After a drive of 50 miles around town running errands yesterday in 100 to 103 temps I heard and could see via the site holes on my QFT carb the fuel boiling off in the float bowls. At the end of my trip the water temp was reading between 200 and 210 with the puller fan doing its job as I was running the AC. My habit following any lengthy drive is to pop the hood to help in cooing down the engine bay. Out of curiosity I checked for fuel percolation, and frankly was not surprised to find it occurring. The engine and bay was very hot. FYI, I removed the splash shields months ago to increase air flow.

I researched the threads/posts on fuel percolation and see some are addressing the percolation by adding Phenolic spacers. However, most applications in Summit and Jegs cite the following "Not for use with stock manifolds with heat crossovers, can withstand temperatures to 400°." I assume my stock manifold has heat crossovers. I bought the car restored, and recently changed the Holley 4160 R3810A carb out due to an incessant whistle coming from the front left barrel's idle bleed jet. I gave up trying to fix it. Tired of the annoying and embarrassing whistle I changed the carb to a QFT HR-680-VS Hot Rod Series. So far I'm very pleased with the car's performance.

My Vette is a '67 with the 300hp 327CI engine. As I only saw fuel percolating in the fuel bowls and did not see any fuel percolating in the carb's barrels as shown in the video associated with this post can I assume that after sitting for 24 hours there will be no starting issues due to fuel soaking the cylinder and/or entering the oil pan? I pulled and sniffed the dip stick when the car cooled down and could not detect any smell of fuel.

Do I live with this condition, or address it with heat spacers and/or carb heat shield?

 

The only spacers I've ever seen with that warning are the ones from CoolCarb that have some kind of plastic stuff sandwiched between two aluminum plates. The plastic will turn to snot if it gets too hot....I would NEVER use that. Wooden or phenolic spacers will work just fine. Holley even sells some that fit the Quick Fuel carbs. Here they are on my '67 Chevelle 396.

The longer ones that go all the way fore and aft under the fuel bowls work the best. I also use nylon washers under the carb mounting nuts to further reduce heat transfer. You will need to check hood clearance when you add spacers. The second pic shows how I do it.

The effects of mild percolation should be gone in as little as an hour of cooling down time....its just fuel boiling over which then puddles on your intake runners and causes a 'rich' starting condition. It eventually evaporates...

 

Thanks for replying with pictures. What's the saying, one picture (2 in your case) is worth a thousand words! As for starting after float- bowl percolation and boil off, my vette started without any incident.

Did your vette come stock with AC, or did you install an aftermarket kit?

 

Errr...the A/C in the '67 Chevelle is a Nostalgic Air install. Installed by the previous owner and marginal at best. I'll never own another one.

My split window has Vintage Air A/C that I installed myself. Freakin' awesome and have to turn it down after its on for a while - its that cool.

 

I added a Vintage Gen IV AC unit last year. It gets cold, but not "freakin" cold. I attribute that to the fact that I use it with the top down most of the time.

Frankie, what are your thoughts on installing intake manifold gaskets that block the hot gas crossover ports that run below the carb on the intake manifold in addition to using spacers? Good idea, bad? Would this create any new heat dissipation or build-up issues elsewhere? What about cold weather performance?

 

I used fuel injection gaskets that already block off the manifold heat riser passages and IMO that made the MOST difference on percolation issues on my '61. It did more than spacers and other techniques. Two issues if you do that though:

1) You must wire open or eliminate the passenger side exhaust manifold heat riser, and,
2) You might not like the change in the exhaust sound...this is why F/I cars had a crossover pipe under the oil pan - I added one on my '61.

Depending on the year/carb/intake manifold there may be two round holes on the intake manifold that you can put small "Welch" plugs in to accomplish nearly the same thing. I don't know if your Holley setup has them but I'm doubting it.

Cold weather performance: Just let the car fast idle a bit longer and everything will be fine. And even that is only needed when its really cold (not Florida cold)...

 

Addressing your last point above first, I plugged off the two holes in the intake manifold that allow hot gases to pass under the carb when I installed the QFT carb as it has an electric choke.

As to the heat riser, I don't think there is one as there is no mechanism at the flange point between the exhaust manifold and the exhaust pipe to attach a wire. My car has side pipes BTW. How can I be sure? What am I looking for?

 

The exhaust heat riser is just a clapper valve (much like a carb throttle plate) that activates by heat and a counterweight. You would see a squarish weight on a short arm at the bottom of the exhaust manifold that you can move with your hand - you might not have it..

 

Thanks Frankie, you've been very helpful and patient answering my questions. I checked, and there is no sign of any mechanism as you've described. There is an exhaust pipe flange and a metal gasket mating up to the exhaust manifold.

I had a heat riser go bad on my '76 Camaro, and it drove me nuts until I (Midas actually) figured out what was "clapping" in the engine bay!

 

Here are some pics of the heat riser and it is located on the passengers side attached to the exhaust manifold & then to the exhaust pipe:

 

Thanks for the picture. There is no heat riser in my car's exhaust system.

 

The second picture is OK except that the closest point for hood clearance is at the right side (passenger) edge of the air cleaner lid. Or, 90 degree position if you look at the lid through the windshield. Need to put another foil ball in that position.

I like to use cylinders of "dum-dum" (caulking strips) or Play Doh, or similar.

 

Thanks for your note on hood clearance locations. My kids are grown and gone, and so my Play Doh days are over! I'll have to resort to something else!

Here's a picture of the ports in the manifold I plugged before installing the QFT carb, and pictures of the carb.

 

If ok, a little digression here. .. Wondering what brand of air cleaner are you running on your 327/300? .. Are any spacers being used to raise the air cleaner base up slightly for throttle linkage clearance? . .

 

There is no brand on the air filter. The number stamped on it is 013102F1, which returns a big zero on an internet search.

I did not use spacers to raise the air cleaner, and there is plenty of throttle linkage clearance.

 

I used plumbers putty on top the air cleaner to check for hood clearance on my 65 327/250hp. I measured only about 1/8th inch of thickness that I could raise the carb with a spacer. I would like to put some sort of heat shield or spacer under the WCFB carb. I'm guessing the WCFB may be a little taller than an AFB or Holley, so any spacer will have to be very thin.

I purchased a Felpro #1205 intake gasket with the blocked off crossover ports but have not yet installed it.........it's on my to do list.

Doug

 

The tin foil works great, readily available, reusable and no sticky residue to clean off shiny parts.
Used the same method to check hood clearance on my aftermarket A/C compressor and
relocated alternator...

 

I used two, 1" pieces of 3/8" double-sided foam-insulated tape stacked on top of each other to form a 3/4" thick measuring gauge. I placed them around the air cleaner and closed the hood. None of the pieces stuck to the hood which indicates I have at least 3/4" of clearance. I may add one more level and check again as I don't want the hood to rub against the air cleaner should it rise due to expansion caused by the engine's heat.

Update - I added one more piece of tape bringing the total thickness to 1.125". I checked for clearance and none of the pieces stuck to the hood.

 

Little ***** of lightly-formed aluminum foil work very well for judging hood clearance too.

 

I'm running a 461 intake and a 2818 holley with the standard L79 air cleaner and have to shim up my hood to perfectly flush with the fenders to keep the filter from rubbing the hood on the passenger front with a 1/4"(I believe) Mr Gasket heat shield. More like Doug's clearence instead of the 1-1/2" clearence the OP shows with his foam strips

Hard to understand this much variation in clearence.