Fuel Blowing Out Carb Vent Tubes Under Heavy Throttle: Expert Opinions Please

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12many

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Carb/fuel system experts diagnosis/opinions please; This is a new problem that just reared its ugly head over a week ago. Stout low 12 second 410 in an A100 van, mild cam, Action+ intake, 750 dp, cold air intake, headers yada yada. Dialed in mostly issue free for over a year. Trying to avoid writing a novel (haha) but what is happening is under either just heavy primary side acceleration or going wot I'm getting fuel spraying out the fuel bowl vent tubes (primaries or both) practically drowning out the motor until I let off, at which ppint the spray/blowing out stops and the motor recovers. This just started one day, drove to work all was fine. On way home went to get on it and this problem cropped up.Took an afternoon with the air cleaner off and test driving to see what was happening. Did this with a dialed in 750 Street Demon (again, with a years worth of mostly trouble free running and drag racing) and is now doing it on recently installed AED 750HOM double pumper. Same issue on both. Been running a Carter 6270 mechanical and a 4594 electrical (at/below fuel tank) with 3/8" hard line, Fram cartridge fuel filter (at/below tank) with Holley 12-803 (no bypass-return) that did fluctuate up and down from a 6psi setting, and at times was showing anywhere from 4 on down to 1.5psi at times. Ditched it the other day for the Holley billet 12-840 (no bypass/return) but got the same fluctuation. Swapped out the cheasy Spectre liquid filled pressure gauge for an old USA made gauge from decades ago, and am getting rock steady 5.5psi at idle, in park holding rpm's at various levels. Checked electric pump output, fuel filter and output through it, up to mech pump, up to regulator, and to carb. All seems good. Still have to road test under load to verify readings. Float levels checked on both carbs, all good. Vent tubes clear, no obstructions etc. Anyone have a clue as to why all of a sudden fuel now sprays out the vents of two different carbs. under primary or wot acceleration, when the pump psi and float levels seem to be good? Tank pressurized, vacuum in tank? Plugged fuel cap vent, issue with either fuel pump? I need to step away and rest my brain, before I end up blowing a gasket! Any insight greatly appreciated:BangHead:
 
I would lower float levels. Its almost free.
Something you would like to fix sooner than later, gas fumes in engine bay arent good in any amount.
 
Is it possible that your trusted fuel gage is wrong, and now your fuel pressure actually higher? Are you running different fuel than before, possibly with more ethanol? I agree, drop the float levels to slightly below the holes. Did you recently switch to the cheesy plastic float level sight glasses?
 
Like I mentioned, the Street Demon floats were set to spec, never an issue till last week, swapped to an AED 750, with sight glasses (carbs are wet flow tested and set prior to shipping) and my levels are midway in windows. But same issue on both. And the pressure gauge I'm now using is giving a steady reading, and I have the regulator adjustment screw out further now then where it was set with the Spectre gauge.
 
SGBARRACUDA said:
Sounds like fuel level is to high. We use to use a piece of 5/16" hose run from one vent to the other with a slice taken out of the topfor air. Or there is this.
Holley Carburetor BOWL VENT TUBE TRUCK AVENGER OFF ROAD RACE DEMO DERBY A23 | eBay
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I am aware of the hose mod, I'll look into that and maybe lowering the floats a little. I'm just baffled why the issue cropped up on the other carb, and is doing it with the new. Gonna drain tank flush fuel from lines, get some fresh fuel, and see what happens.
 
We use to do the trick I mentioned in the 70's when upon launching off the line we had the same problem your having. I would try this trick, it's cheap and may solve your problem. If it doesn't than yes you need to look else where. I will tell you that since I went to a bypass fuel set up, I've never had this problem.
 
Set the float levels to the bottom of the sight glasses. You should just be able to see the level lap a little if you rock the car. IMO and experience mid way up the hole, in a Holley based carb, is too high.
 
Years ago i had a holley on a 440, eventually switched to an edelbrock. We set float level to bottom edge of sight holes.
 
I always adjust the Holleys until the fuel was just at the bottom of the fuel level holes on the side of the bowls. Half way is too much In my opinion. Recheck your fuel pressure with another gauge. 5-7# would be my comfort zone.
 
No need for fancy clear sight glass plugs, you set to bottom edge of the holes, then thread the brass plugs back in.
 
moparmat2000 said:
No need for fancy clear sight glass plugs, you set to bottom edge of the holes, then thread the brass plugs back in.
Click to expand...
The fancy-schmancy AED 750HO-Modified dp comes setup that way.

Aed-750-Ho-Double-Pumper-Holley-Carburetor-Red.jpg
 
Your new carb was set by the manufacturer, but at what fuel pressure, with what fuel, and with or without an aircleaner? You need to set them again, on the vehicle, in real life conditions.
 
d55dave said:
Your new carb was set by the manufacturer, but at what fuel pressure, with what fuel, and with or without an aircleaner? You need to set them again, on the vehicle, in real life conditions.
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Totally agree, I've been working on Idle and transition circuits, on way to accelerator pumps step by step, using midway sight glass float levels, but will readjust and do the tube deal, and start fine tuning again.
 
12many said:
The fancy-schmancy AED 750HO-Modified dp comes setup that way.

View attachment 1715319736
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Yep fuel level set too high I bet. The vent passage goes through the top of the metering block. Too high of fuel level and some healthy acceleration and out it comes. Holley has bowl vent extensions available. Does your carb manufacturer have them too?

"Holley fuel bowl vent baffles are used with the center-hung style fuel bowl. It extends the fuel bowl vent to near the wall of the fuel bowl and can effectively control fuel splash on the primary side when accelerating hard, or on the secondary side when braking hard."

holley bowl vent.jpg
 
toolmanmike said:
Yep fuel level set too high I bet. The vent passage goes through the top of the metering block. Too high of fuel level and some healthy acceleration and out it comes. Holley has bowl vent extensions available. Does your carb manufacturer have them too?

"Holley fuel bowl vent baffles are used with the center-hung style fuel bowl. It extends the fuel bowl vent to near the wall of the fuel bowl and can effectively control fuel splash on the primary side when accelerating hard, or on the secondary side when braking hard."

View attachment 1715319744
Click to expand...
Yes, the carb has those I believe whistle vents they're called? Also, being my engine/trans sits in the van at a pronounced angle vs sitting close to level like in most vehicles means the fuel may be "level" but it will be higher against the primary metering block and lower against the front of the bowl, as well as higher on the secondary rear bowl, possibly with that vent submerged in fuel......hmmm. And hmmm again
 
12many said:
Yes, the carb has those I believe whistle vents they're called? Also, being my engine/trans sits in the van at a pronounced angle vs sitting close to level like in most vehicles means the fuel may be "level" but it will be higher against the primary metering block and lower against the front of the bowl, as well as higher on the secondary rear bowl, possibly with that vent submerged in fuel......hmmm. And hmmm again
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If it is sitting at a steep angle, you could try a marine carb angle wedge under your carb.
 
Yep, if the engine is tipped up you will aggravate the problem especially if the fuel level is too high.
 
Will look at actual angle. I see 5, 8 and 12 degree plates. Limited on room for spacers with doghouse engine cover, might get away with a 5 though.
 
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