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Old 09-22-2011, 10:37 PM
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Tuning?

Got my 350 chevy broke in the other day and need to fine tune it now. it runs alright but has a rough idle and wont idle very good and then when i give throttle quick itll backfire through the carb... need some tips to fine tune it???

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Old 09-23-2011, 04:11 AM
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G'day Linez35,

Before anybody can make any finetuning tips/suggestions you will need to post at least some basic info. on your current setup.

Compression ratio, Camshaft specs, Heads, Inlet manifold and Carb info.. Distributor set up and your basic tune info. as it is at the moment i.e. Initial advance settings and idle speed, and total advance at say 2500-3000 rpm would be real helpful. Carb settings if you know i.e. jet sizes and metering rod sizes etc.. Once this is known there are plenty of members here that will be more than willing to advise.

Having said that, Backfiring through the carb when the throttle is "blipped" could mean a lean fuel mixture, not enough accelerator pump (resulting in a leanout) too much advance, sticking valve etc.. Also you didn't say if it backfires when under load (i.e. when driving) or simply blipping the throttle when stationary ?.

Cheers.............Barry.
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Old 09-24-2011, 01:02 PM
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it has a holley dominator intake, edelbrock carb 600 cfm, howards cam 284intake/284 exhaust 1700-5800 rpm range stock distributer no headers, dont know how to do advance or timing yet just trying to learn any help would be much appreciated thank you..
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Old 09-24-2011, 05:01 PM
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I would like to offer this advise-get a Haynes or Chiltons manual for your car and read it thoroughly. Lots of good technical knowledge to be gained . It helped me to get started when I was about your age.
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Old 09-24-2011, 05:06 PM
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Check you plugs to make sure none are fouled.
Double check your sparkplug wires to make sure you have the firing order correct.

Do you have a timing light? What about a vacuum gauge?
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Old 09-24-2011, 06:08 PM
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dont have a timing light need to get one.. i have a vacuum gauge the style with the pump up handle thing, where do i hook that too?
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Old 09-25-2011, 09:24 PM
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G'day Linez35,

Yes you are now at the point where you need to buy or borrow a timing light if you don't have it already. You can't set the timing properly without it.

If you are going to buy a Timing Light, make sure it is Xenon globe type, very bright for use in daylight conditions. Many cheap lights are too dull and you need to make the work area over the motor dark so that you can see the light on your timing marks.

A Timing light with an Advance check function is a good option and better still is a Timing Light with Advance Check and Dwell check. Look for a Timing Light with a Rubber Nose piece so that when you get too close to the rotating fan, you only chop up the rubber not the timing light. Trust me, you will do it at least once !

You could also do with a vacuum gauge, not the pump up style like you have. That one you have is used for testing vacuum circuits for leaks and function. Vacuum Test gauges are quite cheap, under $20. Many can be used for testing fuel pressure as well. They are a simple bit of kit, and great for diagnostic work, especially for tune ups.

Buy the Best Tools and Equipment you can afford and look after them. Good Quality gear will last for life.

Thought for the day:- My old man (sadly departed over 20 years ago) always used to say to me "son, Never loan out your Tools or Woman, not even to your best friend, as they will always come back F&@%*D"

Also the suggestion of a Service manual is quite correct, buy or borrow one that covers your era of motor. I naturally assumed you had one ?.

Once you have these, you are good to go on basic tuneups on most Spark Ignition engines. There are also publications on Basic Tuning principles, perhaps invest in one of these as well. Knowledge will never go astray.

Can't advise much more than that until you get some basic equipment. As far as your engine mods, that is quite a "tuneable" package.

Cheers............Barry.
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Old 09-26-2011, 01:20 PM
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First thing is, dial in the carburetor according to this Edelbrock Owner's Manual. Go to Walmart and get a thin, white 3-ring binder, along with a 3-hole punch, a magnifying glass and a black magic marker if you don't have one. My plan is to get you started making your own reference materials and this will be the first one you make.

Click this link and print off the 23 pages in the manual, hole-punch the pages and put them in thin, 3-ring, white binder. Write on the front, "EDELBROCK OWNER'S MANUAL" with your magic marker pen. You will then have a reference manual to Edelbrock Performer series carbs and Thunder series carbs that you can use for years to come.
http://www.edelbrock.com/automotive_...ers_manual.pdf

Note that on page 23 is a data log. You will need to partially disassemble the carb to get to the main jets and remove them to record the numbers off them. The magnifying glass will be helpful in properly reading the small stamped numbers on the primary and secondary metering jets and metering rods and the seat number. Make certain you have a new cover gasket to go back together with.

Check the fuel tank for junk inside. Clean as necessary. Install or change the inline fuel filter. Minimize rubber fuel line. It will disintegrate on the inside of the rubber hose and can cause tuning problems if allowed to get into the fuel bowl. Use as much hard line as you can in the system. Lengths of 3/8" brake line from AutoZone work well as fuel line in a stock or slightly warmed-over system and rubber line should only be used in very short pieces (1"-2") to join two pieces of hard line together or to begin a line from a pump, for instance.

You absolutely need to know the fuel pressure that the carb is receiving from the pump as you are driving. Temporarily mount a mechanical, liquid-filled, 0-15 psi fuel pressure gauge onto the cowl of the car/truck. Use duct tape or tie wraps or whatever you can find to tie it to the cowl. This is only a temporary installation that will allow you to read the fuel pressure through the windshield while you drive and allow you to make changes to the pump or system to limit fuel pressure to 5 psi. Let me say this again, EDELBROCK, CARTER AND ROCHESTER CARBURETORS WILL NOT TOLERATE FUEL PRESSURE ABOVE 5 PSI. More pressure than that will overpower the needle and seat and blow raw fuel into the intake manifold. If that is happening, you will have a tuning nightmare that you'll never get fixed.

If you are using a mechanical fuel pump driven off the camshaft and the pressure is over 5 psi, replace the pump with a bone-stock one from the local Chevy dealer or AutoZone or other parts outlet. Pressure regulators don't work well with mechanical pumps due to the pulsing action of the pump. An electric pump utilizes a regulator well because the pressure is steady, not jumpy like a mechanical pump.

Once you get the carb and fuel pressure dialed in, begin work on the ignition system. The cam you are using will want about 16 degrees of lead at the crank. You will need to change the centrifugal advance curve in the distributor to limit total advance to whatever your combustion chambers call for. Early production heads will run better with 36-38 degrees initial and centrifugal, with maybe another 10-12 degrees of vacuum advance. It is not at all uncommon to have a total of 50 degrees of advance when cruising down the road under a light load. Later cylinder heads with a more efficient chamber, such as the L31 production head and most current aftermarket heads will want more like 33-34 degrees initial and centrifugal. It doesn't take as long to light off the mixture and get a complete burn with the later, more efficient chambers.

If there is ANYTHING that you don't understand about what I have said here, PLEASE ASK.

If there is ANYTHING that you don't understand about tuning the carb, call up and talk to the tech guys at Edelbrock....
http://www.edelbrock.com/automotive_.../contact.shtml

Don't be bashful. You need to know this stuff dead nuts, well enough so that you could teach it to someone else. That's what I'm doing with you.

I think you're going to be disappointed with your cam choice unless you actually do have a 9.75:1 to 10.0:1 static compression ratio, which I doubt. When you get it all together, dialed in and running as good as it will, do a compression check and report back here with the numbers. Here's how to do a compression check.
1. get a screw-in spark plug adapter type fitting that screws into the spark plug hole and a pressure gauge that reads to 200 or 250.
2. Disconnect the coil wire so that you don't have a fire during this operation.
3. Remove all spark plugs so that the motor will turn over easily with the starter.
4. Wire the carburetor primary throttle blades wide open so that the engine can breathe.
5. Turn the motor through 4 or 5 "chuffs" or until the reading on the needle of the gauge stabilizes for each cylinder.

Here's a down and dirty chart that will give you an idea of how much static compression ratio you will need for different cams to operate properly....
http://www.crankshaftcoalition.com/w..._compatibility

Last edited by techinspector1; 09-26-2011 at 01:46 PM.
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Old 09-26-2011, 07:31 PM
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okay thank you.. im really lacking the timing part of it i dont really get some of it.. like 16 degrees at the crank i dont understand what that means and yes the heads arent very good ive got another set that have smaller combustion chambers would that help a little?
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Old 09-26-2011, 11:21 PM
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Quote:
Originally Posted by Linez35
like 16 degrees at the crank i dont understand what that means
When you set the timing on on a motor, you begin with what is called initial lead or crankshaft degrees of ignition advance, before top dead center (TDC) when the spark plug fires. You adjust where the plug fires in the ignition event by turning the distributor housing in the intake manifold hole, with a timing light connected to the #1 spark plug wire (driver's side, closest to radiator). Here's a neat little tune-up animation from boxwrench.net.....
http://www.boxwrench.net/specs/chevy_sb.htm
You can see that by turning the distributor housing counter-clockwise, you would allow the rotor to hit the next terminal sooner. That would advance the spark event relative to crankshaft rotation. If you turned the housing clockwise, the rotor would hit the terminals later and start the fire later, relative to the crankshaft position. If the manual calls for 8 degrees before top dead center (BTDC) and you time the motor at 10 degrees BTDC, then you are advancing the spark by starting the fire sooner. If you time the motor at 6 degrees BTDC, you would be retarding the ignition timing, starting the fire later. There is a very delicate balance where the motor will like that particular advance or retard and will reward you with increased performance over any other setting. That's your job as a tuner. Do all your research and talk to other racers at the drag strip, find out what works well for the majority of them and try it on your combination. A savvy tuner will change only one thing at a time, then check the results.
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Old 09-27-2011, 09:29 PM
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thank you very much you have been very good answering my questions and ill take what you told me and use it..
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