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Chasing my tail with jetting. Need some guidance.

Old 05-20-2024, 08:00 PM
  #31  
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Originally Posted by articfriends
Heres a old post on a dyno job i did WITH a QF marine carb that was way lean:
Dyno session w QF 850 "marine" carb LEAN
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. Iím not against running a PV in the secondaries either (seems thatís the Holley marine thing to do).

Should have run these on the dyno.
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Old 05-22-2024, 06:54 PM
  #32  
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I pulled the carburetors last night and looked over the emulsion bleeds. The way theyíre setup might now, the top bleed (#1) maybe be just above float level
when running. Not sure itís 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 Iím 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 thereís a pretty good dip in the transition phase from 1500 and 2500, the engines wonít spend much time in that range so not really an issue. Itís 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. Weíll 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.

Weíll see what happens.

Last edited by TomZ; 05-23-2024 at 11:10 AM.
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articfriends (05-23-2024)
Old 05-26-2024, 10:03 PM
  #33  
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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. Weíre really rich now, andÖ
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 10ís to low 11ís when giving them the beans. Iím 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 thatís what I got. I think weíre now on the way.

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Old 06-04-2024, 10:42 PM
  #34  
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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 Iíll 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.



Iíll leave the mains at 88 and bring the secondaries up to 96 and see what she does. Thatís 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 Holleyís 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.
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Old 06-10-2024, 01:39 PM
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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.
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SB (06-10-2024)
Old 06-10-2024, 02:02 PM
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Reminds me why I went to EFI, best of luck to you !
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Old 06-10-2024, 03:42 PM
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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.
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Old 06-10-2024, 05:08 PM
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Originally Posted by TomZ
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.
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TomZ (06-11-2024)
Old 06-10-2024, 05:31 PM
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Originally Posted by TomZ
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.
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.
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Old 06-11-2024, 07:02 AM
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Originally Posted by GPM
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!
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