vaccum too low?? 357 with comp 280H cam - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 06-24-2008, 07:19 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 06-24-2008, 07:24 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,088
Wiki Edits: 0

Thanks: 4
Thanked 539 Times in 455 Posts
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
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 06-24-2008, 07:28 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,286
Wiki Edits: 326

Thanks: 831
Thanked 1,147 Times in 946 Posts
Install Rhoads lifters.
http://www.jegs.com/p/Rhoads+Lifters...10002/-1/10213
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 06-24-2008, 07:39 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 06-27-2008, 01:27 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,285
Wiki Edits: 0

Thanks: 6
Thanked 400 Times in 394 Posts
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.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 06-27-2008, 05:30 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 06-27-2008, 06:03 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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??
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 06-27-2008, 08:19 PM
SSedan64's Avatar
Registered User
 
Last photo:
Join Date: Dec 2007
Location: Macon, GA.
Age: 50
Posts: 6,016
Wiki Edits: 0

Thanks: 0
Thanked 46 Times in 46 Posts
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; 06-28-2008 at 12:27 AM. Reason: correction
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 06-27-2008, 08:47 PM
Registered User
 
Last photo:
Join Date: Jun 2008
Location: USA
Posts: 1,172
Wiki Edits: 0

Thanks: 0
Thanked 2 Times in 2 Posts
******************
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 06-27-2008, 11:12 PM
4 Jaw Chuck's Avatar
Hotrodders.com Moderator
 
Last photo:
Join Date: Mar 2002
Location: Manitoba, Canada
Age: 46
Posts: 5,095
Wiki Edits: 0

Thanks: 2
Thanked 114 Times in 92 Posts
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.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 06-28-2008, 10:15 AM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,285
Wiki Edits: 0

Thanks: 6
Thanked 400 Times in 394 Posts
You do not need to drill the primary throttle blades. (Ever)
Not on any motor (short of full race) that is street driven.
A super radical, full race motor with a full race cam maybe.
Not this motor. A holley carb only requires minor readjustment of the seconday throttle idle stop, the correct idle timing (with modified advance stop limit) and the right power valve that will stay closed at idle in gear.
Edelbrock carbs and Q jets idle with the sec throttles fully closed at idle.
Edelbrocks and Qjets use power piston springs to control the metering rods.
Lighter power piston springs are required with big cams for a clean idle.

Holleys share the idle airflow and fuel flow on all 4 throttles at idle.

All street carbs with a 3/8" PCV vacuum port on the base were designed, factory calibrated and intended to be used with a functional PCV.
The PCV provides part of the total of the idle airflow thu the carb at idle.
If you eliminate the PCV (even on a racey street motor), the carb will idle too rich and require further opening the throttles at idle to compensate.
T

iming curve for the 280H cam:
24 initial base at idle.

+ 12 degrees of mechanical centrifical advance
requires disassembley and modification (welding or stick something in the slot) of the advance pin slot length to
limit centrifical advance travel limit. (available bushings will only limit it to 18deg)

This gets you 36deg total mechanical advace. 24+12=36
Then you want 12-15deg of cruise vacuum advance.
Get the crane adjustable vac adv with stop limit disc. Install with the stop limit disc (12deg) and play with the rate (allen key) adjustment.
Most stock vac advance is 20deg. Too much with this modified curve.

I prefer ported vacuum source. Use what ever you like best.

24+12=36 36+12 to 15deg vac adv at cruise gets you 48 to 51deg total combined at high part throttle cruise.

The increased idle base timing will correct the (slow combustion) egr effect of the cam overlap at idle, allowing correct throttle blade opening and throttle response.

If you depend on vacuum advance for the required additional idle timing, the vacuum drops (and so does the timing) when you put it in gear.
The idle timing and vacuum fluctuates up and down with idle speed. This tends to make the idle hunt up and down and flair as you go from idle in gear to idle in neutral.
The timing also drops (with vaccuum as you suddenly open the throttle) right when you need it to rise.

A more radical cam with more overlap needs even more idle timing (and a shorter centrifical curve).
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 06-28-2008, 02:56 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 06-28-2008, 03:13 PM
F-BIRD'88's Avatar
Yada Yada Yada
 
Last photo:
Join Date: Oct 2005
Location: Ontario, Canada
Posts: 9,285
Wiki Edits: 0

Thanks: 6
Thanked 400 Times in 394 Posts
Everything you heard about, or assume is wrong when it comes ot carbs and fuel mileage.
A 750 is no more likely t have x fuel mileage than a 600 will.
For a high performance 350 a 750cfm carb is best overall.

You just don;t want to tear down the distributor and fix it. You want some one to tell you its ok as is so you don;t have to get dirty don't you.

you want 24deg at idle inital with no vacuum advance connected.
you want 36deg of total mechanical advance all in at 3000rpm ish.
You want an additional 12-15deg of additinal vacuum advance while cruiseing
adjust the timing with a timing light at idle and with the engine idling slow enough so you are not already on the mechanical advance curve.
rev the motor up and ensure that the timing doesn not go past 36deg at high rpm with vac adv disconnected.
reconnect the vacuum advance to ported vacuum and shut the hood.

You can improve the fuel mileage of the manual choke 750AVS carb by swapping the primary metering rods to dial in the part throtttle cruise air fuel rate.
Download the edelbrock tuning book and read it.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 06-28-2008, 04:16 PM
Registered User
 
Last photo:
Join Date: Apr 2007
Location: Seattle, Wa
Posts: 7,088
Wiki Edits: 0

Thanks: 4
Thanked 539 Times in 455 Posts
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
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 06-28-2008, 05:24 PM
Registered User
 
Join Date: Jun 2008
Location: Atikokan, Ontario
Posts: 8
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
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.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Comp Cams xtreme energy cam - big Block JamesRS Engine 7 11-18-2011 08:22 PM
Anybody dealt with this guy on ebay? Or through Dirttrackthunder.com? killerformula Hotrodders' Lounge 20 08-31-2008 06:12 PM
Cam break-in and zinc Frisco Engine 34 03-13-2007 09:22 PM
Split duration cam for low compression? newchevyman Engine 5 07-09-2003 01:43 PM
Cam Help black66 Engine 3 05-12-2003 07:15 PM


All times are GMT -6. The time now is 06:34 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.