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

Have you tried removing the HVH or are they installed upside down?

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.

Have not tried removing it yet… it’s installed correctly. :)

Running without the hatch was done as a means to preventing being blown up, lol. I do understand what you’re 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. I’m 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 I’m not the expert.

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.

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.

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.

What does a plug check look like at 3500-4000 RPM to verify the AFR's are correct.

Sorry. Couldnt resist!!

https://cimg3.ibsrv.net/gimg/www.off...668d06dece.jpg

I’m with you! lol Just tired of spending money!

I’d have to run a fresh set of plugs because these have been through it. Dark but from fuel. Even bank darker than odd. No pictures but next time I go out, I’ll have fresh plugs and I’ll take pictures.

I put the data into a graph… interesting to present it that way because the fuel curve can be visualized. For me that helps. Have been doing a lot of reading tonight and I think that there are a couple of things happening:

1. Down low, the IFR is still too big. I made some 33 and 34 IFR jets so I could give those a try, and if needed, I might be able to lean it out just a little with a 71/72/73 bleed. That could fix the fuel curve on the bottom.

2. As rpm goes up so does the A/F. Demons do run some serious jet due to the venturi and booster setup (so I’ve read). Next thing to try would be some smaller main air bleeds, 24 or 25.

Once the curve ends begin to flatten, I can mess with the jets more accurately. That’s the thought anyway.

Tom,
For reference my Barry Grant 4150 carbs on my old 590HP 454's ran 76/82 jetting with a 6.5PV, they flowed 950 cfm and ran an AFR of 12.2-12.8 with your existing cams.
I'd put a couple new plugs in convenient cylinders and go run it.

crazy how some want lots of jet and some dont to achieve afr target. Do you remember size of your PVCR feeds? and bigger question, regular bowl vents, j hooks or this newer bridged holley vent thing? I just installed one of these on a 850 carb on dyno and afrs went down a full point, had to pull all kinds of jet on a 600 hp 509. https://i.ebayimg.com/images/g/W2gAA...X/s-l1600.webp

Willy’s vent ?

I'm no expert in regards to jetting but they seem pretty high to me.
With all you've been thru already I hate to bring this up, but have you verified you're maintaining fuel press at high RPM?

[QUOTE=zz28zz;4898278]I'm no expert in regards to jetting but they seem pretty high to me.
With all you've been thru already I hate to bring this up, but have you verified you're maintaining fuel press at high RPM?[/QUOT theres a big difference in jetting between J hooks, regular vents and the "truck" vent I just posted, 10 years ago I wouldnt have believed it, thats why Im curious what kinda vents he has

something a customer just bought for 2 engines Im dynoing, says to be a holley part #

[QUOTE=articfriends;4898282]This makes sense cause the J-hooks would produce a slightly neg press in the float bowl due to venturi effect, where-as the straight vents and the bridged thing would have a slightly pos press in the float bowls from the ram air effect.

My comment wasn't directed towards the vents, just a general comment abt leaning-out at higher RPMs.. I went thru a similar situation when I first got mine. In my case tiny strips of teflon tape (from prev owner's handy-work) got caught in the fuel pump check-valves so fuel press dropped at high RPMs.

in a way this goes to my point of the hatch being off. on a dyno the air is straight down to the carb, in the boat with the hatch off the air is a big whirlwind of crap. in the case of the modified boat the carbs had regular straight vents and the air thru the arrestor was so fast that it would draw fuel out of the bowls. seems those vents would negate a lot of the straight down air or cross air or what ever the carb is seeing at the time.

PVCRs are .070.

I'm using J-tubes to make there, err, USCG compliant.

Reading up on these carbs in particular, they do seem to want a lot of jet.

Going to methodically work through the configuration... starting at the IFRs to fix the super-rich transition circuit and go from there. May even consider a PV in the secondary. Will have to test at each point.

Fuel pressure was good. I was standing at the back of the boat with the hatch off watching.

Might have to give this a try - easy to fab up for a test.

From my experience, IF a typical holley carb with regular vents needs to have say a 82 main jet to reach target afr, with J hooks that same carb may need a 88 or so from negative pressure in bowl screwing up the jetting and IF has the crossover tube like I just pictured, will be the opposite, that 82 would go down to about a 76.
Just wrapped up a dyno job for a member off here larose481, a pair of rebuilt HP500 mercs. he switched to promaxx 290 ovals, edelbrock performer rpm intakes, stock carbs off the HP 500s. We were at our target afr on tailpipe and had reasonable enough cylinder balance with 79/80 jetting in front and a 88/91 jetting on rear. After putting the vent on (j hooks were removed before dynoing), our afrs dropped a point or so. Had to go to 74/75 front and 84/90 rear to get back on target and get power back at same air denisty

How/Where are you measuring your AFR numbers. Only on one side of each engine? Or both sides of each engine. Have you done steady state cruise at a given rpm for several minutes, killed the ignition and checked the plugs. ( I do realize for many set ups this is no t always possible.)This will give you an idea of how equal the distribution is in each cylinder. Also if your checking port engine in one manifold, and starboard in the opposite manifold could explain your differences in readings between the two engines.

Okay, looks like most everyone selling those tubes now.
Referred to as

Holley Bowl Vent Tube Crossover Off Road Truck

AFR readings taken on port side of each engine. O2 sensors are installed in the tailpipes a few inches down stream from from the flange where it connects to the header elbow.

I have not done any real steady state runs for given rpm though have let it settle before going up to the next rpm point. We're testing in a small area and tend to run out of room. On the days we've gone out, the weather wasn't cooperating so this has been the compromise. But, even if I was out in the Chesapeake, anything over 3500 under load is getting to the point where meltdown could happen. It's just too lean above 3000 rpm. So I need to understand why everything begins to go so lean when increasing RPM.

I agree about checking plugs - need a fresh set to test with as these are a bit on the done side.

When I first talked about using these carburetors, there were some comments about it being pig rich if I increased the jetting over stock (stock with BGs emulsion was 86 PMJ/93 SMJ, .030 emulsion in 1-3-5, and .070 IAB, .028 MAB). It was lean from the get-go and needed 88s and 96s to run but still produced a lean pop at peak torque. And now, here we are.

So J-tubes would increase the jet requirement or reduce? At first the explanation was that it would dictate a need for more jetting, but then it looks like you had to take jet out of larose481's once the J-tubes went back on.

I have the original vent tubes to try to se what we get. I can definitely understand the effects the moving air would have on the J-tubes (and its effect on the float bowls). My tubes have breaks mid-way that should negate the effect but by how much is an unknown.

In your testing over the years, have you come across a fairly mild build, 500 cube, 600-ish HP setup needing so much jet? 91s with a 070 power valve on the primary side (and still be lean into the secondaries) seems seriously excessive, don't you think?

On laroses, the J tubes were removed before ever dynoing and never put back on. My carb expert /dyno partner Norm has said for years the J tubes impart a vacuum on the bowl, disturb the fuel in bowls and to the jets and cause the need for carbs WITH them to have jets approx 5 numbers larger to yield the same AFR, especially at higher rpms. Since they are prone to cracking and falling off, he always removes them. So simply removing them and putting regular straight tubes in carb body richens the tune on same engine. In Laroses case, he also removed them BUT felt the need to have something on the carbs for his customer. So he brought me both engines to dyno and ordered those "truck" tubes pictured BUT we started dynoing without them. I had tune pretty much dialed in on engine one and the truck tubes were scheduled to be delivered the following day so we left engine one on the dyno to see what happened when they went on. Well it was a good thing we did because the afrs plunged pretty much a full point, we were still debating how to get #6 cylinder out of the high 13s too as it was the leanest BUT paired to 7 which was the richest. We had afrs ranging from 10.5 to almost 14-1 and tailpipes were 1.7 points apart side to side from all the lean cylinders on even side. So surprisingly the truck tube not only richened the tune overall even at part throttle, it also closed the gap some between leanest to richest which was a plus. so I ended up pulling 5 jets sizes out of the primary's just to get back to the averages we previously had ( from 79/80 to 74/75), I also pulled secondary jetting down 4 sizes from 88/91 to 84/ 87 BUT between pulling fuel from front and rear, it drove # 6 back into high 13s, almost 14s. So I went back to a 90 in THAT corner, in end it had 74/75 in front and 84/90 in rear. I had a post 3 or 4 years ago on a 502 with a different brand 850 that needed all kinds of jet to get to a safe afr, I ll see if i can find it, Smitty

Heres a old post on a dyno job i did WITH a QF marine carb that was way lean:
https://www.offshoreonly.com/forums/...carb-lean.html

I remember that thread.

The guy I got the metering blocks from did open them up to match the boosters so at least that part is in my favor.

I think my issue has a couple of faults to deal with:

1. The j-tubes (easy enough to test), and ..
2. Air bleeds.

The emulsion is now the same as a standard Holley double pumper from way back when. I might benefit by moving the number 1 and 4 bleeds to number 2 and 5 (and replace 1 and 4 with a plug) but I think testing the air bleeds would be an easy first before having to open them up. They have 28s now - perhaps dropping to 25s will see some benefit. I’m not against running a PV in the secondaries either (seems that’s the Holley marine thing to do).

Should have run these on the dyno.

I pulled the carburetors last night and looked over the emulsion bleeds. The way they’re setup might now, the top bleed (#1) maybe be just above float level
when running. Not sure it’s a problem but it would be confined to the low range anyway. The part that might be an issue is where the last emulsion bleed comes into play. But for now I’m going to leave it.

I drilled some .025 main air bleeds to replace the .028s. This is a change of area of 24 percent (significant) and should drastically bring down the numbers. The Idle is where I want it and even though there’s a pretty good dip in the transition phase from 1500 and 2500, the engines won’t spend much time in that range so not really an issue. It’s possible the change in main air bleed may delay the mains a little. Upping the idle air blleed a touch may calm that down, too. If this works (it goes really rich) then I can take some jet of it. We’ll see.

Currently, the jet area between the primaries and secondaries is way off. Primary total jet area (with pvcr-12%) is .01246. The secondaries are .01150 so something is way way off.

** I also pulled the J tubes and put the stand vent tubes back in.

We’ll see what happens.

Getting there!

What an absolute effort to get on the water today! Late start, flat on the trailer on the way out, ramp closed because full (we are surrounded by water here and there are only two ramps - two - ridiculous, seriously), and then some nasty pop-up thunderstorms. Finally got out around 4 pm but lost the DLG because the entire cockpit got soaked in said thunderstorms while trying to get out.

The DLG eventually came around and we got some good data.

1. We’re really rich now, and…
2. The fuel curve is flat.

Emulsion is 026/blk/026/blk/blk (last test was with 028, same positions).

Started with 025 main air bleed but changed back to 028 on the water. Brought the ratio up about half a point.

A/F ratio in the upper 10’s to low 11’s when giving them the beans. I’m going to start pulling some jet and retest later this week. Hoping to drop the primaries in to the mid 80s/secondaries low 90s. With the fuel curve now flat, the jetting should be able to move the whole scale up or down.

From my studying on this, big swings are needed to establish baselines and that’s what I got. I think we’re now on the way.

Went out Saturday with a new tune but had to drop back some.

Started off the day with 86/93 and 6.5 pv. In the primaries.

Was real fat off idle so installed a 081 t-slot restrictor in the t-slot feeds on the main body. My baseplates are QFT pieces that have larger transition slots and 081s were a good starting point on a 1.56 main body. Both engines seemed to like the restricted t-slot - I left the idle screws and throttle plate openings where they were when I took them off the boat. Idle was good after warmup and going into gear only dropped idle by a maybe 200 rpm if that.

Running out in the Bay for a brief time, the engines did not like the combination of t-slot restrictor, reduction of main jet nor the change to the later opening pv. Ended up putting the 8.5 pv back in it but was still running lean. I was trying to keep everything in the upper 12s/low 13s and lighter load. That helped some but testing was difficult due to a bunch of slop so we headed back in to stay in-shore.

Swapped to 88s in the primaries. Better but still leaning out above 4k especially as the heat soak got going.

Going forward, I think I’ll go back to the 6.5 pv and watch my vacuum gauge to tune for load. Smitty gave some good direction in another thread I found that correlates vacuum/load and acceptable a/f ratio. Up to 13.2/13.5 max for light load cruising through 3200 or so based on vacuum (6-8 hg). Above that shoot for 12.5 to 12.8 (3-6 hg), and low 12s when vacuum is below 3 inches.



I’ll leave the mains at 88 and bring the secondaries up to 96 and see what she does. That’s pretty close to equalized. The 80443 Holley 850 also uses a power valve in the secondaries. I may try to mimic that solution - would be helpful to have all of Holley’s specs on emulsion and pvcr - very hard to find. With the above jetting, I think they were setup with a standard emulsion config (028s in first and third, the rest blocked), 028 PMAB, 025 SMAB, and 067 pvcrs (maybe 063 in the secondaries). Have to run it and see.

Ran the boat for Harborfest. On the trip up the bay, cruise AFR was in the mid 13's. I can probably work on the air bleeds to richen it up a little but overall she ran well. I did not run the boat hard at all but did see on a couple of occasions where AFR dipped to 11.8/12.0 when getting off the gas and then jumping back on it hard to maneuver around some traffic. Thinking I might take a little out of the PVCRs so that cruise stays good. PVCRs are 070 and low in the blocks. I used 8.5 PVs in the primaries and 4.5s in the secondaries. I ordered some 3.5s that I could try out to delay the delivery. Has been a fun learning experience.

Reminds me why I went to EFI, best of luck to you !

Thanks.

I did contemplate EFI but considering what I would have to had to put out for a real system (not some sniper junk), I decided the carburetors would do what I needed them to do.

With a digital timing system and AFR, you're most of the way to FI system. Carbs can be well tuned good runners, and hard to beat the reliability.

Carbs are fine, I was never good at tuning them, can't say I've mastered EFI either. I would rather push keys than dump gas everywhere changing jets, air bleeds, power valves, and whatever else is in there.

i can attest to it being no fun especially trying to change them in the boat stuck in between troughs and being pitched all over the place. Close to blowing chunks a couple of times too!