This will be long...

So I've had somewhat a summer's worth of time to run the boat with the 509s. I've got about ten hours on everything and so far performance has been hit or miss. Idle depending on the day is so/so, the thing runs really rich most of the time, and outside of one time where the boat really worked well, I have not been very happy with it.

Leading up to Labor Day weekend, I was chasing an issue where I was experiencing a lean pop getting into peak torque (47-4800) when getting on it. I doubled checked fuel pressure (7 lbs) jetted up to a 94 on the secondaries, and installed a 50cc pump on the secondary side. That Saturday, I took her out and she seemed to be right. Acceleration from just beyond planing to 65 was the best it had ever been. 65 to 80 was also great - still had some trim to go, too. Was enough to scare the Mrs so the rest of the day was spent just cruising (give her some slack... she's fine going fast - she just hasn't been on the boat with the new power and it surprised her - was also a rougher day on the water). Before anyone asks, I do not have AFR figures for the day outside of idle and low-speed cruise. The sun was washing out the display badly. Idle AFR was in the mid 11s. Cruise in the mid 12s.

The next week, I pulled the oil filters to check engine health just coming into the 10 hour mark. Thankfully, no surprises there, but while I was there, I thought I'd check plugs. The front half of the plugs on both were very dark and wet - the rear half was more of like what I'd expect from a rich mixture - acceptable. I took one jet number from the primaries, threw it back together, made sure the floats and fuel pressure were good, and adjusted idle AFR on the hose (11.9-12.3). Made sure manifold bolts and carb stud nuts were snug. Snappy throttle, good idle - done.

Took the boat out - idle back down to rich again (11.5). Occasional stalling especially on port engine. Cruise seemed okay but WOT performance was non existent. Another rough and busy day, but when I went to really get on it, it seemed like the throttle wasn't doing anything and it seemed to fall on its face. Again, AFR was washed out by the sun. Idled through a no wake and went to cruise again. The AFR on the port engine was in the 13s and climbing getting on plane (starboard was also up but not like port). Eventually, it would become sort of acceptable - I took it easy the rest of the day and put it back on the trailer. It's possible I messed up something putting the metering block back on - I will check. At home flushing, I saw the fuel pressure was really low on both, but I'm pretty sure the liquid-filled gauges were just hot. I checked a couple days later and they were reading properly - idle AFR around 11.8-11.9.

I do plan to scope the intakes to see if there's any evidence of leaking. My AFR 325 heads were cut down to 112cc. With the block work done, I'm thinking the ports were lowered .040. My intakes fit pretty tight at the china wall, but I did not have any issues with the bolt holes lining up. I'm pretty sure my head guy took material off the intake face to compensate - I'd have a real fitment issue if he had not. Still, I'm going to look. Before taking it apart, I will check idle vacuum at operating temp.

This leads me to these Demon 850 carburetors.

I had a lot of issues with them last year - ended up being ethanol contamination as well as a wide-open main well (plug was missing - I never got a chance to load the engine so who knows what that would have done). I have done a lot of reading about these things, mostly over at racingfuelsystems.com (favorite quote seems to be that Barry Grant was taking carb calibrations from out of his ass). The carburetors have the ability to flow over 1000 cfm so they're good for my 509s, but the calibration is awful - over emulsification leading to huge jets and crap performance - very rich on the low-end with the potential for dangerous lean conditions at WOT. The general consensus is that these things need about four jet sizes richer, primary and secondary, to run safely (stock jetting is 85P/93S - I'm at 85/94 now).

So, lots of reading - it has been recommended that in order to fix these, they need to be recalibrated to a more standard Holley double pumper setup. Currently, the carbs are set-up like this:

IAB: .070
HSAB: .031
Emulsion 1: .031
Emulsion 2: Blocked
Emulsion 3: .031
Emulsion 4: blocked
Emulsion 5: .031
PVCR .059
IFR: .037
PMJ: 85
SMJ: 94
PV: 6.5

Suggested to get it baselined properly (based on a 1.560 venturi and 1.75 bore - typical 850DP):

IAB: .070
HSAB: .025
Emulsion 1: .028
Emulsion 2: Blocked
Emulsion 3: .028
Emulsion 4: blocked
Emulsion 5: blocked
PVCR: .067
IFR: .035 (moved to bottom of the idle feed passage).
PMJ: 80
SMJ: 90
PV: 6.5 (might kick it up to a 8.5 to kick it off sooner).

Test from there.

Evidently, this isn't a problem with Demons only - seems that a number of the big dollar carburetors are over-emulsified (per discussions anyway). I've got a set of brass set screws coming in as well as the proper drill bits to do the work. I'm actually pretty eager to do the work - the parts on these things are great - I just don't see needing to shell out another $2k for carburetors when I can use what I have.

Would love to hear some thoughts around this plan. Thanks for reading!

-Tom

You may have a transfer slot problem with too much fuel at small throttle opening. You may have to install a restrictor in. I would start around .056

Coud go that way - in the beginning I tried getting idle set around 13.0 and it would die going into gear no matter what I tried. Idle has been more of a just leave it where it falls (within reason), but perhaps this is direction to take.

A big advisement has been to move the IFR to the bottom of the metering block.

Might see if I can find some old double pumper metering blocks and just modify those.

Yes, moving the IFR to the bottom will help. If you keep the three emulsion bleeds i would go to .022. Also when idling make sure the boosters are not dribbling.

Doing so with the emulsion bleeds will then really richen it up across the entire rpm range and make it more responsive to jet/pvcr tuning overall, right? I think that's how I'm understanding it.

One of the functions, too, of lowering the IFR is to eliminate the chance of fuel dribble at idle, right? (I've had to lower my floats to combat that issue in the boat)

Lowering the IFR is to keep the fuel in the idle circuit from pulsating. The emulsion is to make the fuel lighter.

Bummer.
My 850 sea demon was fine. On the dyno and in the boat. 502na. Motor + boat long gone. Sorry. Maybe time period thing ? I bought and used that carb new 20-21yrs ago.

the early Demons were not bad but the later ones (when BG decided that living large was more important than quality and he was behind on bills) were barely good for parts. we were forced to use them in Super Cat and you would go through 20 of them to find 6 that worked good and even then you had to take the new ones apart to remove chips and fix machining flaws. his shop was on the other side of town from me and my old neighbor was his book keeper for awhile till he stiffed her. shame they went down hill as bad as they did.

Yep, mine is a newer one but built before Holley took over post bankruptcy.

The main body (outside of the bleeds) and throttle plate are good. It’s the metering blocks/configuration that need help.

I’ll bet stuff is trapped in the wells. Might just buy some new ones from BLP or Quick Fuel. Shoot, standard Holley metering blocks would work - if anyone wants to donate any, I’ll gladly take some charity!

Sucks too… she was running well… for a couple weekends here and there. Really strong outside of the carb sh!t.

Taken from an old BG discussion on Team Chevelle (from Tuner over at racingfuelsystems.com). For anyone reading, I hope this is useful - it is for me.

Emulsion Circuit
by Tuner

Incorrect two-phase flow is at the root of all this aggravation. People who have drill bits but don’t know why to use them have been molesting innocent carburetors for a long time. Now some of them are in charge of the manufacture of new carbs and they think they have improved them by using larger drill bits to make the air bleed and “emulsion” orifices. I guess the guys that engineered the original carburetors on the old muscle cars were pretty stupid or they would have “improved the emulsion” 40 or 50 years ago when they had their chance. After all, they had the awesome power of the single-point ignition system at their disposal, they shouldn’t have been afraid of a little soot.

It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.

What I’m going on about here is “correct two-phase flow”. That is the description of a fluid flow that is made up of a liquid and a gas flowing together in the same conduit. As the ratio of gas to liquid increases (more gas, less liquid), at some point the gas bubbles coalesce from many small ones into a few big ones and the flow starts to “slug” and become erratic. The carburetor nozzle spits like a garden hose with air in it when there is too much “emulsion” air.


An emulsion of air and fuel has reduced density, surface tension and viscosity compared to fuel alone. This increases the flow of fuel considerably, particularly in low-pressure difference operation, at low throttle openings or lower engine speeds. Just how much of an increase (richer) is dependant upon where and how much air is introduced into the fuel flow.

Mainly, what must be understood is that because the fuel discharge nozzle connects the venturi to the main well, whatever the low pressure (vacuum) is in the venturi, it is also the pressure in the main well. The air bleed is in the carb air horn or somewhere else where it is exposed to essentially atmospheric pressure, which is higher than the venturi pressure. This pressure difference causes air from the air bleed to flow through the emulsion system into the main well and to the nozzle. The flow of air can have very high velocities, approaching sonic in some orifices. The airflow literally blows the fuel toward and through the nozzle. A larger main air bleed will admit more air to the emulsion system and that can increase or decrease fuel flow to the engine. The size, number and location of the other air holes in the emulsion system, the size of the main well flow area, the size of the nozzle and the specific pressure difference at the moment are the determining factors. The ratios of air volume to fuel volume to flow area, with the air volume's expansion with the venturi velocity induced pressure reduction being the key. The bubbles expand as the pressure drop increases with airflow. Suck on an empty balloon to experience the effect.

The fuel flow through the main jet is the result of the pressure difference between the atmospheric pressure in the float bowl and the venturi air velocity induced vacuum acting on the nozzle and the main well. The venturi vacuum in the well is reduced (the pressure is raised) by the "air leak" from the air bleed. This reduces the pressure difference that causes the flow through the main jet. If the air bleed were big enough, the pressure in the well would be the same as in the float bowl and no fuel would flow. Think about drinking through a soda straw with a hole in it above liquid level. Bigger hole, less soda. Suck harder, not much more soda. Big enough hole, no soda. This is the means by which the emulsion system can "lean it out on the top end". Incidentally, the vacuum that lifts water up a soda straw is in the most sensitive operating range for emulsion systems.

It is in the lowest range of throttle opening, at the start of main system flow, that the effect of adjusting the introduced emulsion air (and it's effect in increasing the main fuel flow) is most critical. Small changes can have large and sometimes unexpected or counter-intuitive consequences. The goal is to seamlessly blend the rising main flow with the declining idle/transition system fuel delivery to accomplish smooth engine operation during opening of the throttle in all conditions, whether from curb idle or any higher engine speed. The high speed and load mixture correction is usually easily accomplished, in comparison.

The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.