Holley hot stop fuel perc issue

[DanEC]

I went through both carbs today and set all floats per Holley tech sheet except I set all of them 1/32 inch lower. Car runs fine but after a test drive I'm still getting wet primary throttle plates after about 10 minutes heat soak. Fuel bowls temp is under 120 deg F, engine in running about 180 F. I always open the hood after a drive.

Today I finally noticed some spitting and bubbles from the shooter nozzle so now I'm sure that's where the fuel is originating - the accelerator pump circuit. I tried to shoot the pump cover with my IR gun but of course it's pretty much out of sight down there. Best I could tell it was around 140 F.

Any ideas on how to stop this? I'm running a 1/4 inch composite insulation base gasket now and that is pretty much all I have room for. I guess I could wad up some aluminum foil and stick between the housing and the manifold but I'm hoping for something a little more elegant.

Also - here's a weird one. I grabbed my S&H air cleaner from the front carb and removed it and found the whole under side was covered in an oily substance. It was clear with a reddish tint and no odor. I removed the top and found the K&N air filter and the inside of the base coated with the same oil. It's apparently leached out from the filter which are about a year old and only have a few hundred miles on them. The other one was OK. I guess they must have really soaked this one when they treated it.

[PeteF]

I seem to remember that the accelerator pump has an adjustment on it. Have you checked that? It's a spring loaded screw that can be adjusted with carbs on the car, using a feeler gage to get a specified clearance on the lever arm. I think the Holley website has a video.

[rodneym]

DanEC,
I've had similar issues with my 660s.
I went at least 1/16" lower than sight plug.

[lippy]

I tried a ton of insulation on the lines, spacers, etc. My conclusion is that it works fine with summer gas, but with crappy CA winter gas, none of these fixes prevent percolation after heat soak, and hard warm starting.

[vatdevil]

Try blending in race gas or ethanol free pump gas.

[DanEC]

I'm about 98% certain the fuel is coming from the acceleration pump shooter - which makes sense with only the primary plate getting wet. Unfortunately the accelerator pumps are down underneath the carb getting radiated with heat off the intake manifold. They are definitely getting a good heat soaking. I set the floats a little lower but I'm not sure that will do anything about the heat buildup in the accelerator pump.

I'm trying to figure out if I can fabricate an aluminum shield to slide under the bowl and accelerator pump and secure with the base plate bolts. But it's pretty crowded and tight under there with a LR intake and a breather sitting under the rear-most carb fuel bowl.

Fortunately it's not a lot of fuel peculating - but just a few drops of fuel can set up a gas stench that lasts for a day or more in the garage.

I sent the Holley tech site an email asking for any solutions - it looks like it will be next week before I get a response.

[Learjockey1]

I have the same problem with two fours with and HM manifold. I was going to try the DEI Design Engineering Inc Cool tape under the float bowls. I don't have enough room for an insulator block or aluminum shield under the carbs. Just enough room for a thin gasket and 2 1/4 " blue thunder oval air filter and top which clears hood 1/8". I read a street rodder article on this method. Worth a try. DEI has some nice products to cover and insulate fuel lines etc

[DanEC]

Thanks - I'll check them out.

[Gaz64]

It all depends on what accelerator pump inlet valve the bowls have, AND what outlet check valve weight is under the nozzle.

http://www.bob2000.com/acccheck.jpg

[DanEC]

I was wondering if a heavier check ball/weight might help this. But, I also wonder if that might affect the whole set up of the fuel shot slightly when the throttle is opened. I haven't been inside the accelerator pump on these carbs so I'm not certain what type of valve it has.

Lacking a better idea at the moment, I may just stick a square of EZCool insulation on the bottom of the accelerator pump cover with a few dabs of Strip Caulk and see if that works.

[Tom Wells]

Dan,

Gasoline with alcohol seems to start vaporizing (not actually boiling) at about 140 degrees F. So the carb bowls don't need to get very hot to cause problems.

Without alcohol the onset of vaporization seems to move up to the 170-180 degree F area. A lot depends on brand, mix, quality, phase of the moon...

I'd try the suggestion above regarding non-alcoholic gasoline first to see if it helps.

Then there's always the switch to EFI to get the pressure up from 6-8 psi to 43.5 psi which raises the boiling point and can, in most systems return any excess to the fuel tank. Problem solved - at least that's what I went to after years of trying to beat the underhood heat related flooding.

Wells's corollary states that as the chemical substance laughingly known as gasoline gets crappier, the vaporization temperature goes down. And down.

Tom

[DanEC]

Since I resolved that the fuel percolation is coming from the accelerator pump circuit I experimented with building a couple shields for the pump covers from sheet aluminum. The angled corner on the top one is to clear my heater hoses.









I wish I could say this was an outstanding success but I still got a couple dribbles from the front carburetor accelerator pump shooter after about 10 minutes hot soak following shut down. It did seem to lower the temperatures on the accelerator pump covers when I read them with my IR gun but it's hard to get a clean reading on them. I would say it is quite a bit less overall, but it only takes a couple drops of fuel on a hot surface to create a fuel stench in the garage. I might try putting a small piece of EZcool insulation on the bottom of the shields.

[Gaz64]

Looking at your carbs, I'd put money on it the bowls have the late model umbrella inlet valve, it took over the early design because it would be way cheaper to mass produce.

The early bowls had a hanging ball inlet valve that would shut when the pump operated. The inlet valve was open at rest, so the pump circuit could not develop pressure.

The umbrella inlet bowl "may" have a .013 hole in parallel with the inlet valve. It's purpose is as above, allows percolation bubbles to vent to the bowl and not pressurise the accelerator pump circuit.

If it doesn't have the hole, I would add it, it makes the accelerator pump a little less sensitive to gentle opening rates, so the car may use less fuel, and not load the plugs up as much from light throttle driving.

[carmine]

Hello Dan.

Get rid of the composite gasket and replace with layered aluminum heat shield kit from Mr gasket.
I have tried same as you with composite gasket and aluminum shielding of bowls with minimal results.
The Mr gasket shielding eliminated the problem completely.

Heres the link; Mr. Gasket 3712, Mr. Gasket Carburetor Heat Shield | Mr. Gasket

[DanEC]

Originally Posted by carmine

Hello Dan.

Get rid of the composite gasket and replace with layered aluminum heat shield kit from Mr gasket.
I have tried same as you with composite gasket and aluminum shielding of bowls with minimal results.
The Mr gasket shielding eliminated the problem completely.

Heres the link; Mr. Gasket 3712, Mr. Gasket Carburetor Heat Shield | Mr. Gasket

Do you know how tall the gasket stack is? I only have room for a hair over 1/4 inch thick base gasket because I have a smooth hood. I considered the Mr. Gasket heat shield set up but with dual carbs and all the accelerator linkage and rear mounted breather I thought I would end up cutting the shields up into pieces.

Gaz64 - I haven't looked but I suspect it has the rubber umbrella seal in the accelerator pumps. On another forum someone suggested taking a dremel and grinding a very shallow grove from outside the umbrella to the relief hole under it. The theory was that it would allow vapor pressure to vent back to the fuel bowl and bowl vent but would seal off when the accelerator pump was activated.

I haven't had a chance to look yet but I wonder if Holley makes a composite accelerator pump cover that doesn't absorb heat so easily?

[carmine]

Stack approx 3/8 inch high.
I had hood clearance issues as well so i eliminated 1 gasket and 1 of small aluminum shield which brought total stack down to approx 1/4 inch.
The larger shield can be notched out in order to clear any linkage obstacles.
In your case you would be using 2 kits because of dual carbs then trim excess on large shield that overlaps between carbs.

[DanEC]

Carnine - thanks, I will probably order a couple and try them.

[Gaz64]

Originally Posted by DanEC

Gaz64 - I haven't looked but I suspect it has the rubber umbrella seal in the accelerator pumps. On another forum someone suggested taking a dremel and grinding a very shallow grove from outside the umbrella to the relief hole under it. The theory was that it would allow vapor pressure to vent back to the fuel bowl and bowl vent but would seal off when the accelerator pump was activated.

I haven't had a chance to look yet but I wonder if Holley makes a composite accelerator pump cover that doesn't absorb heat so easily?

Yes, that would work BUT extremely risky, way more accurate to drill a hole.

The place for the bleed orifice is already there just not drilled through.

.013 from memory, number 80 drill.

Gary

[zrayr]

Originally Posted by DanEC

I went through both carbs today and set all floats per Holley tech sheet except I set all of them 1/32 inch lower. …..".

thats not low enough. You can go a lot lower without ANY danger of draining the float bowls, especially if you install some main jet extensions with. the notched nytrofil floats. Even w'o that set up you can go lower on the fuel level.

Your engine will run better, and not run rich around a fast corner when you lower the fuel level some more.

I'll repeat what I said recently here, I'ver been working on Holley carbs attached to Ford engines for over 50 years. I haven't seen one yet that didn't run better with the floats lowered from the Holley specs. position. Don't take my word for it, try it and see,

Z.

[lovehamr]

Dan, Occam's razor being what it is I'd try what Tom was talking about. He's dealt with more heat/carb issues than most here because he's a little off center in his mountain motor builds. I'd try a tank of good marine gas (no alcohol) before I went to too much more trouble.

Hey Tom,