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Lars’s Tuning Secrets: The Common Tuning Tour Observations and Fixes

During the Tuning for Beer World Tour Event Series, I 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 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 with Moroso part number 26140 (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, but most engines will respond best (and run cooler) with the vacuum advance attached to manifold vacuum.

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 an increase in primary jetting by 2 to 4 jet sizes with an 8-jet split to the secondary side.

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 GTOs, the most common performance issue is that the throttle linkage does not allow enough travel to fully open the secondaries. Check throttle travel on any GTO. 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
 
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GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I
 
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Lars’s Tuning Secrets: The Common Tuning Tour Observations and Fixes

During the Tuning for Beer World Tour Event Series, I 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 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 with Moroso part number 26140 (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, but most engines will respond best (and run cooler) with the vacuum advance attached to manifold vacuum.

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 an increase in primary jetting by 2 to 4 jet sizes with an 8-jet split to the secondary side.

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 GTOs, the most common performance issue is that the throttle linkage does not allow enough travel to fully open the secondaries. Check throttle travel on any GTO. 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
I would NEVER set a float level with a hot engine,,you will spill a little gas
 

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Discussion Starter #7 (Edited)
GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I
I've never seen a real GTO with F.I. Are you sure you're on the right Site...? All classic GTOs use carbs - F.I. was never used. And those of us who run them know how to spell it, too... :lol:
 

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I would NEVER set a float level with a hot engine,,you will spill a little gas
Nonsense. Float levels are always set on a hot engine. Just don't light a match when you're doing it. Gas spillage is not a problem - the engine surface temp is below 200 degrees. Gasoline does not ignite at 200.
 

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GOOD INFORMATION, but most of the people on this site do not have carborated engines they are most F.I

Is that like a carbonated engine? Gosh I never heard of a real Fuelie GTO. I just have three of those carbonated things.............:rolleyes:

Lars, good write up. Any tips for tri-powers? I am going to be dropping in a 421 SD with 1967 2.11/1.77 ported heads (yes we're using 428 pistons to align the valve reliefs) and am wondering how best to approach setting up the carbs; any pearls of wisdom?
Mike
 

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Hey Lars, great post. Normally, I take my engines to a pro to dial them in. I do a lot of work myself, but every time I have tried to dial in my own engines, they run worse, use more gas, and usually wind up pi--ing me off.

I had my son's 350 in his 69 chevelle rebuilt with all good performance parts from bottom up adn replaced all bolt on items under the hood. I couldn't believe how crappy the car ran when I got it back. I took it to a guy in the area known to have the best carb shop. It costed me $400, but it was probably the best $400 I spent. He found 2 different size secondary jets in a brand new Edlebrock carb. recurved the elect. distributor to match the cam, changed the gap on the plugs, and a few other tricks and the chevelle seemed like it had an extra 100 horsepower.

I have a question, what is the difference between manifold or ported vacuum? Is it the difference between taking it from the base of the carb or from the manifold?
 

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Lars,
Where do you connect to get ported or manifold vacuum?

Second question: I was checking my stock Q-Jet the other night and when I open the throttle, the secondaries don't open. I hit the throttle on a hard romp and they looked like they wanted to but just barely moved. After looking at the carb, I noticed the lever on the passenger's side of the carb (that keeps the rear flaps from opening) was not releasing. If I manually lifted the lever and hit the throttle, the 4 barrell opened up completely.

Is something wrong witht the release lever?
What makes the lever release so the 4 barrel will kick in?

Thanks for all the great info.
Dale
 

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Ported vacuum will be on the carb, one of the ports at the base of the carb. Manafold vacuum will be plumbed into the intake manifold.
The secondaries will not open untill the motor is warmed up and the choke is fully open. You should be able to hold the choke plate fully open and then that locking lever should move out of the way and allow the secondaries to open.
 

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Thanks, I don't think my choke is working because, it doesn't idle up at start. I have to keep it running when it first starts. It stays running after a few seconds, but never has a high idle. Could the choke cause the 4 barrels from kicking in?
 

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Hey Lars,

Thanks for all of the info. Another question: How are you at auto trannys? When I give my goat enough gas to down shift (I don't have to be romping hard even), it won't shift back into the higher gear until I am almost letting completely off of the gas. Like if I punch it and it downshifts into first, it is almost like I have the shifter pulled into first until I let completly off of the gas. But when I am just cruising easily, it upshifts perfectly. It did seem to shift a little better after I drove about 40 miles home from work the other night. I thought of maybe a vacuum leak, but wouldn't it have problems shifting altoghter?

Dale,
 

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When was the last time the fluid and filter were changed? If it`s a turbo 350 in there you could try to adjust the detent cable. There should be a little like a horse shoe sleeve thing on the outer cable at the back side of the bracket that holds it at the carb. Pull that adjusting sleeve up with a screwdriver or whatever then roll the throttle to wide open, let the detent cable slide in or out where ever it wants to be, then while still at WOT push the adjusting sleave back down to lock the detent cable in place. If that doesn`t help, check the vacuum hoses and module at the tranny for manifold vacuum. If it has vacuum check to be sure there is no tranny fluid leaking into the vacuum line, if none, then check inside the hose nipple there, some vacuum modules are adjustable with a thin screwdriver, you may be able to adjust it in or out to make it up shift better to your liking.
If it`s a turbo 400, check the kickdown switch at the gas peddle (electrical) to make sure it`s adjusted right and doesn`t stick in the kickdown position. I`ve seen some that stick and won`t slide easly back and forth
 
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