Frankenstein Mighty Demon 850 - Marine - Cannot richen up.

[TomZ]

Long long time lurker and knowledge gatherer, first time caller. I have followed a lot of the recommendations here to recalibrate my BG Mighty Demon 850 and it has fought me every step of the way. Thought I'd go ahead and just ask here instead of beating my head against the wall.

Twin-engine go-fast boat, 31-foot Formula with sterndrives and transmissions.

509 CI
AFR 325s cut down to 112cc, non-CNC chamber, but CNC intake and exhaust porting
9.4:1
235/241 113 LSA .646/.646, hydraulic roller, 2/3-4/7 swap - Comp 953 springs.
Merlin intake with 1-inch HVH Super Sucker
Demon 850/1000 body, downleg boosters, QFT 850-1000 throttle plate (the BG plate tore when I was adding PV protector), Holley billet blocks.
Timing, 35 degrees, all-in @ 3k rpm
Innovate DLG-1 AFR monitoring with O2s installed on odd banks.

All testing done with engine hatch off the boat (should richen up 2-3 the the closed)

Originally, the problem started with the carbs running overly at any RPM. After going around and around with rebuilds, etc, found that the power valve vacuum feeds were never drilled in the base plates. These were standard, untouched Mighty Demon GC carbs. More inspection found missing well plugs, bad drilling, etc. I replaced the metering blocks with Holley HP parts. Carbs both using progressive linkage rod - also have 1:1's in my carb kit if needed.

First outing for shakedown from a few weeks ago, we tried 88s and 98s with the following in the metering blocks:

63 PVCR
37 IFR - high position,
70 IAB
31 HSAB
Emulsion - 32/30/28/28/blocked

Results:

[b]RPM > Port > Starboard >[/b]
1500 11.5 12.2
2000 11.8 12.2
2500 12.4 12.8
3000 13.6 13.6
3500 13.8 14.0
4000 14.4 14.8
4500 14.8 14.9

In above test I was running with the flame arrestors off and my buddy was at the sticks stepping the RPM. The primary mains didn't even start to come on until just about 3k rpm and very sputtery to start. The secondaries weren't coming on until about 4400 and also very sputtery.

Prior tests on the primaries only saw similar results and wide open of 4500 with a lean pop. Decided it would be best to rework the carbs by going back to basics with Holley's 3310-style emulsion.

Testing this week with the following setup:

Primary 88/Secondary 97
PVCR 70 w/ 8.5 PV (no PV in secondary)
HSAB 28
IAB 70
IFR 35 (moved to bottom position)
Emulsion bleeds (top to bottom) 28/blk/28/blk/blk/blk (from some reading - may want to change to blk/28/blk/28/blk)
Siphon break 27
Pump nozzles: 31 primary, 35 secondary
130 needle and seat (flooded even with lower fuel pressure)
Floats at middle of site glass (Demon float bowls)
Fuel pressure at 5 psi (pressure cycling due to 130 n/s).

First run:

[b]RPM > Port > Starboard >[/b]
1000 14.0 14.7
1500 12.4 13.0
2000 12.1 12.4
2500 12.8 12.5
3000 13.7 13.5
3500 14.2 13.7
4000 14.1 13.9
4500 13.7 14.0

Had real issues with fuel pressure and maintaining float level. Went to 110 needle and seat due to flooding. Readjusted fuel pressure to 7 PSI and reset float levels.

Primary 98/Secondary 97
110 needle and seat
Floats at middle of site glass (Demon float bowls)
Fuel pressure at 7 psi (steady fuel pressure)

Second run:

[b]RPM > Port > Starboard >[/b]
1000 13.2 13.4
1500 12.1 12.9
2000 11.8 12.3 (primary mains coming on strong - no sputtering)
2500 12.8 12.8
3000 13.4 13.4
3500 14.1 14.0
4000 14.0 14.1 (secondary mains coming on right after this data point - no sputtering)
4500 13.8 13.5

Beached and changed primary jets to 91s.

Third run:

Primary 91/Secondary 97
110 needle and seat
Fuel pressure at 7 psi (steady).

[b]RPM > Port > Starboard >[/b]
1500 11.8 12.9 (always something going on with port - richer in the idle transition - built the same so who knows)
2000 11.6 12.0
2500 12.2 12.5
3000 12.5 12.8
3500 13.5 13.4
4000 13.5 13.6
4500 13.7 13.2 (might have misheard port number)

Pushing any harder, the A/F ratio jumped into the upper 14s - I didn't want to hurt anything so I got out of it. It felt like there was plenty of stick left but it wasn't really doing anything and at that A/F is wasn't making any power. Boat was trimmed up to run high speed and the torque was there to carry the bow, just not enough fuel to do any more.

So... still fat down low and lean upstairs even with monster jetting compared to others I've seen. The goal is to have the entire working rev range, 2500 to WOT, to not exceed the upper 12s and richen up to about 12.3-12.4 when running at WOT for a few miles. Right now we're at the point of burning down stuff. When I plot the fuel curve, I'm getting an exaggerated V versus a steady-to-declining A/F ratio.

Not sure what to do next here and running out of good ideas. Maybe add power valves on the secondaries to have them start dumping fuel (vacuum at 4500 is 3hg so a 4.5 would definitely do it). Good idea? Close up the HSAB? Close up the emulsion bleeds some more? Cruise seems to be coming on-line. But man does does she lean out up top! Engines are new with less than 20 hours - all testing time. The boat is still running 70 mph at 45-4600 - it ran the same speeds with the stock 400 hp Mercruiser 502s (47-4800 rpm). These should easily make 600+ and top out around 56-5800.

Probably need tslot restrictors on the primaries, too, to control the rich transition. Or maybe just open up the IABs a little? This isn't a huge concern though considering the engines don't spend any real time in the 1200-2000 RPM range (was only recorded because we were step-testing in 500 RPM increments from idle).

I am surprised by how much jet these carbs need. I've seen many 502-based engine setups with 1000 4150s and 1050 Dominators - I don't recall hearing anyone that I know of getting into jet numbers this high on a 600 hp 502. It seems really excessive.

Appreciate the help. Thanks. -Tom

[TomZ]

Site is having issues with replies. I also posted this on the old Tapatalk site.

[TomZ]

In case anyone is watching this thread...

I dropped the emulsion bleeds to 026s in the first and third holes and blocked the rest (5-hole emulsion metering blocks), and then dropped the MAB to 025.

Ran the boat on Sunday in between thunderstorms.

Taking a large swing at the emulsion setup was a step in the right direction. Instead of it being lean as RPM increases, the curve seems rather flat but very rich.

In-gear idle is 12.8 or so - just enough to keep it from stalling when engaging the props. Taking off to get on plane, the AFR dips into the mid 11's - I might be able to clean that up some with a larger IAB but that wasn't the area of real concern. Once on plane and coming into the primary mains, I was seeing low-10's on the meter throughout the operating range. I took out the 25 MABs and replaced with the 28s that I already had. That brought the ratios into the upper 10's low 11's. The key though is that the we were able to safely high-load the engines to about 5k rpm - we couldn't do that before (in the end should see 5800 but we were out in some very sloppy 2-3 ft chop plus swell so didn't trim up much at all - running very flat and "in the water").

So what next? I think I'll run her with the power valves plugged to see how much jet can be taken out of the primaries and secondaries. I've been following the fixes of others with 1.56 venturi carburetors and think that we're getting closer than we've ever been with this.

If anyone wishes to comment, please feel free.

[TomZ]

[quote="TomZ"]Site is having issues with replies. I also posted this on the old Tapatalk site.[/quote]

Seems if you choose, "do not automatically parse URLs" it will let you comment.

[rgalajda]

test-------

[CDN]

Test

[TomZ]

Have been working toward getting these things working. So far…

PMJ 88
SMJ 96
PVCRs 070 (Holley HP blocks - lower than stock Holley)
Emulsion 026/B/026/B/B
PPV 8.5
SPV 4.5
TSR 100 (probably not needed)
IFRs 035
IABs 070
MABs 027
Acc Pumps P-30/s-50
Squirters 31/35
Progressive linkage

Tried to emulate the 80443 from very limited knowledge of the actual calibration.

Idle is rich at 12.5 and can probably be leaned out some. I need to keep the 035 IFR because of load just off idle.

I need to check vacuum with AFRs to understand load correlation, but still running a little lean under 4k rpm. 13.7 stable. I’d be happier with upper 12s/low 13s.

I did not test WOT but did observe 11.9 when jumping on it to avoid a safety concern so it seems that heavy load is much better. Possible to take a jet or two out of the secondaries.

I get that most of you here deal with street and drag configurations. Would still appreciate any additional advice provided.

[CDN]

Im surprised you have a sec power valve and that much a sec jet thats a big spread when using a sec pv
Idle rich yes pbly a .073 with .035 ifr will get you in high 13’s
How is cruise afr ? Rich ? Id think a TSR Around .082 would be closer .100 is pretty big

[TomZ]

I had a 081 TSR but thought I may have added it prematurely. I have QFT throttle plates but they measured out closer to a standard Holley so I went back to 100. I was worried that I may be interfering with lower rpm cruise. There was really no change between the two so the TS is probably right-sized as is.

For the type of load that a boat engine has the following is a general guide on safe running AFRs at different vacuum levels throughout the range. The key here is that the engine is always under heavy load, like an extremely heavy car accelerating up an endless 30-degree grade.

Light cruise = 6-8 inches of vac, 2600-3400 rpm, 13.5 max

Fast cruise = 4-6 inches of vac, 3400-4700 rpm, 12.5-12.8

WOT = 0-3 inches of vac, 4700+ rpm, 12-12.3.

Pretty far off the mark currently. Mine is a twin engine setup so there’s a little bit of leeway but not much, maybe a couple tenths at cruise.

My jetting is box stock what for a marine 80443 850DP come with. The configuration of the 80443 is to have two 6.5 PVs (mine are 8.5 and 4.5). My PVCRs might be a little larger but not by much. I’m imagining the over fueling on the secondaries is meant to keep the combustion chambers cool. Primary mains could be increased perhaps to 90s and so the trick? I do not have a choke tower and I understand that the Holley 850 with the choke tower (or remnants of) flowed a little less on the primary side compared to carburetors with fully contoured air entry.

Is my logic reasonable? Thanks!

[TomZ]

Forgot to mention that a high 13 idle in gear is not tolerable. The engines will stall. Low 13s are probably doable but only after being really warm. Screws are 3/4 out on all four screws.