if the engine is pulling 17" HG at part throttle cruse @ 4100 RPM;
the total advance would be 49 degrees BTDC!

So maybe the engineers in 1968 had it all wrong... or maybe the guy thats
running 10 degrees initial at idle and 36 degrees total at cruse RPM might
need to rethink his tune-up!
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Old 04-10-2010, 11:31 PM
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Interesting

Hey buddy we have the same engine basically.
I got a same carb also and mine works like a charm. You sure you tuned it correctly? Dual or Single plain intake? This will make it or brake it on this engine BIG TIME.. I had a single plain with this carb and had same symptoms.
I went dual plain and all was fine.
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Old 04-10-2010, 11:35 PM
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More more thing

Like firebird88 stated already the timing curve will due a number on this engine. I made that mistake and I also had the vacuum hose plugged into the incorrect vacuum port. There is a specific vacuum port for the timing advance on this carb.
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Old 04-11-2010, 12:29 AM
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I see you have the AIR GAP and 750 DP. You said you need to be well warmed up to run well. This is very true. I found my air gap required a 190 thermostat and that I run about 1/2 hour before any fine tuning. The DP is going to fight you all the way also. A vac sec 750 set to begin to open at about 3000 and open all the way at 5000 or so is really what you want. The Air Gap works great when it is warm but cold it is worse than terrible. I'd just use a standard non air gap RPM manifold given I had to do it again. You will need to work with the shooter a bit too. I ran about 3 sizes larger than the original in the 3310-3 carb. One size larger main jets.

I had 16 deg in the the dist and all in by 2600. 36 deg total. Initial fell where it might. I always set the timing at max adv travel. The only time I consider initial is if the motor kicks back against the starter. Then I just add a few extra deg in the centrifugal advance and still set it at max total advance. I also ran the vac advance which was absolutely necessary for drivability. 10 deg. An MSD or similar multi spark box helped out at low speed.

My car was 2600# and the 3000 10" converter was perfect for it. You may have to go to 3500 with the heavier car. I still use the 3000 conv with the 6-71 and it works even better.

Mainly I'd ditch the 750 dp and get a 750 vac sec 3310-3, get a set of springs, a set of accelerator pump cams, and the next 3 larger shooters for beginning tuning. Get the dist recurved for a faster advance and a good multi spark ign box. Install a vac gage so you can see what is going on and determing whether you wil need to change the power valve.

I'm also a big believer in the AFR gage and a vac gage. It would have taken all summer to sort out this motor were it not for the AFR. Instead it only took a couple weekends. Most of the time I was chasing carb parts for testing.

So far you have been given everything you need to make this motor run by the guys here so let's see you make it run.

One last thing, if you must run that DP then add a spring to the secondary and learn to stay out of the secondarys untill 3500+ rpm. It will add a new complexity to driving but that's what you will have to do to make it work.
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Old 04-11-2010, 12:59 AM
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750 DP should be fine on this engine, but like nearly all applications will not be correct with out of the box parts. As has been stated you need to increase the shooter size and possibly the pump cam size also. That "bog" you have is a lean hole in the fuel curve, when you slam the throttle open the engine takes a big gulp or air, if the accelerator pump shot isn't big enough to "cover" this gulp the engine will struggle until the airflow through the carb is fast enough to pull enough fuel through the jets to run correctly.

Lots of people think this type of "bog" is from the carb "dumping" too much fuel and "drowning" the engine but this is very rarely the case, 99.9% of the time it is too little fuel.

The beauty of the Holley design is that it is easily and quickly able to be altered to exactly match every engine combination.

People need to realize that every carb, no matter who makes it, will rarely be correct in as-delivered form. This is why there are hundreds and hundreds of different Rochester Q-jet carb #'s, every one has been fine tuned by the factory for its specific application. Holley gives you the basic carb, it is up to you to configure it to work for your application. Holley's modular design makes it easy.
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