2bbl still boiling fuel

[Blue71]

Could t find my old post so here goes an update. 71 stock Bronco. In my other post.....Had problem starting after running it. Fuel boiling in carb, Flooding, etc....well it’s not flooded since I put the 1/4” heat spacer in. So it’s 2bbl motorcraft with 1/4” heat spacer, new water pump, new radiator, timing set. Sometimes it still acts like it doesn’t wanna start when hot. Pretty common with this Bronco so I do the half pedal thing and she fires right up. I noticed the other day after 30 min of driving that the clear aftermarket filter just before the carb had a few bubbles in it so I felt it and it was hot and fuel seemed to be slightly boiling. I checked fuel line to see if it was close to the exhaust anywhere and it’s not. Any more ideas?

Thanks,

Blue71

 

[Apogee]

Do you have an IR temp gun or probe? That can make chasing issues like this more fun. I kept boiling the fuel out of the float bowls on my Holley on shut down, but a 1" phenolic carb spacer seemed to fix the issue. I'm not sure how much you're gaining with a 1/4" spacer, but it's gotta be better than nothing. I'm not sure if an aluminum manifold makes it better or worse, but FWIW, I'm running an aluminum intake.

 

[ryan97]

On my 76 with stock 2150 carb (I just rebuilt) it had the lead/metal gasket on the intake then the 1” aluminum carb spacer then a 3/8” rubber gasket them carb. Look at the stock carb spacer kits on RockAuto, O’Reilly etc

 

[Blue71]

I’ll check out the spacers. Might help. Did u have to replace the carb studs with longer studs? I’m Also running a 190 thermostat... that was what I had on the shelf at the time. Any chance a 180 would help any? I usually tun a 180.

Thanks guys and keepem commin

Andy
Blue71

 

[73azbronco]

I'd check pump is putting that air in.

 

[Pa PITT]

bACK IN THE EARLY 70'S WHEN I 1ST GOT INTO THE PARTS HOUSE BUSINESS. wE sold a tin metal gasket for mostly the Q-Jets .. The gasket was very thin like 1/16 or maybe .040 These gasket were shinny on one side & dual on the other side ... Seems the shinny side went toward the intake too push the heat back to the motor & away from the carb.

 

[Broncomt]

Another item that can hold heat and boil gasoline is rubber hose from fuel pump to carb. The rubber really holds the heat. I had same problem with my 68 stang, cured it by installing correct steel tube from pump to carb..

 

[Rustytruck]

Here's a little light reading on the subject.

http://www.tandfonline.com/doi/pdf/10.1080/10473289.1994.10467294

Just remember that before the fuel pump, fuel is under suction. Any loose connections or bad hoses or even the o-ring at the manual fuel switch can allow air to suck into the fuel line and that is why you are probably seeing allot of air in your fuel filter.

Another thing to remember is fuel flow and heated fuel going into the engine is pretty stable while the engine is running. But once the engine is shut off the the heat in the engine compartment really spikes. Expanding the fuel and vapor and causing the fuel and vapor to boil out of the carb and into the engine.

 

[NC_Pinz]

FYI, the 2 bbl Bronco carbs always seemed to vapor lock when you get the engine temp up and are driving in the summer. Stop for 2 min and it is hard to start. At least ours was that way.

Our '75 came with a exhaust heated spacer right below the carb. I never knew whether exhaust gas is circulated only when it is cold out or all the time. I'll eventually remove that and put a phenolic spacer to help keep the carb a bit cooler.

 

[ntsqd]

On my '67 Ranchero I didn't have the room to go with a thick insulating carb spacer so I used the Chevy DZ302 trick. I made a carb spacer from 1/8" aluminum. I started with a sheet that was bigger than the footprint of the 302's intake manifold & trimmed to fit from there. When finished it fit up tight against the rear of the distributor and covered the rear-most intake manifold bolt heads, and nearly touched both rocker covers.

It worked well for me. I later realized that it was working like an exhaust heat sheild and keeping heat radiating off the intake from reaching the carb. It also was acting as a radiator and conducting most of the heat conducted thru the carb pad on the intake away from the carb.

Fuel still boiled off. Particularly summer fuel. Especially ARCO fuel. Learned to only buy there in emergencies. The reason for those carb fuel bowl hoses to the charcoal canister is to send the fuel vapors back to the fuel tank and not where you can smell it.

 

[blubuckaroo]

Blue71,
The air bubbles in the fuel filter, and your hard-start are separate issues, but both heat related.

The fuel line shouldn't have any air bubbles in it. That can be from leak somewhere in the fuel system. A loose hose, leaky selector valve, or leaky fuel pump can cause that.
Also, those bubbles could be an early sign of vapor lock. That's where the fuel actually vaporizes in the fuel pump suction line.

Your hot hard-start issue is probably because some of the fuel is boiling out of the carb after shut-down. The fuel can boil out of the fuel bowl and right through the venturis and into the intake. So, on your next start-up the engine will be flooded.
This is a bad thing for engine life. It washes oil from the cylinder walls and contaminates the motor oil. You really need to fix this, and not just learn to live with it.
I've even blocked the exhaust heat crossover in the intake. That and a good spacer helped, but even if the heat isn't transferred directly to the carb, the under hood temp gets hot enough to cause this problem.
You might consider lowering the bowl fuel level a bit to see if it helps. That way there will be a bit more space for the boiling fuel.

 

[ntsqd]

The 2100 series carb top with the canister vent over the fuel bowl (I'm assuming that there was a version like this) would mostly eliminate the flooding since those vapors would be directed to, ideally, the charcoal canister. It won't eliminate the hard hot start because that is a symptom of a dry fuel bowl.

Know that today's fuel really isn't formulated with carb's in mind. It is made for EFI systems. An electric fuel pump with a "bump system" (one that you can run for 10-30 seconds w/o cranking) will eliminate a lot of cranking, but the only sure cure will be to go EFI.

 

[blubuckaroo]

ntsqd,
When a float bowl boils over, it's not something that the charcoal canister can handle. That's just made for vapor.
When a carb boils over, it dumps a huge bunch of fuel down the intake.

 

[ntsqd]

Then why do smog era carbs all have a bowl vent that goes to the charcoal canister?

The vapors off the top of the fuel in the bowl is why. If you can vent that off then it can't build enough pressure to push the liquid fuel thru the mains and into the engine.

 

[blubuckaroo]

Then why do smog era carbs all have a bowl vent that goes to the charcoal canister?

The vapors off the top of the fuel in the bowl is why. If you can vent that off then it can't build enough pressure to push the liquid fuel thru the mains and into the engine.

Those vent lined to the canister are for vapors. When a carb boils over, it's a lot more liquid fuel than vapor. The fuel just spills down the intake.

 

[ntsqd]

And if the vapors keep getting removed then the pressure in the fuel bowl that pushes the liquid thru the mains and into the engine can't build up. Without that pressure how is the fuel going to go uphill to get to the boosters get into the engine?

I'm sure it isn't 100% effective, but all of the OEM's did this for a good reason. Smog rules drove it, but it had collateral benefits.

 

[blubuckaroo]

And if the vapors keep getting removed then the pressure in the fuel bowl that pushes the liquid thru the mains and into the engine can't build up. Without that pressure how is the fuel going to go uphill to get to the boosters get into the engine?

I'm sure it isn't 100% effective, but all of the OEM's did this for a good reason. Smog rules drove it, but it had collateral benefits.

To simplify what's happening...
Take a pot filled to within 1/4" of the top with water, and put it on the stove. When it boils, a lot of the water will overflow the pot and end up on puddled on the stove. It was never vapor, but water propelled by vapor.

The same happens with a engine when it get shut off. Heat builds in the carb because it has no air going through it. That heat boils and dumps the fuel.
That dumped gasoline has nowhere else to go than right down into the engine.

 

[ntsqd]

And if you remove the heat by removing the hot vapors....

oh forget it.

 

[jckkys]

Heat soak after shut down is inevitable. It can be less problematic tho. The Ford 2100/2150 carbs are no more vulnerable than others. In fact the float bowls don't hang out over the hot intake as far as Holley 2300s. Here, https://scoutparts.com/Two_Barrel_carburetor_base_gasket_heat_shield_New_Old_Stock/p18970, is a heat shield that was OE on Scouts with Holley 2300s. That and a phenolic spacer as thick as you can fit also helps. Stay away from clear plastic fuel filters. The OE type works better and is less likely to cause a fire. Bubbles in the filter are meaningless. The 195 degree thermostat is better for the engine and doesn't contribute to percolation. The '73-'77 EBs had EGR and required an EGR spacer under the carb. They added heat. But this was an unintended side effect, not the purpose. Headers also add a LOT of under hood heat. Winter blend fuel in hot weather makes fuel percolation worse too. As does alcohol blended fuel. In short do the things the help, and avoid the things that make percolation worse.

 

[blubuckaroo]

There are also aftermarket heat shields for four barrel applications.
I've used one of these "Mr. Gasket" shields in the past.
But a word of warning...
This shield uses a stack of gaskets and aluminum to dissipate heat. They come only with the four hole design. If you have a divided or open plenum intake, you need to trim out the four holes from the aluminum and gasket stack. I've had the engine suck a gasket chunk off the stack on my '70 SS. It eventually ate the gasket chunk, but the aluminum was trying to break loose too. That wouldn't have had a good ending.
I think the best way to deal with it would be to just use the shield along with a wood or phenolic spacer with a divided plenum design.
https://www.summitracing.com/parts/mrg-3710/overview/

Also, here's a video that demonstrates what happens with the fuel in a carb bowl when you shut the hot engine off. Even with a thermostat set at 195*, the engine temperature gets much hotter than that when you shut it off. Maybe more than 220* in the summer. And the gasoline boils at around 160*
I don't know what the elevation or gasoline blend is in the video, but you get the idea.
It's sort of a long video, but the real action starts at around 10 minutes.
https://www.youtube.com/watch?v=d7AkcjnCTVU