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Dyno session w QF 850 "marine" carb LEAN

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Dyno session w QF 850 "marine" carb LEAN

Old 02-04-2020, 08:28 AM
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Default Dyno session w QF 850 "marine" carb LEAN

I had a new 502 short block I sold to a customer, he provided a set of port matched dart 325s, mild hyd roller cam, Vic Jr intake, carb spacer and Quick Fuel 850 ,marine carb, all new stuff in boxes. I assembled this combo and put it on my dyno. Right off the batt, hp/tq numbers were WEAK and afrs were LEAN. Floats set aggresively high, fuel pressure almost 7lbs. so initially we had some fairly cold, dense air (1200 feet dry air density "DA") with 55 to 60 degree intake air temp at carb. We were seeing numbers in the 15-1 to 17"s. Throttled shop air supply, turned up heat to get air temp motor was seeing into mid 80s (like it would in a boat), DA was now in the 2600 to 2900 range, CF went from 3 or 4 % to 6 ish, AFRS went into the 14 to 16 range. BSFCS in the .420-.450 range (too good to be true for a 9.5-1 motor). Out of the box this "marine" carb's initial calibration is what I would call "blow up lean". It had 82 primarys, #70 mid bleeds, #28 hi speed bleeds, w 6.5 PV. On secondary side no PV, #94 jets and same bleeds as primary. The Jet numbers seem "reasonably" large enough, keep in mid though, the circuits that feed them , internal circuits can make those jets act small or large. So we started adding jet which in turn gained hp/tq. Went from 82's to 93 primarys, went from 94 to #100 secondarys. One step before the 93/100s, our HP flattened out around 580/585, our tq was about identical. With the biggest jets mentioned, we gained another 10 ft lbs of tq and hp at least maintained (tells us were in happy range) our afrs were between 12.8 and 15-1 (#8 cylinder at worst spot), remember, I have individual cylinder 02 sensors and this is a NON fire swap cam (they seem to even this stuff up). I did NOT spec any of this top end.
Being this is getting much more restricted exhaust, the afrs will probably drop about a point in boat, would probably be ok. I could have put #25 high speed bleeds in BUT I didnt have any that small to try at this time. Sooo, we progress, now its time to drag my dyno brake like a guy drivinga boat at part throttle. So being my dyno is manual, I drag brake form idle to 5000 progressively loading it and giving it more throttle/higher load, ie, 250 ft lbs at 2500, 350 at 3500, on up. Do a graph, holy fawk, part throttle is between 14-1 and 17-1 (hottest/coldest cy;inders). I graph/correlate torque vs MAP, convert map to vacuum, my 6.5 PV isnt opening even at 400, almost 450 foot lbs of tq! Wrapped this up for the day that day, needed some #25 bleeds, 58/63 bleeds and a 8.5 PV to go any further. End of day yank mufflers off, we see 590 hp/595 ft lbs tq, of course, afrs are even leaner but hey, its FUN!
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Last edited by articfriends; 02-04-2020 at 09:04 AM.
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Old 02-04-2020, 08:35 AM
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Day two, I consult with my dyno partner/carb expert Norm over coffee, I decide to take primary metering block to him. He mods the whole jet circuit (opens up jet feed from .135 to .158 with drill then a reamer, wire gauges the low bleeds in plate, mods them slightly, mods circuits, feeds etc). He also supply's me with more testing supply's, ie, appropriate bleeds, power valves, some modded 100 Main jets that would be like 104's. I really have hard time wrapping my head around how opening up a feed hole from lets say .140 to .160 is going to do SQUAT when its feeding a jet that's about .105 ID BUT it does! Soooo, back onto dyno, make open header pull, before anything else (how I ended day previously) to establish a new baseline vs where we ended. #25 hi speed bleeds in rear/#63 mid bleeds in front, 8.5 PV . Motor is LAZY, RICH, and down 25-30 hp even with open headers . Graph afrs, into the 10s to 14-'s, FKNG IMPOSSIBLE. We didnt mod THAT much stuff! THEN I remember, I used vaseline to get bleeds to stick to screwdriver , I look, sure enough, still plugged with GOO (we all make stupid mistakes lmao). Clean that **** out, re pull it, miraculously we are still a little on rich side, powers down a tiny bit. I do some part throttle drags, motors wailing and blubbering, bsfcs into the sixes at PT, well, at least now we can start PULLING jet back out instead of trying to max them out on a little piss ass 600 hp build! (shouldn't be necessary). I end up going back to #70 bleeds in front, #90 then #88 primary jets, I rejet the rear, keeping in mind #8 is on fire at wot like most these open plenum deals, I stagger jet rear down from 100s to 98/96. Thing makes almost 600 ft lbs tq, same hp at 590. AFRS at wot actually close up a little and are between 12-1 to a little over 14-1, dragging it at part throttle, Im in a range of 13.5 to 12.5 average (still have hot and cold cylinders). I leave the # 25 bleed in rear, if customer feels its too rich in the boat, that could go to a #28 or 32, possible primary's could go down to 87 or maybe a 86.
The point of THIS write-up is to show how important PROPER carb tuning is on a marine application. IF you slapped a set of anybody's decent aluminum heads, intake, cam etc on a 502 and you bolted THIS carb on out of the box, you would most likely see ALUMINUM when you pulled the plugs on water after 20 minutes to get a reading! Smitty

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Old 02-04-2020, 08:51 AM
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And I bet it goes over 600hp
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Old 02-04-2020, 08:57 AM
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Originally Posted by F-2 Speedy View Post
And I bet it goes over 600hp
IF I had put some 89 in , hooked a msd box up we would have seen about 605 to 610 with open headers, never did though, very close to 600 though! The customer was told he would "blow his drive up IF he went over 600 hp" so, were keeping it at 595 ish, LMFAO!
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Old 02-04-2020, 09:05 AM
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Originally Posted by articfriends View Post
IF I had puts some 89 in , hooked a msd box up we would have seen about 605 to 610 with open headers, never did though, very close to 600 though!
It wasn't long a go some were bagging on the Vic Jr intake ? if I recall.........so stock Merc 502 dimple rods and forged crank, what rpm @ the 590hp pull
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Old 02-04-2020, 09:23 AM
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Originally Posted by F-2 Speedy View Post
It wasn't long a go some were bagging on the Vic Jr intake ? if I recall.........so stock Merc 502 dimple rods and forged crank, what rpm @ the 590hp pull
Not my favorite intake. 5800 w mufflers, 5900 w open headers. I would have liked to tried a performer rpm, would have probably made 10 less hp on top and picked up 30 to 40 ft lbs of tq where it coulkd be used. I did a overlay of the best open header pull vs best open header pull of Jeff Hoekstras 502 w chinese valeko heads and MPI intake I heavily modded, Jeff's 502 kicked this motors ass from top to bottom! Now, to be fair, jeffs had more cam so its not a real good comparison but even with his bigger cam, he out torqued this motor EVERYWHERE.
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Old 02-04-2020, 09:30 AM
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Norm doing his "magic"


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Old 02-04-2020, 09:32 AM
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Was there a hand written note in the box when he gave it back...............he didn't like my C&S carb
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Old 02-04-2020, 10:48 AM
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Yep. Most "newer" carbs have way too much emulsion circuit area. Usually, only 2 in each channel are needed depending on the application. To make things worse this usually causes light load rich conditions (automotive) and high load lean conditions.

Sounds like you are getting it sorted out.
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Old 02-04-2020, 11:40 AM
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Originally Posted by SS496 View Post
Yep. Most "newer" carbs have way too much emulsion circuit area. Usually, only 2 in each channel are needed depending on the application. To make things worse this usually causes light load rich conditions (automotive) and high load lean conditions.

Sounds like you are getting it sorted out.
This one was pretty much too lean everywhere but at dead idle, i was just shocked how far off it was out of the box. If a average joe skipped the dyno tune, didnt bother to put a 02 meter on boat, went out and drove this, unless it just broke up to point they knew it was lean, i strongly feel motor would get damaged in a fifteen minute drive trying to get good ol fashioned plug readings! Clearly this carb was really meant for a stockish bbc, not something modded in the 600 hp range! I would have chosen a bigger carb , smaller heads and prob a fire swap cam had i been one picking the parts combo, Smitty

Last edited by articfriends; 02-04-2020 at 08:41 PM.
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