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Old 05-11-2009, 12:06 AM
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Holley 750 Vacuum secondary flooding at WOT

So I'm leaving work Friday night heading to the drag strip for the first time with my current project (84 Regal w/383 sbc) and it starts running very rich at high rpm's (monitored on wideband) and would stall. It had been running flawlessly since I put it together a few months ago so this was strange. I figured that it was a problem with one of the needle and seats, so I replaced them both. Started heading for the track again and it flooded once more, this time it was bad. It stalled and puked fuel all over the intake then caught fire. Bubbled the 2 week old paint on my hood pretty bad and melted a bunch of wiring. I was steaming at this point. The carb is a basic Holley 750 w/vacuum secondaries and choke milled off. Fuel pump is a Holley red that makes 5 psi at all times. It would only flood when I got in to the secondaries so I am thinking that it has to be the rear float. I took the carb apart and inspected the float. It is the black Nitrophyl one and I could not find anything wrong with it. The carb is an older Holley with the flat head bowl screws. Probably at least 12-15 years old. I went through it with a rebuild kit a few months back and it had been working fine (did not replace floats). Does this sound like a bad float? Can these things really absorb enough fuel to sink? I can't think of anything else that could cause this and YES the floats were set right If it makes any difference I am running leaded race gas.

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Old 05-11-2009, 12:20 AM
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Where is your fuel filter?

Did you carefully remove the bowl to see if there was any foreign matter on the floor of the bowl? Most times this situation is dirt in the needle and seat.

Could be the float is saturated. Could be the float pivot pin is sticky/rough/rusty? Could be float assist spring missing or improperly installed? Could be float bracketry tweaked or bent? Could be nicked o-ring on needle/seat body assembly. Did you lube the o-ring on the new parts?
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Old 05-11-2009, 12:37 AM
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When I took the carb apart it was flawless. Fuel filter is a large canister style mounted on the inner fender. It is 3 months old. O-rings were lubed with assembly grease. All float hardware was intact and like new. Spring was in proper position. Float moves very freely. Is there anything else it could be?
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Old 05-11-2009, 12:49 AM
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Pretty much all I can think of. The flood came out the bowl vent tube?

Good needle and seats(Holley, AED, Quick Fuel) and not from a generic kit like Echlin or NAPA?

Service garages and speed shops used to have a float scale to weigh floats to check for the saturated or "waterlogged" condition, but it has been 25 or so years since I've seen one. Maybe just replace the float, they are fairly cheap. If you are set up to be jettable on the rear or think you may want to be in the future get the pre-notched float so you can run jet extensions.

Last edited by ericnova72; 05-11-2009 at 12:51 AM. Reason: more info
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Old 05-11-2009, 01:05 AM
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I'm pretty sure the flood comes from the vent tube because it went ALL OVER and would kill the motor at full throttle. Other than the bowl overflowing, I don't know of anything else that could make it spew gas like that. It was off the charts rich on my wideband. Once I'd clear the flood and start it up it wouldn't act up again until I was well in to the secondaries and the rear bowl was filling. All of the parts I've put in this were Holley brand from Summit. My friend is sending me his extra 750 Demon to try but I want to fix this carb for the sake of principle! I had this carb working great with perfect AFR's all the way to redline. Its just a f'n Holley, there isn't much too it. I just don't like taking something apart and not finding a problem
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Old 05-11-2009, 01:24 AM
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It is possible that the float has become "heavy" due to fuel absorbsion. The newer fuel formulations are harder on old floats. I/ve also seen the little tang on the float arm that closes the needle get bent or worn and slightly wedge on the needle in the dropped position(fuel entering) causing the float to stick. You never know what a previous owner might have ham-fisted.

I always pre-set the float so that it is "squared level" with the top of the bowl when held inverted. Then, when checking with the sight plug on the car, if I have to make a drastic change more than a full turn and a half then I know that something might be "hinky" in the bowl and to remove and inspect again.
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Old 05-11-2009, 05:33 AM
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If it was the float wouldn't flood all the time and not just when in the secondaries fuel pressure is always present in the rear bowl even if not opening secondaries.Possible something is wrong with the metering plate or gasket? then when the secondaries open and fuel starts being called for something allowing way to much fuel to go through the boosters?
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Old 05-11-2009, 08:29 AM
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If the floats are "heavy", the needle will be very easy to be held off of the seat by the incoming fuel.

I'd replace the floats.

If the carb sat dry for a long time, the floats would have dried out. Then when the carb was used again, it would have worked fine until fuel re-saturated the plastic, IMHO.
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