Demon 850 Baseline Jetting
 

Guys, I'll have my 509s in the boat soon and will be ready to test. I have not dyno'd the engines, but one has already been test fired and the other will be done this weekend. We couldn't get dyno time worked out and I'd rather get the boat back together than wait. The starboard test fire was good, strong oil pressure, not leaks, etc.

Engines are 509s with as-cast CNC chamber AFR 325s, 112cc chambers, .646/.646 235/241 113 hydraulic roller (Rookie's old cam), Merlin 4150 intakes, HVH spacer, and Mighty Demon 850s (down-leg boosters not annular).

I've baselined the carbs per Demon's instructions. Based on what I've read regarding the Mighty Demon 850 I went ahead and increased jets to 88 primary and 96 secondary with a 4.5 PV on the primary side (originals are 80/88 and 6.5 PV).

Good start? I will be tuning with a A/F meter in the boat but curious with regard to how close (or far) I may be.

Why the 4.5 Power Valve? Me think you going the wrong direction on that.
Tom, what afr system did you go with?

Oooh, long time ago but I believe my 502 w/Canfield heads was very close to out of the box jetting with 850 DP Sea Demon. I had tuned it on the dyno w/wet exhaust. I'll see if I can find my notes. May take me a bit. ie hrs or days.

Ordering the Innovate dual gauge that you mentioned the other day.

I had the 4.5 pv that TomR sent with the carbs so I used them. What are your thoughts? Bring them in earlier?

Under load I think you are going to lean out at the top of the primaries. I would start with the 6.5 or even higher. Don't want a lean condition under load.
Did you get two of those gauge kits? Don't forget to use 90* spacers/heat shields, every try without them cost another $80-$160 for new sensors.
I got mine on ebay for about $20 a se\t. I'll try to find the link.

I believe these are the ones, it does not say stainless but I think they are, call them to verify.

2x Oxygen O2 Sensor Spacer Extender Adapter M18x1.5 90 Degree Bung Pipe CEL Fix | eBay

I don’t have the room to use those so they’ll be direct into the tailpipes right after the water bridges. My tailpipes are TRS to I’m really far from the water outlet as it goes into the tips.

Thanks!

I read the Might Demon liked a lot more jet across the board. I’ve got enough jets to play with everything. Maybe I’ll just put it back together all stock and see what we get?

Possibly, i dunno. Most 850 everything are in the 80’s for hetting

How can those 90* things can be accurate when they aren't directly in the exhaust flow ? seems they would be getting stagnant uncirculated fumes

There is nothing stagnant going on. Cars do it that way and they work in my marine set up.

i TESTED W/O THEN WITH, NO CHANGE IN AFR

Those engines are gonna be pig rich with that increase in jets. You'd be pretty close with no power valves at all now. Rule number one in carb tuning is start with it the way it comes out of the box. Then make incremental changes. You've already confused yourself making changes without knowing what you had to begin with.

Well Said

Tomz / as a maybe heads up, i did some quick searching around, and in 2 different instances they mentioned holley changred some things (air bleed sizes and etc) on these carbs. Therefore a pre Holley vs current holley made BG may need different jetting requirements.

I’m going to put them back to stock and see what I get.

Bringing this back to the top.

Current setup is close to stock jetting (85/93 - two up on the primaries) with a 8.5 pv., 5.5 lbs fuel pressure (Mighty Demon 850 down-leg boosters).

I got out yesterday for the first time this season. It was really crowded - more than I've ever seen outside of one of the major holidays. Lots of idiots out yesterday - so getting a chance to really run her out didn't happen until later in the day. Overall the boat ran well. I need to fix up the idle a little on the port side but that was only a minor inconvenience.

One thing that popped up was after getting out into the big water. We were clipping along about 4500, mid 60's and I thought I'd try running her a little harder. I pushed her some more (not WOT) and she popped out of the intake on starboard. AFRs were generally in the 12.5 range, but I cannot confirm what they were running at when it popped. We were in a pretty good head sea getting some air at times, and being that I'm afraid of melting things, I didn't want to press my luck. Water temps were averaging about 160-165 at speed, oil temps 200, 50-60 psi oil pressure. All good signs other than the lean pop.

One thing I thought of is that maybe I'm running out of pump shot on the secondaries.

The 9022 800 uses a 30cc pump on the primary side and 50cc on the secondary. Mine are both 30cc pumps. I think the carbs are pretty close to being good. Jetting up on the secondaries to a 94 or 95 might bring the AFRs down to a rich/safer level. But I don't want to change too many things at once and think increasing the pump shot will help sort it out some.

What do you guys think?

Wow this thread is reading my mind, I too am looking to fire my fresh 509 this weekend, it has a Holley 850 Marine carb and I was curious what to do with jetting. I am an EFI guy so this jetting thing I have never played with. eventually the boat is going to Holley HP but since it is on B/O Holley sent me a Marine 850 to get me on the water. I installed an AEM wide band in the new dry tip risers so I can see what its doing for tuning and my plan was "don't touch a thing" until its on the water since I was not able to get it to a dyno.
What jet kit to I buy to take with me so once on the water I can tune? My buddies father is an old school drag racer so he's coming with me as he knows carbs but I want to be prepped so what do I buy as a for jets and things so we have what we need?

502 brand new GM block
.030 over
stock iron heads
Stock magnum rods and crank
Holley EFI single plane intake but with a carb on it and the injector bungs plugged

Isky cam

• Hyd. Roller
• Valve Lift Int/Ext. 0.553/0.578
• Valve Lash Hot Int/Ext. 0.000
• ADV Duration Int/Ext. 275/284
• 0.050 Duration Int/Ext. 228/238
• Lobe Center 112

Im thinking the carb should be close out of the box but again going by what im told I have no real experience with carbs.
https://www.holley.com/products/mari.../parts/0-80443

I'd try it as is. I'll bet it's pretty close.

Couldn't hurt to have some jets on-hand. A few above and below what's in it now.

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so that carb comes with 88/96 jets so you should be plenty safe for a starting point, Smitty

Not recalling the specs on the HP 500 or the Merc jetting. I built a HP 500 clone and used the HP 500 staggered jetting and things were fine for years with that carb.