Carb Tuning

Tony Mele · Jul 21, 2018

Hey gang,
So it's time for my stupid mechanic question of the week.

Truck runs great on level ground, climbing a hill, or on the gradual downhill. But if I hit a sudden downhill slope - I'm talking about when I drop the front end into a mud hole or deep rut when off-roading - the truck immediately dies. Pump the gas twice and it fires right back up. Very clearly a fuel issue.

Running a Holley 600. Open the float viewing screws and fuel is just at the lower level of the threads. Rock the truck a little bit and fuel spills out.

I've read in a few threads that the trick is to lower the float bowls. Anybody got any thoughts or suggestions?

Tony

spap · Jul 21, 2018

They make off road float springs which are stiff that might help and parable lower the floats too. Depends where they are at now

SteveL · Jul 21, 2018

I expected to hear you were running an edelbrock carb. They run great flat but horrible on angles. Some guys smarter than me, or just more patient, have been able to make the holleys work ok. I'm lazy so I just went with a q-jet

Rustytruck · Jul 21, 2018

If you must stick with a Holley off road run lower fuel pressure in addition to lowering the floats. Quadrajet for me too.

Tony Mele · Jul 21, 2018

Thanks everyone. I'm not ready to make the switch to EFI yet. But I spend as much or more time these days on the back trails as I do on pavement. I suspected it was a flooding issue, as it never starves for fuel when climbing.

I'll lower the floats to start. Not sure how to lower the fuel pressure on a mechanical pump, but I'll research that. I need to install a fuel pressure gauge at least. I suspect it's too high.

blubuckaroo · Jul 21, 2018

The Holley carbs with side mounted floats control fuel pressure better than those with center hung floats.
It's the leverage the float puts on the needle valve that makes the difference.
That's the biggest problem with an Edelbrock carb too.
Any bouncing or angle change will allow the float to reduce pressure on the needle valve. The off-road needle valves do little to remedy this.
The best fix is to install a fuel pressure regulator.

SHX669 · Jul 21, 2018

Which holley carb do you have ?
When/if you adjust the float levels a little goes a long way so no " more is better" doesn't apply - LOL.
When you initially lower the float adjustment the fuel level will actually rise because you're shoving the float deeper into the fuel so you have to use up some of the fuel by idling or a fast idle before you recheck the fuel level

Tony Mele · Jul 21, 2018

Holley 1850 Double Pump 600 cfm

SHX669 said:
Which holley carb do you have ?
When/if you adjust the float levels a little goes a long way so no " more is better" doesn't apply - LOL.
When you initially lower the float adjustment the fuel level will actually rise because you're shoving the float deeper into the fuel so you have to use up some of the fuel by idling or a fast idle before you recheck the fuel level

blubuckaroo · Jul 21, 2018

Tony Mele said:
Holley 1850 Double Pump 600 cfm

That's got a center hung float. A street carb. But with the right fuel pressure and proper tuning, it'll work great.

Tony Mele · Jul 21, 2018

blubuckaroo said:
That's got a center hung float. A street carb. But with the right fuel pressure and proper tuning, it'll work great.

Just looked up fuel pressure valves on Summit. Do I need a pressure gauge to go with it as well? Can't quite tell if a pressure gauge is included in the valve.

Tony Mele · Jul 21, 2018

Trying to find that right balance. You know half the time I'm on the street. Half the time I'm off road.

blubuckaroo · Jul 21, 2018

Tony Mele said:
Just looked up fuel pressure valves on Summit. Do I need a pressure gauge to go with it as well? Can't quite tell if a pressure gauge is included in the valve.

You'll need a pressure gauge, since most regulators are adjustable.
After you've set the pressure, you should remove the gauge.

Rustytruck · Jul 22, 2018

Try 4 psi on the street. When going offroad at slower speeds try 2 psi that way when the floats start bouncing less fuel squirts past the needle and seat. My cagle fuel regulator does this automatically.

blubuckaroo · Jul 23, 2018

Rustytruck said:
Try 4 psi on the street. When going offroad at slower speeds try 2 psi that way when the floats start bouncing less fuel squirts past the needle and seat. My cagle fuel regulator does this automatically.

I use a Cagle too, but can't really recommend them, because they're practically nonexistent anymore.
Too bad, because they were really the most practical answer to excess fuel pressure.

Tony Mele · Jul 23, 2018

I've got the parts ordered. Pressure regulator and gauge.

On a related note I dropped the float bowls a little bit as well. At first too much. Truck was starving for fuel. I've got it set now and it's running well on the road. I'll update as soon as I get the regulator on.

pcf_mark · Jul 23, 2018

This is a total hack let me start with that. If you put a short length of rubber fuel hose on your bowl vents you can reduce the stalling on extreme angles and abrupt maneuvers also. If you add a short 3" piece of hose (that still fits inside your air cleaner) you prevent the slosh of fuel from going out the vent into the engine flooding it out. Classic drag racing trick because on the green you hit so hard the fuel sloshes out of the front bowl and the engine goes stupid rich and falls on its face.

Here is a post with some trick and copper line for extending the vent.

http://classicbroncos.com/carb_tricks.shtml

Justafordguy · Jul 23, 2018

pcf_mark said:
This is a total hack let me start with that. If you put a short length of rubber fuel hose on your bowl vents you can reduce the stalling on extreme angles and abrupt maneuvers also. If you add a short 3" piece of hose (that still fits inside your air cleaner) you prevent the slosh of fuel from going out the vent into the engine flooding it out. Classic drag racing trick because on the green you hit so hard the fuel sloshes out of the front bowl and the engine goes stupid rich and falls on its face.

Here is a post with some trick and copper line for extending the vent.

http://classicbroncos.com/carb_tricks.shtml

You mean like this? I love hacks when they work. ;D

pcf_mark · Jul 23, 2018

Justafordguy said:
You mean like this? I love hacks when they work. ;D

That's it!

Tony Mele · Jul 23, 2018

Gotta try that!

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