jimmies63

I've got a bone stock, fully restored, all-original 63, maybe 150-200 miles on it, and in as little as 30 minutes in 60 degree weather, I get fuel boiling after shut off.

Everything is 'new' - well, rebuilt date correct at least. This has to be essentially a brand new 1963 corvette.

Did new cars do this? You can hear the gas boiling like a coffee maker!

I read all kinds of posts with people trying to solve it with spacers, shields, ductwork, fans, drive with the hood open, put in a gas cooler..... No way people did this in 63 with a brand new corvette.

Something else going on here. What am I missing?

Is there a way to solve this without bubba-ing the carb?

65 vette dude

I've never had a problem with it before, and I live, and was born in South Florida. The best way I know how to get rid of, or prevent percolation is to replace the intake manifold gasket, with a F.I. intake gasket, that will block off the exhaust crossover that's under your carb. Of course then you will need to put an H,or a X pipe behind your tranny, to tie your exhaust pipes back together again, or you will have this funky exhaust sound.

66jack

Also...

Is the splash shield still in place on the bottom of intake?

claysmoker

Take a look here, it will shed light on your problem even though you aren't at high altitude like I am.

http://www.eps-hane.com/techtips5.html

The gas we run now has different properties than when our cars were new.

63atlast

A simple half inch "non-metal" spacer solved my problem and is hardly noticeable.

Beggers

Agree, this is all it took with my 63 A Mr. Gasket carb insulating spacer will do the trick as long as your timing is not too advanced and the engine is running at a normal temperature. Brian

Frankie the Fink

Percolation and vapor lock have been around since the dawn of carburetors - modern fuel's lower boiling point has made the issue more prevalent today. All of the above is good advice. Also check your passenger side heat riser valve at the bottom of the exhaust manifold and make sure its working - I wire mine wide open...this contributes to heat under the carb. The most effective fix is the F/I gasket as mentioned but you can try a carb spacer and you might get lucky. A phenolic or wooden spacer works best...but you have to watch hood clearance closely.

A $23 Harbor Freight I/R temp digital temp gun can be used to "shoot" your carb bowls when this happens and confirm percolation.

I've never had it happen that quickly in such low temperatures though - makes me wonder if you have something else going on. I've never had perc problems in 60 degree weather.

One other thing you might want to check is your fuel pump pressure (any vacuum gauge will also serve as a fuel pressure tester). Since your car is "all new" many of the repro pumps and rebuild kits produce over-pressure situations due to stronger springs/diaphragms....and fuel pump pressure can temporarily spike on engine shut off to make matters worse. Sometimes lowering float levels 1/16" helps too.

C-1 Generation 2

Possibly not for you since it seems your trying to keep everything correct. What I did on my 79 Z28 which had similar problems was use this heat shield: http://www.jegs.com/p/Mr-Gasket/Mr-G...rPage=90&pno=1 plus added two switchable mini fans to the base of the air cleaner. http://www.northerntool.com/shop/too...8151_200308151. All hot starting problems went away. I'm fortunate in that early 2nd Gen Camaros with California emissions came with a heat shield. I even stamped all the correct numbers on the heat shield. You can see the engine by looking on My Garage

Powershift

You probably have a tankful of "winter" gas, which has more butane and lights than "summer" gas……especially since you live in NY. Try filling up with a summer blend in the next few months and see if that makes things better.

Wire heat riser open, or replace heat riser with fuel injection engine exhaust spacer. Cheap $.

Install an insulating spacer under the carb. If the heat passages under the carb are open, you can plug them or install an intake manifold gasket that has the heat passages blocked. I did this years ago, and didn't like it, so I removed it and installed the regular gaskets. But it is worth a try.

Run a 180 thermostat. Check that your engine is operating at this temperature using IR gun.

Make certain that the intake manifold has the valley pan attached on the underside of the intake manifold.

Most of these suggestions/recommendations are also stated by others above. Pick the easy ones first.

Larry

MikeM

As previously stated, check the fuel pump pressure. First.

wmf62

mine 'boils' in the gas tank from the heat soak from the mufflers being directly under the tank; you can hear it when you take off the cap....

OR, it could be 'perc' from the EFI return line...

you pick..
Bill

Plasticman

As for being an "old" issue, we bought our 62 in 1999. Test drove for an hour in Wis., where we purchased it. I had it trailered down to SW Chicago suburbs, where we lived. Took it for a drive, and after about 45 minutes (note that it ran great during the test drive and after the trailer ride), we got gas (Amoco premium Chicago area oxygenated fuel which included 10% ethanol). 2 miles after that, it stalled pulling up at a stop sign. Got it fired up, and drove the rest of the way home. It would not idle at all (had to keep giving it throttle to keep it running), so pulled the air cleaner, and the fuel was percolating (boiling) into the carb throats. This is with a 160 deg. thermostat, on a nice mid Sept. day (mid 70's), with no other signs of overheating. Too early for "winter" gas. Tried other brands of gas (still Chicago area fuel), looked at the float settings, and "rebuilt" the carbs. Checked the fuel pump, and replaced it (only due to a small leak, and old one being of unknown vintage). No change.

To get it to run properly on that fuel, I blocked off the heat riser passage (FI type intake gaskets), added 5/16" thick intake to carb insulator gaskets (have dual AFBs), and added a full length Moroso heat shield under the intake. Have not had a problem since......

Plasticman

856666

I agree with Vette Dude about the gaskets.

There is lots of posts on percolation on the Forum - mostly is caused by all the additives in unleaded gas so the boiling point is lower, assuming you checked all the mechanical reasons.

My 65' would empty the float bowl after a run on a hot day and do the same while idling at a extended stop light on a hot day.

I had to install the FI exhaust manifold gaskets to keep the heat out of the intake manifold and while I have a slightly changed exhaust not at idle, it's not really funky.


Phil

AZDoug

It started in the mid 1990s when fuel formulas started to change.

As more cars went to EFI, perc in EFI cars wasn't an issue.

I never encountered hard hot start problems in the 1970s, and 1980s.

As far as WMF62 tank boiling, that is why my EFI doesn't return through the engine compartment, the return line is right after the regulator, which is right in front of the rear wheel, so my motor fuel line is essentially dead headed, which works fine as long as you know how to compensate for it.

Doug

SDVette

For you guys having the percolation: How hot is your carb getting?

Even after a good hard 30 minute drive, in 80 degree weather.. running a 180 T-stat, indicating 180-200 on the temp gauge, my IR gun shows:

- Intake at t-stat: 180
- Exhaust: 500+
- Overflow tank: 300 (it's attached to the exhaust manifold)
- Carb fuel bowl: 120

My carb stays cool for some reason. It's a standard WCFB on a standard intake.. No spacers.

Are you guys making that much more heat?

MikeM

WCFB's have a cast iron base which doesn't soak heat like an aluminum based carburetor.

I don't have my heat riser blocker in the intake and I use a 180* thermostat with my WCFB and I don't have any problems. Never have.

wmf62

Doug
my thought at the time for having the return line originate in the engine compartment was that the fuel would be constantly circulating through the fuel rail and therefore not be deadheaded like with a rochester system, and i still believe that to be a valid reason.

the only 'oddity' is the 'boiling/bubbling' sound eminating from the fuel tank, and other than 'odd', it causes no apparent problem.
Bill

Frankie the Fink

Percolation problems did not start in the mid-90s. The Carter WCFB had anti-percolation features as far back as the 50s to address the issue...

AZDoug

Well,

what about those Holleys and rochester Q jets i had (and have) that didn't perc over in the the '60'-'80s, that seem to perc over now? They didn't have or need airgap manifolds back then, either.

My '72 Corvette came with a heat insulator due to its higher T-stat rating for emissions, all the other Q'jet cars i had that were '60s vintage didn't need insulator gaskets with their 180-185" tstats, but they perc over now with the same tstat if not isolated.

My '60s cars all started right up, anytime, summer or winter, with no heat induced flooding, so either fuel is percing at a lower temp now, or the carbs made today don't have anti perc features, or the old ones have all been incorrectly rebuilt and now perc.

My 308 with its Weber 40 DCNFs will hot flood now, it didn't in the '80s.

Bill, the Rochester FI was an open dribble valve, your EFI injectors are simply closed when off,a nd can't leak unless they are defective. Warmer fuel at the injector is an advantage on EFI, warm fuel in the tank, is not.

Doug

Ironcross

What are/is FI exhaust gaskets?...and on the same subject what is a intake FI gasket...We sold over the years these brand name gaskets from Detroit Gasket, Victor, Felpro and Mc Cord....none referred to any gasket set specifically as a FI exhaust set....What about the intake, those intake gaskets include the dog house to the sub manifold and intake base