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Chasing my tail with jetting. Need some guidance.

Old 05-13-2024, 07:51 PM
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Default Chasing my tail with jetting. Need some guidance.

As the season approaches I've been working on getting this thing ironed out. To refresh everyone's memory here's what I have:
  • 509 CI (twins)
  • 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 - Comp 953 springs.
  • Merlin intake with 1-inch HVH Super Sucker
  • Demon 1000 body with downleg boosters, QF 1000 throttle plate, Holley billet blocks.
  • CMI Etops exiting inside at the transom tips (TRS)
  • Timing, all in, 35 degrees @ 3k rpm
All testing done with engine hatch off the boat.

Previous setup from a few weeks ago AFRs with 88s and 98s and some crazy emulsion that was recommended by a friend that I bought the metering blocks from, .063 PVCR, .037 IFR - high position, .070 IAB, and .031 HSAB:

RPM > Port > Starboard >
1000 13.8 12.6 (ignore - idle screw issue)
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 - the emulsion was whacky and the HSAB was .031. I was running with the flame arrestors off and my buddy at the sticks stepping RPM. The primary mains didn't even start to come on until just about 3k rpm. They also were very sputtery. The secondaries weren't coming on until just at 4400 and also very sputtery. I had him shut it down. Decided to rework the whole thing by going back to basics with Holley's tried and tested 3310 emulsion.

Testing last week with the following setup:
  • Primary 88/Secondary 97
  • PVCR .070 w/ 8.5 PV (no PV in secondary)
  • HSAB .028
  • IAB .070
  • IFR .035 (moved to bottom position)
  • Emulsion bleeds (top to bottom) .028, blocked, .028, blocked, blocked, blocked
  • Siphon break .027
  • Pump nozzles: 31 primary, 35 secondary
  • .130 needle and seat (bad idea)
  • Floats at middle of site glass (Demon float bowls)
  • Fuel pressure at 5-6 psi (needle bouncing due to .130 needles).
First run:
RPM > Port > Starboard >
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.

Second run:
RPM > Port > Starboard >
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:
RPM > Port > Starboard >
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 - we were running out of room).

Pushing harder started to raise the A/F ratio and I didn't want to hurt anything so I got out of it. Also felt like there was plenty of stick left but it wasn't really doing anything. Boat was trimmed up about 6-7 on the card - but this boat trims well beyond what's listed). Spinning 27P Mirages (not pluses).

Oh, also noticed that I was getting some accelerator pump shot pullover as the rpms were getting up there. Had not seen that with the whacky emulsion tuning last time out.

So... still fat down low and lean upstairs with monster jetting. 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? But how does that affect the fuel curve from 2500 on up? Cruise seems to be coming on-line. But man does does she lean out up top!

Probably need transition slot restrictors on the primaries to control the rich transition. Or maybe open up the IABs?

I am surprised by how much jet these carbs need. I see many 502-based engine setups with 1000 4150s and 1050 Dominators - I don't recall anyone here talking about jet numbers this high. Anyone running jets over 100 on the secondary side with a 1000-1050 cfm 4150? Would really love to hear some thoughts and suggestions on direction.

TIA

Last edited by TomZ; 05-13-2024 at 08:50 PM.
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Old 05-14-2024, 07:39 AM
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Have you tried removing the HVH or are they installed upside down?
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Old 05-14-2024, 07:53 AM
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may not be the problem but something we learned in the race boats was that you cannot have a hood scoop open in the front and the back, one end only. the reason was the air flow straight thru was causing a vacuum in the flame arrestor and pulling fuel up out of the carb. running with the hatch off may be causing an issue with the air flow. try with the hatch on or make a shield for in front of the arrestor to block the air some and see if it changes something.
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Old 05-14-2024, 08:14 AM
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Originally Posted by Rookie
Have you tried removing the HVH or are they installed upside down?
Have not tried removing it yet its installed correctly.
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Old 05-14-2024, 08:21 AM
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Originally Posted by compedgemarine
may not be the problem but something we learned in the race boats was that you cannot have a hood scoop open in the front and the back, one end only. the reason was the air flow straight thru was causing a vacuum in the flame arrestor and pulling fuel up out of the carb. running with the hatch off may be causing an issue with the air flow. try with the hatch on or make a shield for in front of the arrestor to block the air some and see if it changes something.
Running without the hatch was done as a means to preventing being blown up, lol. I do understand what youre saying though, and know that the hatch being back on should richen things up.

Talking with a couple of carb builders - one strongly believes the carbs are too big for the engine combination and weight of the boat. Im not against dropping the venturi size. The goal is to turn 56-5800 - I would think a 1000-1050 would be what it needs, but Im not the expert.

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Old 05-14-2024, 12:46 PM
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with the hatch on it could richen it up some but more of my point was just the way the air flows around in the engine bay with the hatch off could be causing a problem. we had a modified boat with the scoop open front and rear and the carb was up in the scoop a little. found out the air flow thru the arrestor was actually pulling fuel up and out of the vent tubes due to the vacuum it created as the air flowed over. it is possible that with out the hatch the air is doing something wonky that is not letting it flow into the carb correctly. btw I dislike the BG parts as we tossed a lot of them in the trash due to quality issues. those were the later ones before they closed up.
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Old 05-14-2024, 05:22 PM
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The carb body and the bowls are the highlights at this point. One thing that the local guy here mentioned was that the Demon body, though it looks really good, can have trouble getting a good signal especially at lower RPM due to the bowl - Holley's stepped HP body is supposed to enhance signal because of the tumble created by the step. The Demon really shines in a WOT situation so long as it has enough fuel. I'm close to scrapping the bodies and using a stepped-style - will be basically all Holley from that point.

Have any of you seen one need this much jet? The original Demon 850/Race Demon 1000 came with a 86/93 jet combination and a .059 pvcr/6.5pv. With the factory emulsion bleeds many would jet up a bit - but not this much! Isn't there an old saying that if you have to jet up a certain number of sizes, the carb is too big? Not enough airflow early to make it work?

Should have waited and put these on the dyno to get right. Oh well.





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Old 05-14-2024, 05:25 PM
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Originally Posted by compedgemarine
with the hatch on it could richen it up some but more of my point was just the way the air flows around in the engine bay with the hatch off could be causing a problem. we had a modified boat with the scoop open front and rear and the carb was up in the scoop a little. found out the air flow thru the arrestor was actually pulling fuel up and out of the vent tubes due to the vacuum it created as the air flowed over. it is possible that with out the hatch the air is doing something wonky that is not letting it flow into the carb correctly. btw I dislike the BG parts as we tossed a lot of them in the trash due to quality issues. those were the later ones before they closed up.
I'm sure it will richen up some with the hatch back on - at least half a point if I had to guess. Still not enough to make it live upstairs. Maybe hatch back on and drop HSAB to 25 on the secondaries? May delay them coming on but may richen it up, too.
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Old 05-14-2024, 08:10 PM
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What does a plug check look like at 3500-4000 RPM to verify the AFR's are correct.
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Old 05-14-2024, 08:48 PM
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Sorry. Couldnt resist!!


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