I've had a 4781 Holley 850 on my car on and off over the years, removing it mostly from frustration due to it flooding out (big time) on me every now and then. The flooding was usually worse in the primary side, but IIRC occasionally it would pour out from both the primary and secondary sides. Years back in my youth I initially assumed it was a piece of dirt/tubing debris that caused the needle/seat assembly to be wedged open. I'd clean everything in the primary bowl area, drain and change the fuel diluted oil, and drive a bunch more miles again. This would happen again at varying intervals, and I'd do the clean-the-carb and change-the-oil routine again. After too many times of doing this I finally shelved the 850 and put a stone reliable 3310 780VS carb on it. Worked great for a boatload of years. The last few years I wanted a bit more power so I put the 850 back on. Ran fine for a year or so, then proceeded to take a major crap (primary circuit flooded again) while enjoying a track day. Hauled the car home and cleaned/drained/filled the systems and put a new float in for insurance. Worked great for a couple years and several track days, and then it crapped on me again last month, six laps into the first track session of the day. Loaded up the car and went home.
I borrowed the primary bowl off the reliable 780 and put it on the 850 carb this afternoon. Started the engine and the carb proceeded to flood the engine again. Is it possible that the (mechanical) fuel pump is suddenly or intermittently increasing the fuel pressure (but why doesn't the secondary circuit puke out fuel like the primary circuit)? I'm trying not to think this carb is possessed by demons. The carb is several decades old, and I'm seriously thinking of buying a new 850 for hopefully better reliability, and slightly less weight up front and up high.

Cliff notes: Holley 850 floods out on a somewhat regular basis. Doesn't appear to be vapor lock. Changing floats seems to be just a short term fix. Haven't measured fuel pressure this latest failure, but the fuel pressure was about 6psi (IIRC) a couple years ago when I checked it. Is it any way possible for the mechanical fuel pump to have intermittent pressure changes that might be causing this? However, this flooding has happened with two different fuel pumps (both AC brand) over the decades.

Thanks for any help or insight.

 

Have you got a pressure gauge in the system? What does the tip on the needle look like?
The intermittent/random thing baffles me. The only Holleys I had flood like that, one had a chunk of goo in the needle and seat, and the other one had way too much fuel pressure.

 

I've got to round up a tee to put in the line and recheck the pressure. It was "in limits" when I checked it a couple years ago (when it wasn't flooding out at the moment ).

I haven't looked at the seat yet. I just took the quick route of swapping out the whole bowl assembly, but it turned out to be a waste of time anyway.

I'm still in the middle of moving stuff from the old place to our new downsized location (half my stuff is in a box somewhere unknown). Today has been the first day I've taken a break to try to get the car running again in an attempt to sneak in another track day before everything shuts down at the end of this month.

 

The Holley needle seats have an o-ring on them and when that o-ring fails to seal gas will flow past it and overfill the float bowl. Another cause of flooding is the hollow brass floats will often fill with gasoline; making them heavier and allowing the carburetor to overfill (flood). The solution is to use the premium Nitrophyl floats because they're a solid foam material. Regardless of the type of float you're using be sure to set the float level right at the bottom of the sight plug (or slightly less). Mechanical fuel pumps have a spring inside that provides constant pressure of 5 to 6 psi and the pressure will never increase. Do you have an inline fuel filter?

A 427" engine can only consume 742 cfm @ 6000 rpm at 100% volumetric efficiency so an 850 cfm carburetor wouldn't produce any gain at all. A mechanical secondary carburetor will give a false impression of producing more power because it's providing more throttle opening for a given amount of pedal travel than a vacuum secondary carburetor. As it's impossible to achieve a 100% volumetric efficiency with an exhaust system attached it's pointless to install an excessively large carburetor.

 

is it safe to assume you are setting your floats correctly?

 

Put a modern float in there, new needle and seat, and check fuel pressure is below 8 psi, 6 is pretty standard, and 8 is pushing it for the needle to hold. Possibly the secondary holds because it is already closed when the pressure spikes, and yes it is possible for a mechanical pump to do that.

 

Assuming your pump isn't over-pressuring the system, I have to ask what kind of needle and seats you're using.

The reason I ask is that I had the same thing happen this spring for the first time. Luckily I was in the garage with the carb uncovered and the hood up so I could see it overflowing right away. I had just replaced the accelerator pump diaphrams and the needles and seats. I always count threads when they come out and match that with the new ones so that float adjustment is always very close. Upon start up the flooding began and I shut it off and disconnected the fuel line ASAP. I also had to drain the oil- 6 quarts of brand new Amsoil. Not cheap.

Now to the point- this was also the first time I used Holley N&Ss. It's a Barry Grant modded Holley from back in the day- great carb! So, I pulled the Holley units and replaced them with some from a Barry Grant rebuild kit like I had always done before. Problem solved. I can't tell you anything else except that both brands have the same dimensions and look identical. It's been fine since then.

 

I'm using down flow. .128 with my 5 ish psi 830 cfm with down leg boosters in the past I have used steel and titanium 130 and 150 s with out any problems

http://blp.com/cart/index.php?main_page=product_info&cPath=6 5_108_268&products_id=578

 

Yes, Howard.

 

I disagree with your last paragraph comparing engine CFM and carb CFM. Apples and Oranges. Carburetors are rated a certain CFM at a known pressure drop (1.5" Hg). I've gone to great lengths to minimize the exhaust backpressure for less residual exhaust mass in the cylinders (and slightly improved VE), and I also prefer to minimize the restriction/pressure-drop in the induction side (improving VE there too).

 

That's on the schedule today.

 

Been a while since I have had the Holley fuel bowls off. But I recall when changing floats there is a hinge pin the float rides on, correct? And on that hinge pin is a tiny spring that comes in contact with the float, correct? Question: Does the spring help raise the float or does the spring help lower the float? I had a float spring get cockeyed after installing new float. Had a gusher issue. Just saying, maybe that little spring is shot.

 

Could it be as simple as a pin hole in the bottom of the float. I chased one for a while. The float would fill with a little fuel and stay at the bottom of the bowl. When I would pull the carb to trouble shoot it the fuel would drip out of the float and appear fine. New floats and no more problem.

As mentioned above, It could also be an issue with the pin and spring at the bowl as well.

 

Sounds like either extreme heat or fuel pressure/seat (as stated above).
I had a customer once whose 402 big block after 15 minutes would boil the bowls....drop the float and it woulds spew fuel through the vent.
Turns out he had Mark V head gaskets on a Mark IV and the top of the engine was an oven......just a thought.
Is it coming from vent or boosters?

It would be easy to hook up a cheap Holley regulator to test......

Tell us what you find.....

Jebby

 

Boosters.

 

I'm not comparing anything. The rule of thumb for sizing a carburetor for a street engine is 1-1/2 cfm per cubic inch of displacement and that's what all of the world's factories have always done. For off road racing with open headers 2 cfm per cubic inch of displacement is recommended. Are you running your 427" off road with open headers?

 

Whose rule of thumb? I recall Grumpy Jenkins running two 660 CFM carbs on his 331" small block.

I'm running it "off road", with 8" H2O backpressure in the exhaust.

 

Just measured the fuel pressure (dead headed, as I didn't care to flood the engine again). It was 7 psi, with roughly +/- .5 psi fluctuations from the engine starting momentarily due to all the fuel fumes still in the engine. With the engine stopped the pressure is 7 psi even.

 

The 660s were in the first or second year of PS. Grumpy went to Dominators on the 331 around the second year. Just my recollection on the years, but Dominators did find their way to the 331.

And you are absolutely correct on questioning the CFM formula. If it were a valid computation in all regards, then Chevy and Ford would never have specified 750cfm Holley carbs for their 302s. The formula works well for a mild combination...at least well enough for you to make a safe pick.

Some years back, I tested with a spec 600cfm to a 3310 on a Ford. Both were unmodified. The engine picked up speed and et with the 750 and you could feel it. On the street, the 600 was a little better just putting around.

 

Who knew that 500 cubic inches couldn't handle 2 1150s
Maybe I should put a 750 on this