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Old 06-24-2008, 06:19 PM
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vaccum too low?? 357 with comp 280H cam

I recently put together a sbc 357, put on a all new parts that contained flat top pistons .040 over, AFR 180cc fully CNC ported heads, comp cams 280H, crane cams 1.5 roller rockers, stock length comp cams chrome moly pushrods, edelbrock rpm air-gap intake, edelbrock 600cfm manual choke carb, and a Mallory HEI distributor. I now have around 2000 miles put on it so i'm wanting to start the fine tune process. I have a vaccum gauge installed, and it's getting its feed from the carbs rear full vaccum port. How ever the engine seems to have a low Hg reading from what i have read for a 230duration camshaft, as i'm only pulling 10-11Hg's at around 800-900rpm. I've tried playing with the A/F mixtures screws up front, but can't get much of a change. I'm running 18degrees base timing, and 34degrees total. I had the distributor runnng of ported vaccum, but recently switched it to full vaccum in hope of a change, but that wasn't the answer. Motors runs good at high rpm, doesn't seem to be the most responsive down low however. I can't hear any whistles of a vaccum leak, but i'm still wondering about this vaccum, is 10-11Hg too low for a 280H comp cam with 230duration??

The cam was put in straight up, i have also wondered an timing issue. its a 80's style 4 bolt main block, using a 6 3/4" harmonic

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Old 06-24-2008, 06:24 PM
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Quote:
Originally Posted by 357 Cutlass
I recently put together a sbc 357, put on a all new parts that contained flat top pistons .040 over, AFR 180cc fully CNC ported heads, comp cams 280H, crane cams 1.5 roller rockers, stock length comp cams chrome moly pushrods, edelbrock rpm air-gap intake, edelbrock 600cfm manual choke carb, and a Mallory HEI distributor. I now have around 2000 miles put on it so i'm wanting to start the fine tune process. I have a vaccum gauge installed, and it's getting its feed from the carbs rear full vaccum port. How ever the engine seems to have a low Hg reading from what i have read for a 230duration camshaft, as i'm only pulling 10-11Hg's at around 800-900rpm. I've tried playing with the A/F mixtures screws up front, but can't get much of a change. I'm running 18degrees base timing, and 34degrees total. I had the distributor runnng of ported vaccum, but recently switched it to full vaccum in hope of a change, but that wasn't the answer. Motors runs good at high rpm, doesn't seem to be the most responsive down low however. I can't hear any whistles of a vaccum leak, but i'm still wondering about this vaccum, is 10-11Hg too low for a 280H comp cam with 230duration??

The cam was put in straight up, i have also wondered an timing issue. its a 80's style 4 bolt main block, using a 6 3/4" harmonic
Sounds like a typical big cammed motor. Putting in about 4 degrees of advance on the cam would help, but it's just a big stick, that's the way they are.

Bogie
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Old 06-24-2008, 06:28 PM
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Install Rhoads lifters.
http://www.jegs.com/p/Rhoads+Lifters...10002/-1/10213
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Old 06-24-2008, 06:39 PM
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anything that can be done to the carburator to help me out, motor doesn't like to have normal idle speeds when cooled down, cooled down but still at summer time temps. I aslo have a 2800-3200 torque converter in my TH350-C, and as i let off the brake to take off at 3grand, there's is the, BUH type hesitation, clears out then takes off. Is this only the down fall of having a carb, is or sumthing not quite set. I've only been really hard into this for the last year, learned alot, but i'm still in the dark kindof when it comes to tuning.
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Old 06-27-2008, 04:30 PM
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Quote:
Originally Posted by F-BIRD'88
The more intial timing at idle the more idle vacuum you'll have.
For that cam I like 24deg at idle. To do this requires recurving the distributor advance curve ( shorten the mechanical advance travel limit) to around 12deg.
24 inital 36 total mechanical. then add 12-15deg of vacuum advance
24+12 +36 + 15 is 51 total combined at highway speed vacuum cruise.

Swap the primary metering rod power piston step up springs for a lighter tension set that keep the rods down in the jets (lean ( cruise mode) at idle. 4" vacuum rated.

The idle quality and throttle responce will be much better.
may need to change the accelerator pump shooters too.

so i need a tuning kit for the carburator that comes with metering rods and springs?? i pulled the plugs today and they are pretty black around the ring showing the idle mixture. I have already tuned the distributor, but havn't played with the limit on the advance curve, how is this done?? Just an adjustment of something, or modifications to something?? Also if i run 24initial, do i run manifold vaccum to the distributor or ported?? manifold vaccum would bring it up to 36degrees at idle, 24initial+12vaccum
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Old 06-27-2008, 05:03 PM
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also kinda lost on those advance numbers...24(initial) + 12(vaccum??) + 36(total) + 15(vaccum??)...the 24 + 12 + 36 = 48 total at cruise makes sence to me, but what is the 15??
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Old 06-27-2008, 07:19 PM
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Quote:
Originally Posted by 357 Cutlass
also kinda lost on those advance numbers...24(initial) + 12(vaccum??) + 36(total) + 15(vaccum??)...the 24 + 12 + 36 = 48 total at cruise makes sence to me, but what is the 15??
24* Initial + 12* Mechanical = 36* then the Vacuum Canister on Distributor adds 15* at idle & part throttle cruising speed = 51* Total Advance.
You'll need to limit the mechanical advance by shortening the travel in the mechanical advance slot in the distributor. You can also get a adjustable vacuum advance canister for the distributor for tuning. Initial timing is adjusted by turning the distributor. Once you open up the throttle under hard acceleration the vacuum drops and the vacuum advance is reduce to 0* so you have just the Initial 24* & 12* mechanical = 36* Total.

Last edited by SSedan64; 06-27-2008 at 11:27 PM. Reason: correction
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Old 06-27-2008, 07:47 PM
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Old 06-27-2008, 10:12 PM
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Sounds like you need to do some tuning with the carb, specifically the pump shot off the line. If shes bogging when you drop the hammer with that kind of stall speed then shes not getting enough fuel.

A good rule of thumb is you should get a nice blast of grey smoke out of the pipes on launch, if its black its too rich...no puff of smoke means shes not getting enough. Don't richen up the idle mixture to cover up the lack of fuel from the accelerator pump.

The loading up at idle when shes hot is likely from the throttle blades being too far open to idle properly at temp, the fix for a Holley is to drill the air holes in the blades a shade bigger to allow the blades to close so the idle slots can work properly.

I'm no expert at Carter/Edelbrock carbs but I think a set of needles and jets are what you need to get it tuned for your application, they used to sell a needle and jet kit back in the old days that had all the needles and jets in one plastic container so you play/adjust to your hearts content.

Start at the pump shot and she what happens, ramping up the advance rate to get it all in early is another good move but won't solve a bog off the line from poor fuel volume at low RPM. Remember thats a nice big rumpity cam and she needs lots of fuel to get over the hump at low RPM where the overlap is loading it up. If your getting a lot of standoff fuel floating over the carb at idle you might want to put a spacer in there to kill the backflow up the intake if its a problem.

You will probably have more luck with a more performance oriented carb than your Edelbrock 600, I prefer something in the 750cfm range for this size motor with that big a cam in it. Lots of custom carb mfg's out there with something more suited for that cam, Barry Grant makes a nice Holley knock off with all the tricks built into it that you can just bolt on and tune out of the box without having to become an expert on carbs to get it right.
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Old 06-28-2008, 01:56 PM
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Okay, so the hesitation out of the hole is lack of fuel, which I can see being understandable. Only way to change that is for a bigger carb?? I have a 750AVS that I will use for race days, but I'm guessing running the 750 won't be streetable with our gas prices .....I will have to try lighter power piston springs in the carb to help the idle out, as I herd to try this out a few times now. Anybody know what colour springs are in it now (stock springs, 600Edelbrock manual choke), to what what springs should be put in it?? I new this this carburetor stuff, i'm looking for a broken down diagram of the edelbrock carbs that shows all the parts but have had no luck. I'm also running a PCV valve, however i didn't know it would run rich at idle without one.

My timing as is, i have 20* initial, spring and wieght kit that has me at 16* mechanical advance, coming in at about 900rpm, all in by 2800rpm. Giving me a total of 36*. My Mallory HEI distributor has an adjustable vacuum advance, and I have it bringing in 12*. Should the idle be on 24* advance with vacumm off and the idle screw turned in the extra bit, or 36* with vacuum on?? As you said about the idle hunting with straight manifold vacuum. 36 total and 24 intial, i'm right in the ball park.

thanks again for the help guys
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Old 06-28-2008, 03:16 PM
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Quote:
Originally Posted by 357 Cutlass
Okay, so the hesitation out of the hole is lack of fuel, which I can see being understandable. Only way to change that is for a bigger carb?? I have a 750AVS that I will use for race days, but I'm guessing running the 750 won't be streetable with our gas prices .....I will have to try lighter power piston springs in the carb to help the idle out, as I herd to try this out a few times now. Anybody know what colour springs are in it now (stock springs, 600Edelbrock manual choke), to what what springs should be put in it?? I new this this carburetor stuff, i'm looking for a broken down diagram of the edelbrock carbs that shows all the parts but have had no luck. I'm also running a PCV valve, however i didn't know it would run rich at idle without one.

My timing as is, i have 20* initial, spring and wieght kit that has me at 16* mechanical advance, coming in at about 900rpm, all in by 2800rpm. Giving me a total of 36*. My Mallory HEI distributor has an adjustable vacuum advance, and I have it bringing in 12*. Should the idle be on 24* advance with vacumm off and the idle screw turned in the extra bit, or 36* with vacuum on?? As you said about the idle hunting with straight manifold vacuum. 36 total and 24 intial, i'm right in the ball park.

thanks again for the help guys
Hesitation out of the hole centers around a couple issues. One is mixture velocity within the intake and the other is mixture ratio. Ignition would be a secondary issue, though it could be a cause.

1. Let me start with mixture velocity:

A. When the throttles are suddenly opened the velocity of the mixture within the manifold and the intake ports slows. This is why multiple carbs and multi-barrel carbs often use either a vacuum controlled secondary throttle control or a flow control valve above the throttle plates. This feature keeps the air flow up within the intake system by simply not allowing the engine to see more throttle area than its prepared to use.

B. When the mixture suddenly slows, the main metering circuit also slows the rate of fuel being added to the passing air. So the resultant mixture will also fall lean as well as loose the velocity needed to ram mixture into the cylinder. The solution to this will be found in part 2.

C. When the throttles are snapped open, manifold vacuum diminishes which causes the vacuum advance to move toward a retarded position right when the engine is seeing a less dense and leaner mixture filling the cylinder. So with the sudden momentary drop in timing advance with this condition the engine has another reason to hesitate. Using more base advance and less in the vacuum can is a possible help. The vacuum advance can also be adjusted so it stays in with less vacuum being applied. Again from part 1.A.; the presence of vacuum controls on the secondary throttles such as Holley uses or the use of an air-valve in the secondary such as found in Q-Jets and Carters, help hold the vacuum up in the manifold keeping the vacuum advance from totally falling off.

D. A method of progressive opening of additional throttle area needs to be used and properly adjusted. Mechanical secondaries without an air-valve need to use a ratio or a progressive application of throttle by the driver to prevent stalling the air flow within the intake system. Or is a vacuum or secondary air-valve system it needs to be adjusted to a rate the engine tolerates. This can also include adjustment of secondary fuel enrichment circuits as well. So you've got to achieve pretty good knowledge of the the carb you're using.

2. The mixture falls lean when the throttle is suddenly opened:

A. Opening the throttle blades allows the velocity from the venturies to the valves to slow, thus pulling less fuel from main metering. The simple solution is to provide an extra shot of fuel, which is the job of the accelerating pump. Along with the other variables you're fighting with, it needs to be adjusted to provide enough fuel to prevent a hesitation. Keep in mind this isn;t the only adjustment, it needs to be done in concert with other adjustments

B. The power enrichment circuit will come on as manifold vacuum drops. But there is always a hesitation as it takes time for the mechanisms to respond to the pressure changes and more time for the fuel to overcome inertia and begin to flow. On the back side also because of inertia fuel will continue to flow after the need for it is past, so it's possible to have a hesitation from too much fuel. Usually the difference is when the mixture falls lean, the engine wants to backfire out the intake, while on the rich side it just seems soft on power.

3. Adjusting the idle throttle position.

A. The lower part of the venturi bore with the operating zone of the throttle blades there are to be found a usually round idle fuel supply hole which needs to be positioned below the throttle blade when the engine is at idle. Along side and usually a bit higher than the idle hole is a fuel transfer slot. This meters fuel between the opening throttle's idle circuit and before the main circuit begins to feed fuel. With the engine idling at it's proper speed the throttle blade should not expose more than a fourth to a third of this slot. If too much slot is exposed it will be difficult to obtain a clean idle and there will be a hesitation when opening the throttle further as there will not be sufficient fuel feeding during the transition period. This again is a matter of adjusting things to get proper idle speed and these opening aligned all at the same time. Some carbs include an adjustable idle air bleed screw which is most helpful with a big cam as it's possible to keep the throttles properly positioned and get idle air somewhere else, this is more typical of Carter designs. Holley provides an adjustment to the secondary blade stop on some models and a secondary idle system on other models to help with this problem. The Q-jet has a adjustable stop as well on the secondary blades. There is also the oft quoted process of drilling the throttle blades with a small hole to provide enough additional idle air to allow the throttle blades to be properly positioned relative to the transfer slot. Simple changes like moving the fuel level in the float bowl up and down can have an effect. So does the fuel pump, if pressure is marginal say 3 or 4 psi instead of 6 on hard acceleration the sudden decrease in line pressure between an engine driven pump and a rear mounted fuel tank as the dymanic acceleration wants to drive the fuel in that line back to the tank can starve the carb resulting in a lean mixture.

Whew, Bogie
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Old 06-28-2008, 04:24 PM
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Okay, the 750 will go on, its on the mud bog truck right but it can be swapped over. The distirbutor is not a problem changing, but once there is welding involved there is no going back ....i'll do the timing asap so it's eliminated from the question.
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Old 06-28-2008, 04:53 PM
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Here's what I see on this thread.

First =he has his cam retarded 4-5* from what CC recommends. He says he timed it straight up and CC recommends advanced.

Second =the vacuum of 10-11 @ 850 is in the ballpark for that cam with a dual plane intake.

Third =he says his timing is all in at 36* by 3000 rpm and he is launching at 3000 and has the DUH bog. So timing is NOT an issue for the 3000 rpm launch.

Fourth= Since he has an 600 AFB and not an AVS, then the secondaries are probably not opening too fast, since they are air flow sensitive, non adjustable and very conservative.
Neither should a 600 be too small at 3000 rpm. It might not make the best power above 5500, but it sure is not too small at 3000.

Carb mixture adjustments are necessary, start with the piston springs.
I'd go to the 750 AVS, and tune the thing for your engine.

When you get things in the ball park then you need a distributor curve proper for your engine. I suggest you go to the Performance Distributor's web site and read what Steve Davis has to say about distributor curves. They might be wrong, they have only been doing this since 1974.
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Old 06-28-2008, 07:46 PM
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Just a thought, but if taking off @ 3000, he may be already 1/3-1/2 way thru the accel.pump's stroke as well. Perhaps increasing stroke length / delaying activation for pump-shot, or increasing size of accel pump resevoir for larger shot could help as well?
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Old 06-29-2008, 03:01 AM
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Just thought I should clarify my post...

F-Bird is right about adjusting the secondary throttle stops first on a Holley to try and get the primaries closed further to get them onto the idle slot. Drilling the idle hole is a race only modification and is usually only done on very radically cammed engines.

I know I am going to get shot on this but here goes...

I have never liked Carter carbs for HP use, they seem to be a tuning nightmare with so many variables to adjust and they all overlap so it always seems to be a constant source of frustration to get what you want. The CFM ratings are bunk and a 750 Carter carb flows what a 600 Holley flows on the bench. They work fine for street driving on mildly cammed engines but as soon as you throw a big cam at them it becomes a tuning nighmare. They probably get better mileage on the street due to their built in complexity but when it comes to tuning a big cam engine I toss them in favor of a Holley type carb.

Its not like its impossible but once you do finally get it finely tuned the weather will change and your back to square one. I've had enough Carter carb fires from a little sneeze on cranking from lots of initial advance that I just don't trust em anymore.

Generally they are a pain in the butt...

Ok go ahead guys, I got my flak jacket on now. Let me have it!

One thing is for sure, once you do get it tuned and running nicely and spent a month of changing needles and bending rods to get the pump shot right you will get a chance to do the same with a Holley carb and get it done in one afternoon with only a pump cam change and main jet swap.

Sorry guys...been there done that.
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