lars

After installing a new harmonic balancer on Mark's ("warrenmj") '66 L79 327 Vette, (see thread at https://www.corvetteforum.com/forums...blem-post.html) I thought it would be a good idea to give the thing a quick performance tune. While doing the balancer work, I saw several things that I didn't like, so I wanted to make sure the car was set up right... I had a feeling that it wasn't...

First clue that this car was not running right was the way the QuickFuel carb was set up. The throttle rod is installed in the top hole of the throttle lever, and 2 VERY strong springs are attached for throttle return. I have never seen a C2 or C3 Vette with enough throttle travel to fully open the throttles when attached to the top of the throttle lever:


So I pushed the gas pedal to the floor, and the throttle moved this far:


That's barely half throttle, and doesn't even crack the secondaries open. Here is the actual full throttle position as pushed open by hand:


Next item of notice is the distributor vacuum advance. It is hooked up the the "ported" vacuum nipple on the carb. Running ported vacuum is good if you want to lower emissions: It dramatically increases exhaust gas temperature and engine operating temperature, and it lowers hydrocarbon emissions. It can also be used on big-cammed engines that cannot meet the "2-inch rule" for vacuum advance (e-mail me for my paper about this) due to poor engine vacuum. But for a good-running and snappy street engine, manifold vacuum is usually the better choice:


The next thing I did was to check out the vacuum advance. The ethanol in modern pump gas tends to destroy the diaphragms in the vacuum advance units after a few years of operation. So every year, you should give it a "suck test." Here I'm sucking on the hose, and found that the vacuum advance was ruptured, leaking vacuum, and completely inoperable:


First order of business was to remove the carb to get it straightened out. Flipping it upside-down revealed excessive transition slot exposure on the primary side, which means that the primary idle speed is so high that the carb will be discharging uncontrolled fuel into the engine through the transition circuit. The transitions slots should not have any more than .020" of exposure, which will make them look like little "squares":


Another view showing the excessive throttle blade opening:


Compare that to the idle opening of the secondary throttles - the secondary side of the carb should be set to flow the same amount of air as the primary side in order to provide equal idle air and fuel distribution. The secondary side discharges idle fuel, even on a carb with only 2 mixture screws. On this carb, it's discharging fuel on the secondary side, but no air - the secondary throttle is fully closed:


To fix this, the secondary idle speed screw is given a turn. It is accessible from the bottom of the carb. QuickFuel uses an allen-head screw. Holley uses a slotted head screw:


Secondary throttle is now cracked open to reveal .020 of the transition slot, and the primary side idle speed was backed off to produce exactly the same throttle opening. This will produce very equal air and fuel flow through the intake manifold at idle, and will greatly improve idle quality and mixture screw response. CAUTION: On a Holley, you cannot set the secondary idle to expose .020 of the secondary transition slot. Holley has the secondary transition slot shifted upwards in the throttle bore, so excessive idle speed will result if doing this on a Holley. When setting secondary idle speed on a Holley, use the "light gap" method - this is discussed in my Holley and Speed Demon setup papers - e-mail me if you need info:


The QuickFuel carbs, and most Holleys, have a direct manifold vacuum nipple located in the front of the throttle plate. QuickFuel has one nipple for ported vacuum (in addition to the ported nipple in the metering block on the passenger side), and one for manifold vacuum. I'm going to use the manifold vacuum nipple when I re-install the carb:


Notice how the carb base gasket stuck to the manifold and the carb - this destroyed the gasket upon removal of the carb:


Here's a trick to avoid that in the future: Spray a light coating of "Pam" on your gaskets. Do it to the float bowl gaskets and the metering block gaskets on the Holley carbs, too, so you can do jet changes without ripping up the gaskets:


Next operation is to fix the throttle linkage issue. Many aftermarket carbs come universally equipped with Ford transmission kickdown linkage. The Corvette throttle rod cannot be installed in the correct, lower hole in the throttle arm without jamming against the Ford linkage, so it must be removed. Easy enough:


Ford linkage off, and ready for re-assembly of the carb:


Since this car will be used at an altitude of 6000 feet, the stock QuickFuel jetting is probably just a tad rich. Stock jet size is 66. On a stock car with no headers, I have found that a 2-jet size drop for this altitude works well. Popping the float bowl off to check the jetting revealed that somebody had actually beaten me to it: Jets in the carb were 64's, so jetting is good. Spray a little "Pam" on the gasket and put it back together:


Next item to fix in order to assure full throttle opening is the throttle link itself. Even with the link screwed all the way back to its shortest position in the bellcrank at the firewall, the link is still too long. For most aftermarket carbs, and for many stock Holleys, the factory bend in the rod can be modified to shorten the rod just the right amount:


Modified rod with a little more angle to make it shorter:


Modified carb and linkage installed on the engine. The rod is now installed in the correct hole in the lever, much closer to the pivot point (throttle shaft) to assure adequate throttle travel. Since the dual springs were extremely heavy, one spring was deleted. This produces much better "pedal feel" for the throttle:


Pushing the gas pedal to the floor now produces wide open throttle, and the secondaries are going to work. This throttle travel problem is VERY common on Corvettes, and most Vettes I work on do not achieve wide open throttle. It should be checked on any Vette you work on:


With the car running again, the next thing to check is the timing. Throwing a timing light on the engine showed that initial timing was set to 45 degrees, total timing was 61, and the vacuum advance was inoperable. No, I didn't mis-type those numbers... the timing on this thing was way out in the weeds, and it's remarkable that the car has not suffered detonation damage. Good thing the vacuum advance was inoperable and not adding more timing... So the cap was popped off the distributor to check the springs and to change out the vacuum advance. I like to pull all the wires to one bank off the cap so the cap can be layed off to one side. I label the plug wires before I yank them out:


The defective vacuum advance was a "B1" unit. This vacuum advance might work well on a Caprice Station Wagon, but it's way too stiff to work on any performance application. Defective or not, this needs to be replaced:


A very good general performance vacuum advance for mild street engines is the "B26" unit. It is sold by all NAPA stores under part number VC1765. The versatility of this vacuum advance for mild performance engines is also recognized by MSD, and they install this unit in all the performance "small" distributors with vacuum advance. However, the vacuum advance length is a bit "long," with many of these units pulling up to 18 degrees of vacuum advance. This is too much for modern pump gas, and it must be limited to 10-12 degrees. I do that using one of my VAC Limiter Plates. You can e-mail me for additional info on this:


All re-assembled with a good-spec'ed vacuum advance:


With the engine running again, the timing was set up to produce 20 degrees initial, 36 total (all-in by 3000 rpm), and 11 degrees of vacuum advance hooked up to manifold vacuum, thus allowing the engine to idle at roughly 30 degrees of timing. Maximum total combined timing at elevated cruise rpm will now be 47 degrees, which is perfect. Once that was nailed down, the engine was up to operating temperature, and the final carb adjustments could be made. The bench-setting primary and secondary idle speeds were almost perfect, requiring only a 50-rpm adjustment of the idle speed screw. The idle mixture screws were then equally adjusted from the bench-set 1-turn setting, and ended up at an optimum 3/4 turn setting. The setting previously, with the misbalanced throttle openings, was a massively rich 2-1/2 turns.


Road test of the car is now impressive: Idle quality is excellent, and the car has incredible throttle response. At wide open throttle, it pulls like a 350-horse L79 should, and it sounds and feels great up to redline rpm. This Corvette is back to being a Corvette again!



Lars
V8FastCars@msn.com

emdoller

lars

Post the results - I'd like to hear!
Lars

bigearl56

65hihp

How much do I wish you lived here in the Richmond VA area Lars? Seriously, a whole lot! You're good.
I would give anything to have a mechanic like you near by.....
oh well.

lars

Don -
One of the reasons for these posts is to show just how simple and easy most of this stuff is, yet how often it's completely overlooked. Anybody can do this if you know a few things to look for and a few setup tricks. By sharing some of this, I hope all of you are going to spend 2 hours and make your Vettes all run GREAT!!

Lars

GTOguy

Thanks, Lars. This post of yours should really be a sticky. It succinctly covers all the basics of an engine tune-up on one of these cars in terms so clear a 10 year old could perform the task. Well done!!
Jeff

lars

Very kind of you to say, Jeff - Thanks!
Lars

capevettes

Very comprehensive write up Lars. It demonstrates clearly what a few seemingly minor adjustments can do to make these cars run the way they were designed to run. Thanks for sharing.

Lotsacubes

Warren's gonna get a different car! Never having experienced the secondaries! He's gonna be grinnin' ear to ear. Great read and very educational. Thanks. Larry

Looking forward to Warren's first drive impressions.

lars

Unfortunately, it looks like Mark can't come pick up the car until Saturday, but I'll make sure he provides and posts up some driving impressions of his "new" car... if he thought it was running OK before, he should probably bring some adult diapers with him to drive it home now...

Lars

Nowhere Man

it also shows you that out of the box "upgrade" parts need tweaking to make them "right" Im sure there is a whole lot more cars out there that could use a pro tune up.

Tom Austin

Lars,

What an OUTSTANDING report - thanks for all of the details!

DANG . . . it appears I have the exact same (wrong) linkage on my carb - see photo below. Where do I get the correct linkage (I didn't see that in your write up above)?

DansYellow66

Great write up - that car should run like a completely different car now. You had me going for a minute there on that exposed secondary transition slot until I got to the note - this doesn't work on Holleys. Great information.

1Cerberus4u

Wow, lars, great write up.... Long this thread, have you ever worked on the newer EFI stuff? I have an Edlebrock Pro Flow carb that runs on vacuum and am having a dickins dialing it in....
Maybe a bunch of us will pitch in and fly you out east for a week!!

65hihp

A tip for you Lars, though I know you don't need it. Ask one of your kids to give you a Mighty-vac for Father's Day. Much easier than sucking.
https://www.harborfreight.com/mityva...ump-39522.html

lars

I didn't replace any linkage - I fixed the stock linkage as described in my write-up above. Complete description on how to fix it in the story...

No, I don't do any EFI work. I don't work on anything built after 1980... Lots of guys doing that stuff. I think there are 3 of us left doing carb stuff, so I'll stick with that...

I like sucking on the hoses because it gives me a good excuse to wash it down with a Scotch... And I'm an old guy, and that's the way I've always done it, so I'm not gonna change...I impress the younger guys with C3 Vettes by bleeding their brakes by sucking on the caliper bleeds with a vacuum hose until I get a mouthful of brake fluid. I spit it in the oil drain pan, rinse with Jack Daniels (which is not fit for serious drinking), spit again, and then wash it down with good Scotch. Do that at each of the 4 calipers in about 10 minutes, and the entire brake system is perfectly bled and functional...


Lars

warrenmj

aafadca

Yep. Mine isn't opening all the way either! (Holley). My throttle rod is threaded so I'll try that first..after I check the slots. It'll have to wait until tomorrow when I can put some heat in there.Quick question Lars. Since the Holley vacuum port is roughly in the same place as the Quick Fuel do you recommend moving the line to manifold vacuum also? Thanks
( Don't know how I ended up on that old thread Lars)

lars

Mark -
I hope the roads are dry enough for you to give this a hard-core checkout run... that would be fun! Not knowing how this ran before I'm very anxious about your impression. Maybe I made it slower...

Mark never actually asked me to do the tuning, nor really gave me permission to tear his car apart for the article and photos in this posting... I just did it. I told him I was going to tune it a little, but I don't think he really took me seriously... I'm such an ***...

Yes, the inadequate throttle travel, as noted, is very common. The best way to get another 20hp out of your Vette is to fix the throttle linkage. On C3 Vettes, this can usually be done by removing the aftermarket floor mat: I call this the 20 horsepower floor mat tuning trick... On C2's, the methods shown above are usually needed.

Be careful about the secondary slot exposure on a Holley - you can't set that carb up as shown above. You might want to e-mail me for the complete setup articles. You have to use the "light gap method" on the Holley.

The brand name printed on the carb is irrelevant to the ignition system. See my comments about ported versus manifold vacuum in my article above, or drop me a note for the complete vacuum advance article for a very detailed description of the system. ​​​​​​​