lars

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 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

CoupeKeeper

Thanks for sharing this info, Lars
Steve

Plastic Pig

Nice post.

This info should be made a 'Sticky'

ctjackster

that's a keeper

Johns_65Vette

Lars, Thanks for the great info. I have a question though. Would doing a compression check, before starting any tune up work, be recommended? Just to make sure all cylinders are operating at peak? (within 10% ? of each other)

Sky65

Thanks

lars

Obviously, there is no sense in attempting to tune an engine with internal mechanical problems - ring blowby, camshaft wear and leaking valves will affect manifold vacuum, and this affects all operating systems outside the parameters of tuning. Likewise, you need to make sure you have plug wires in good shape and spark plugs that are actually firing before starting a tuning and setup process.

If the engine is firing on all 8 cylinders, is not blowing smoke out the exhaust, and is not pumping oil up through the PCV system, it can generally be tuned to a very nice level of performance. If you do decide to check the cylinders for integrity, I much prefer a cylinder leakdown test over the more common compression test: The leakdown test will give you much more data and info on condition, and will help you troubleshoot the problem if there is one.

CoupeKeeper

Lars,
Could you please post a brief discription of the cylinder test procedures.

Thanks again
Steve

lars

Here's a good article that covers the usage and benefits of a leakdown tester, as well as how to build one:
http://www.motorcycleproject.com/mot.../leakdown.html

mrg

Thanks for posting this info, lars! .. This will come in real handy. I'm in the process of doing a points distributor refresh. New Vac can (VC1810), shims (.005 end play), new shaft bushings, new cam driven gear, and the Mr. Gasket advance weight/spring kit were installed. I just reinstalled the distributor so I could do an initial engine fire-up. One thing I noticed via the timing light flashes on the balancer is that the timing marks don't jump all around like it used to do.

The pins that the advance weights pivot on are worn. I have replacement pins. I was wondering: What is the best way to remove and install these pins? ..

Old Blazer

Lars... On Holley carbs with secondary power valves, should they be the same size as the primary?

lars

The pins are brazed in place. You have to heat them up to unbraze them, pull them out, and re-braze the new pins.

On carbs with secondary power vlaves, the jetting spread between primary and secondary side is either nothing (same size, aka "square jetted") or a very small spread, like one or two sizes.

capevettes

Thanks for that info Lars. If you ever get to Cape Cod I have a nice big room over my Barn for you to stay with some Corvettes below that would certainly run better after you worked some of your magic. The beer is always cold

6T5C2

You should look into hosting a North East tuning meeting with Lars. I would definitely drive up that weekend from NJ. I'm sure we would all chip in for everything. Might be worth a shot.

Dad's '66

Get one put together, you will enjoy it!

SMR 67

I would volunteer to host but don't have the right facility, but I'll chip in and attend.

Lars, thanks for the helpful post.

mrg

[QUOTE=lars]The pins are brazed in place. You have to heat them up to unbraze them, pull them out, and re-braze the new pins.

I've got one of those oxy/propane rigs that should do the trick .. Thanks, lars ..

C64BOB

Secondary jet sizes on Holleys with power valves depend on the size of the power valve restrictions (the holes in the metering block behind the power valve). The "jetting" is a function of the total area of the jet and power valve restriction combined. Some Holleys use bigger secondary jets and smaller power valve restrictions. Also holleys with airhorns and choke plates will flow less air thru the primaries versus the secondaries and should have the jetting compensated for this.

lars

Good comments - that's correct. As a general rule of thumb, most PV restrictions are equal to about an 8-size jet increase when the PV is open and flowing, but this does vary a little depending on carb design. For this reason, I always recommend starting off any tuning with the stock jetting for the carb number, and make corrections from that baseline if there is reason to do so.

Old Blazer

Thanks for the replies. I can use all the info I can get.