Len and Mindy's Millennium -Money Pit!--Page 6
Welcome Corvette Addicts!
Lar's Tuning Tips
November 2006 courtesy Corvetteforum.com
During the Tuning for Beer World Tour Event Series 2006, I've run across some common themes and problems during the tuning. Thought I’d share the most common performance tuning and upgrade issues that we typically see during the Tours. Here are some comments, tips, and things to look at to help you in your tuning and setup:
First trick to any tuning, and I emphasize this constantly during the Seminars, is to tackle the ignition system first. Never, ever, try tuning a carb without first having tuned the timing curve. 90% of all carb problems are ignition problems.
Get a good dial-back timing light and learn how to use it. This is the most valuable and indispensable tool you will ever own. Sears has them for $69.95, or you can get a nice digital one with built in tach through Summit for around $110. Invest in good tools, and get a timing light.
Unless your distributor has recently been rebuilt and set up by someone competent, your distributor is messed up. I guarantee it. Whether you have a 200,000-mile tach drive points unit, an HEI, or a brand new MSD ProBillet, the distributor does not have a good advance curve in it. Yank it out, shim out the endplay (setting shaft end play to .005 - .007” for cast iron units and .010 - .015” for aluminum housings), clean it up, and make sure the advance system is working right.
Most point-style distributors are missing the advance stop bushing (little rubber bushing on a pin under the cam advance plate). You can get a brass replacement bushing in Mr. Gasket kit part number 928G. Smack the bushing with a hammer before installing it to make it fit very snug on the pin. This kit also has springs that you can use for your advance curve. For HEI distributors, use the 2 gold springs in the kit. For points systems and MSD distributors, use one black and one silver spring.
Set up your timing to 36 degrees total advance, and install a set of advance springs that make the total timing come in around 2500-2800 rpm (faster if your engine can handle it without detonation). Disregard what the initial timing is – initial timing is irrelevant as long as it’s in a general acceptable range. For a performance engine, I like to see initial timing in the 16 – 18 degree range. Mild-cammed engines can get by with a little less. Big cammed engines like 20 to 24 degrees initial. But set total to 36 as a starting point.
Use a vacuum advance control unit that does not pull in any more than 16 degrees of vacuum advance, and make sure it is “soft” enough to pull its full advance in at the vacuum reading you get at idle. You can run off of either ported or manifold vacuum, depending on what your engine wants for best quality idle.
Set your carb up to the stock specs for the carb number you are using and make intelligent jetting choices from there. If you are running headers, free-flowing exhaust, good intake, etc., you can bump the carb jetting up about 2 jet sizes as a starting point for your tuning.
On Holley and BG carbs without secondary power valves, secondary jet size should be 8 sizes larger than primary jet size. The Holley Street Avenger carbs are jetted extremely lean, and respond well to this type of jetting mod.
On vacuum secondary carbs, run the softest spring you can find for the secondary. This is the short white spring in the Holley spring kit.
Beware of commercially rebuilt Q-Jets: These carbs are seriously messed up. Typically, we find that the common screw-ups are: Incorrect floats; cut/modified power pistons; incorrect jetting; and plugged idle air bleed holes. These issues must be corrected before the carb can be made to run right.
Make sure your Q-Jet secondary throttle blades do not open over-center. The secondary throttles should only open to the point that the top edge of the blades points directly at the lower edge of the air baffle. This is short of vertical.
On Q-Jet equipped C3 Vettes, the most common performance issue is that the throttle linkage does not allow enough travel to open the secondaries. Check throttle travel on any C3. Remove the floor mat to correct it in most cases. If this does not do enough, grab the gas pedal and bend it upwards until you can get WOT at the carb.
Holley and BG carbs flow idle fuel on the secondary side. Therefore, you must also have air flow on the secondary side at idle. All Holley and BG carbs have secondary idle speed screws. Use this screw to make the secondary side throttle opening identical to the primary side opening at idle, and make all idle speed adjustments equally to both primary and secondary sides.
Set Holley and BG float levels to the bottom of the sight hole with the engine hot.
Initial bench setting for Holley and BG idle mixture screws is 1 turn out from lightly seated. Make all idle mixture adjustments in small increments, adjusting all mixture screws evenly for best quality idle.
Give the car what it wants: Listen, feel and smell. The car will “talk” to you as you make changes, and you need to listen.
Happy tuning!!
Lars
First trick to any tuning, and I emphasize this constantly during the Seminars, is to tackle the ignition system first. Never, ever, try tuning a carb without first having tuned the timing curve. 90% of all carb problems are ignition problems.
Get a good dial-back timing light and learn how to use it. This is the most valuable and indispensable tool you will ever own. Sears has them for $69.95, or you can get a nice digital one with built in tach through Summit for around $110. Invest in good tools, and get a timing light.
Unless your distributor has recently been rebuilt and set up by someone competent, your distributor is messed up. I guarantee it. Whether you have a 200,000-mile tach drive points unit, an HEI, or a brand new MSD ProBillet, the distributor does not have a good advance curve in it. Yank it out, shim out the endplay (setting shaft end play to .005 - .007” for cast iron units and .010 - .015” for aluminum housings), clean it up, and make sure the advance system is working right.
Most point-style distributors are missing the advance stop bushing (little rubber bushing on a pin under the cam advance plate). You can get a brass replacement bushing in Mr. Gasket kit part number 928G. Smack the bushing with a hammer before installing it to make it fit very snug on the pin. This kit also has springs that you can use for your advance curve. For HEI distributors, use the 2 gold springs in the kit. For points systems and MSD distributors, use one black and one silver spring.
Set up your timing to 36 degrees total advance, and install a set of advance springs that make the total timing come in around 2500-2800 rpm (faster if your engine can handle it without detonation). Disregard what the initial timing is – initial timing is irrelevant as long as it’s in a general acceptable range. For a performance engine, I like to see initial timing in the 16 – 18 degree range. Mild-cammed engines can get by with a little less. Big cammed engines like 20 to 24 degrees initial. But set total to 36 as a starting point.
Use a vacuum advance control unit that does not pull in any more than 16 degrees of vacuum advance, and make sure it is “soft” enough to pull its full advance in at the vacuum reading you get at idle. You can run off of either ported or manifold vacuum, depending on what your engine wants for best quality idle.
Set your carb up to the stock specs for the carb number you are using and make intelligent jetting choices from there. If you are running headers, free-flowing exhaust, good intake, etc., you can bump the carb jetting up about 2 jet sizes as a starting point for your tuning.
On Holley and BG carbs without secondary power valves, secondary jet size should be 8 sizes larger than primary jet size. The Holley Street Avenger carbs are jetted extremely lean, and respond well to this type of jetting mod.
On vacuum secondary carbs, run the softest spring you can find for the secondary. This is the short white spring in the Holley spring kit.
Beware of commercially rebuilt Q-Jets: These carbs are seriously messed up. Typically, we find that the common screw-ups are: Incorrect floats; cut/modified power pistons; incorrect jetting; and plugged idle air bleed holes. These issues must be corrected before the carb can be made to run right.
Make sure your Q-Jet secondary throttle blades do not open over-center. The secondary throttles should only open to the point that the top edge of the blades points directly at the lower edge of the air baffle. This is short of vertical.
On Q-Jet equipped C3 Vettes, the most common performance issue is that the throttle linkage does not allow enough travel to open the secondaries. Check throttle travel on any C3. Remove the floor mat to correct it in most cases. If this does not do enough, grab the gas pedal and bend it upwards until you can get WOT at the carb.
Holley and BG carbs flow idle fuel on the secondary side. Therefore, you must also have air flow on the secondary side at idle. All Holley and BG carbs have secondary idle speed screws. Use this screw to make the secondary side throttle opening identical to the primary side opening at idle, and make all idle speed adjustments equally to both primary and secondary sides.
Set Holley and BG float levels to the bottom of the sight hole with the engine hot.
Initial bench setting for Holley and BG idle mixture screws is 1 turn out from lightly seated. Make all idle mixture adjustments in small increments, adjusting all mixture screws evenly for best quality idle.
Give the car what it wants: Listen, feel and smell. The car will “talk” to you as you make changes, and you need to listen.
Happy tuning!!
Lars
March 16, 2001-------------Here's a picture of the L-88 air cleaner assembly I'm using. This will mate with the air chamber in the hood to take advantage of the low pressure area at the base of the windshield. Should add a couple HP!
June 2001------------------Here's a picture of my current setup. A Holley 3310, 750cfm sits atop a Holley Single plane Street Dominator manifold. In the middle is a TD stepped 1 inch spacer. This should provide the additional height needed to meet the air chamber. Sitting on the very top is a Weiand classic air cleaner. Also added in this picture is the SS braided fuel lines with AN fittings.
This is a picture of the L-88 air chamber and air filter element. It is not yet bonded to the hood. This will attach to the underside of the hood as pictured.