timmah22

Lots of research done over the last month. Knew nothing about carbs. I have a L79 with original aluminum intake and a Holley 4160 (only a few years old). I know holleys are prone to leak. Has the Blue Gaskets. Basic Gasket under the carb. Runs Great. Aftermarket mechanical fuel pump (I didn't install)...never checked the pressure. Valve seats done. 180 degree thermostat. Never overheats. Previous owner had a ugly thermashield on the fuel line to the carb. Said he had percolation problems occasionally since he had it.
Percolation starts after shutdown. Hard start after 30min to an hour of sitting. Not a huge problem, just a little flooded due to the percolation. Appears to leak from the main throttle shaft (both sides)on the intake. Seems to show from the "hot soak" about a day after. Float bowls set correctly. I hear the bubbling after a normal drive over 10 miles. Temperature on the laser gun (after shutdown) shows-170-180 on the intake. 150-160 on the main float bowl.
Done my research on RVP ratings on fuel. I feel if the rating is low (7.8) it might help, but knowing if the station has it is impossible. They seem to change over in June, but no telling when they fill up on the premium.
Here's been my attack on the problem (no relief)
-open hood every time on shut down
-Decided to try non ethanol 93.
-add 5 gal of 100ll to a full tank (runs a little better)

now what? It seems i need to cool the fuel, and deal with the crappy gas.
Carb spacer? Heat shield under carb? Looks as though the Divorced choke will need to be adjusted longer if i install a 1/2 one.
New fuel pump? (no problems when running)
Insulate the air cleaner bottom?
Insulate the breather tube on the intake?
any fuel additives out there to change the volatility (RVP)?

Help and experience is much appreciated!
Tim

magicv8

Lars just posted the solution to the leaky throttle shafts. You will find it with a search, or someone else may help.

I found it
But since you already have a shield, I'm not sure how useful it is

The fuel dribbling onto the manifold at the crossover location is caused by fuel siphoning out the throttle shafts after hot shutdown. This is caused by radiant heating from the engine warming up the fuel in the accel pump housing. When this warms up, the pressure in the accel pump increases, and it will cause fuel to be discharged out the accel pump discharge nozzle - it actually will look like somebody is pumping the gas pedal after hot shutdown. Once the fuel discharges, the pressure in the accel pump well drops, and it re-fills itself and then repeats the cycle. This squirting fuel hits the closed throttle blades and wicks out the ends of the throttle shafts, pooling onto the intake manifold.

The cure is to install a reflective heat shield between the carb and the manifold - the heat shield must be long enough to shield the accel pump well in the primary float bowl from direct radiant heating from the manifold. You can get a shield from Holley - I have a paper that shows how to modify and install the shield for best performance - e-mail me for a copy.

Also -
Don't use any of my papers that are located on the 'net. They are obsolete, outdated, and often altered (the paper posted above is all of these: outdated, obsolete, and altered). Obtain current and legitimate tech papers directly from me via e-mail request.

Lars
V8FastCars@msn.com

John BX NY

tim

On my 67 L79 the percolation issue was helped greatly by keeping the heat riser valve in the full open position with some wire.

Of course today's fuel isn't helping much either.

TheGanzman

I've got the same motor, and had the same problem, even after installing a Quick Fuel 600cfm replacement carb to banish that crappy Holley. I JUST finished installing a "Cool Carb" plate under the carb; also removed the intake and installed blocking plates to block off the heat crossover passage under the intake. My initial test, 2-3 hot restarts, seems to indicate that the problem is solved. This is no small job, though! The reality is this - you take off the intake to block the crossover/install new gaskets, then you figure - might as well replace the valve cover gaskets too. Hmmm...those heater hoses look kinda crappy...Two days later, I was done!

You'll see varying results w/the Cool Carb, but I gotta say that you MUST block the crossover passage on the intake or you will MELT the material within the Cool Carb plate...

On edit, I DID also wire the heat riser in the full open position.

timmah22

Thanks guys. Before I take off the carb to put on the heat shield, how do I adjust the choke rod to accommodate the new thicker gaskets? Bend it? Ganzman...after i wire the heat riser, what/where do you buy the crossover block? Won't this take the divorced choke forever to open? Will I be able to run without anything on the fuel line? Or should I cover that up too? (It ruins the nice look of the fuel line) lol... I'll let y'all know how it goes!

JohnZ

Yup, you adjust the choke rod by bending it, as noted in the Chassis Service Manual.

If you only block the driver's side crossover passage, the remote choke thermostat will still work.

Fat Mike

Ganzman - keep us posted how your car performs as it continues to getter warmer.

Frankie the Fink

On my Chevelle quick fuel with perc problems I bought a 4150/4160 phenolic 1/4" spacer and also used the Holley spacer/heat shield with extensions to protect the fuel bowls instead of the usual carb-to-manifold gasket. Hood clearance is beau coup tight but enough.

Solved my problems (and with an aluminum intake manifold to boot).

Probably around a $25 fix.

Matt Gruber

i'm a member of the fan club.
a $3.25 3" fan and a dryer duct. hard to get a carb hot with a cool breeze.....

Matt Gruber

Mine:
Heat riser blocked.
4 hole wood 0.2" extended 2" under bowls to deflect heat.
Cork gaskets under carb to manifold nuts.
180 T-stat (was 195)
So ifigured (wrong) that since it is breezy and only 70F, i won't need to use the 3" fan. WRONG.
Parked 45 minutes hood closed, it stalled on restart! Then had to rev it 3-5 seconds before driving off.
So the moral of the story, i'll switch on the fan from now on.

Fat Mike

Matt - can you post a picture or two of your fan / dryer duct setup? Thanks,

timmah22

Holley heat shield installed....had to bend the choke rod, modify the shield quite a bit, along with the thick gasket to accommodate my fast idle solenoid (for a/c). Heat riser valve opened. Drove 40 miles in 80 degree weather. After parked... the primary float bowl dropped around 15 degrees during hot soak. No percolation/leaks so far. Summer blend gas is coming around the corner in June so that will bring down the vapor pressure from 9 to 7.8 psi.
The only thing i didn't like was the open gaskets used on the heat shield. They don't isolate the ports. Thought it wouldn't run as well but it seems to run just as good!

wmf62

if the plate itself doesn't have a fully open hole, then you can use the regular 4 hole gaskets
Bill

Frankie the Fink

Sometimes you luck out and it doesn't take that much to stop perc.
You just need to get a few degrees below the dreaded boiling point

AZDoug

FWIW, an air gap manifold solved all the perc problems when I put my old L79 motor into my '37 Fored pickup. Used to perc all the time with a non gap manifold when i had it in the Corvette.

Further evidence you need to isolate the carb from heat. Spacer plus heat shield seems the way to go

Doug

C-1 Generation 2

Frankie do you have any part number for both? I recently installed a QF 650 manual secondaries on the ZZ4 but only using the small gasket. Great idle,cruising and acceleration but after sitting awhile it might take acouple attempts to start.
I let my air gap go with the old engine.Not original.

Frankie the Fink

The Holley spacer-gasket is:
http://www.jegs.com/webapp/wcs/store...erm=510-108-51

The 1/4" phenolic spacer is:
http://www.jegs.com/webapp/wcs/store...erm=074-85-154

Both items have 4 holes (not open) so that issue is avoided. Just to be PERFECTLY clear on what I did. I separated the two pieces of the first item and just used the bottom part with the long fuel bowl protection and the 1/4" phenolic spacer along with one 4-hole normal manifold to carb gasket (as shown). I was T-I-G-H-T on hood clearance so eliminating that little bit of the Holley part allowed me to go with the 1/4" phenolic and the combo did the trick (refer to photo in my post above). If you have hood clearance then just use the two-piece Holley gasket-spacer as is and get even more benefit. The Holley part is marked "FRONT" on one end so pay attention to that.

You'll need the longer carb studs (they come with the first item above); also consider nylon washers (available anywhere) under the carb mounting nuts (as shown in pic) they reduce a lot of heat transfer !

CHECK HOOD CLEARANCE with air cleaner AND wing nut on using some play dough or tin foil before slamming the hood down !

You'll have a lot going on under the carb now so confirm primary AND secondary throttle plates open smoothly with no sticking -- I had to file the phenolic spacer a wee bit just to make sure of this...no biggee.

C-1 Generation 2

Thanks for the quick reply. I shouldn't have any problems with clearance,the hood scoop(engine cover)that is attached to the engine allows a lot of space up or down.
I'll get that ordered right away and put this thread on my favorites for future reference in case a friend that's restoring 67 Chevelle 396 SS convertible runs into that problem.His Chevelle isn't stock and has a Dodge color near to Marina blue.

Lou64

Wait are you serious? I don't know where you put this?

Matt Gruber

i've tried various isolation "cures" over the past 20 years. They all seem to help some of the time, but, sooner or later the carb boils. They just delay heat transfer from a hot engine. The little fan brings in cool air. the duct is a few inches from the front bowl. It draws only 0.18 amps, so it won't kill the battery for days if left on by mistake.
I measured the bowl, 119F after 45min, hood closed.