DaVinci 750 DP Questions

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middleagecrisis

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I came into possession of a low-time DaVinci 750 Double Pumper that was set up for a BB 383 running the 1/8 mile. The carb has sat a while and will need a rebuild. Although the carb is probably overkill for the SB 360 (915J 2.02/1.60 heads, single-plane intake, 230/236 hydraulic cam, 3K Stall/727 and 4.11 gears) I'm putting together, I figure it isn't going to lose horsepower over a comparable off the shelf Holley. I figured on doing a thorough rebuild on it but have a few questions. It currently has 80 main jets and 86 secondaries. I don't recall if the power valves were blocked off, but I will be going back with those on a rebuild. Given that this carb has extra airflow capabilities (no choke tower, blended venturies, etc.) should I baseline with stock 750 DP jetting? The air bleeds look adjustable, has anyone evet tuned one of these carbs, besides Dan DaVinci? I contacted him about a rebuild (no reply yet), but I'm afraid of the cost, given he does such high-end work.
 
middleagecrisis said:
I came into possession of a low-time DaVinci 750 Double Pumper that was set up for a BB 383 running the 1/8 mile. The carb has sat a while and will need a rebuild. Although the carb is probably overkill for the SB 360 (915J 2.02/1.60 heads, single-plane intake, 230/236 hydraulic cam, 3K Stall/727 and 4.11 gears) I'm putting together, I figure it isn't going to lose horsepower over a comparable off the shelf Holley. I figured on doing a thorough rebuild on it but have a few questions. It currently has 80 main jets and 86 secondaries. I don't recall if the power valves were blocked off, but I will be going back with those on a rebuild. Given that this carb has extra airflow capabilities (no choke tower, blended venturies, etc.) should I baseline with stock 750 DP jetting? The air bleeds look adjustable, has anyone evet tuned one of these carbs, besides Dan DaVinci? I contacted him about a rebuild (no reply yet), but I'm afraid of the cost, given he does such high-end work
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You may not realize it but your question is difficult to answer. What does DaVinci do to every 750 double pumper he builds? What if someone swapped his bowls onto this carb before you bought it? All you can do is rebuild it and put it on and tune it like a Holley and enjoy the extra range of adjustability that I'd have to assume it has. After looking at it, you don't know if it has replaceable air bleeds or power valves? These are some reasons why people are resistant to answer the question so, you shouldn't be frustrated with the community so quickly. Assuming it's got some nice features the best thing to do is put it back to its last 'calibration' for a baseline...do NOT put it to production Holley metering. Posting a few pics of the carb can help. Staggered jetting front to rear usually is done 99% of the time, as a 'calibration' which leaves the carb builder, because the primaries are using a power valve.
 
Dan is a great guy to deal with and is extremely reasonable on his pricing. It takes some time for him to get back to you as he runs the shop and oversees almost everything. If he doesn't get back to you shortly, give him another call.
 
Post 3 nailed it. It’s a Holley so treat it like one.

I’ve never had a DaVinci carb apart. Used one on some dyno tests a long time ago but never had it apart.

Without you giving exact measurements of everything including venturi diameter everything said I’d just a guess.
 
Newbomb Turk said:
Post 3 nailed it. It’s a Holley so treat it like one.

I’ve never had a DaVinci carb apart. Used one on some dyno tests a long time ago but never had it apart.

Without you giving exact measurements of everything including venturi diameter everything said I’d just a guess.
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People seem to think Carbs are some mythical creature of the black arts only the guy who put it together can make it work. What a load of BS.

As you well know scientific principles govern how carbs work and since Holley designed it stick with the parameters they set for their design.

Guys there's nothing magical about this stuff just science......
 
IMO
a 750DP is NOT overkill for a hi-compression/high cylinder-pressure 360, with your size cam.
I run about the same size cam (230/237/110), and same size carb (750DP), on the street, but with 3.55s,and with a Commando-M/T, and Alloy heads.
Normal idle-timing is 12>14, Power-Timing is 28*@2800 going to 32>34@3400. In the VA is 22* available. I run MJs of 72/84, and a 10.5PV, (87E10 gas) with fresh cold air from on top of the hood, straight down into the airhorn. I can idle her down to 550 rpm@ 5* timing, and 367 pulls itself around in gear at 4mph. Normal around-town 1-2 shift is 2800@20 mph; at Light-Throttle this will be up to 50* of timing.
That engine has seen two other cams, two other intakes and three other carbs, and; I would run more carb, just to try it but, but, she's a streeter 100%, and she already has more power than the chassis can handle.
I burn nothing but 87E10, and car has gone 93 in the Eighth at my weight of 3457 on that day, which calculated to ~433 hp.

I only ever took it down the track 4 times, and only ever completed two of those 4 runs. and only one of those had an ET, which, with a 60ft in the slow 2.s, sucked pretty bad; 7.92

But that ancient 750DP of mine is friggin'awesome!
 
ValiantS said:
What does DaVinci do to every 750 double pumper he builds?
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What can he do? Even if he changes the booster and the venturii size and shape it will still have an airflow rate and a fuel flow rate that will need to be matched to the engines AFR and LOAD requirements.

If you stick to Holley's original calibrations it wont be too far out.
 
ValiantS said:
You may not realize it but your question is difficult to answer. What does DaVinci do to every 750 double pumper he builds? What if someone swapped his bowls onto this carb before you bought it? All you can do is rebuild it and put it on and tune it like a Holley and enjoy the extra range of adjustability that I'd have to assume it has. After looking at it, you don't know if it has replaceable air bleeds or power valves? These are some reasons why people are resistant to answer the question so, you shouldn't be frustrated with the community so quickly. Assuming it's got some nice features the best thing to do is put it back to its last 'calibration' for a baseline...do NOT put it to production Holley metering. Posting a few pics of the carb can help. Staggered jetting front to rear usually is done 99% of the time, as a 'calibration' which leaves the carb builder, because the primaries are using a power valve.
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Here's the requested pics. Power valves blocked off, 84 Primaries & 85 secondaries. Like I said in my original post, this was previously used in a 1/8 mile drag car. I've spoken with Dan and he's a great guy, I just don't know if my budget will allow his complete rebuild price. If too many things need to be recalibrated to get this 750 streetable for my 360 build, then I may just have to save up.

750DP.jpg


750DP2.jpg


750DP3.jpg
 
Meh, its a Holley HP 750. Calibrate accordingly. Any carb unless it comes on a stock oem engine will need calibrating and fine tuning, that's what hot-rodding is all about.

Enjoy the journey and came here and ask lots of questions.
 
Newbomb Turk said:
Put it back to stock 4779 Holley specs and tune from there.
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My concern is the air bleeds and if there set for fat jetting with no power valves. A 4779 Holley doesn’t have adjustable air bleeds and how do I adjust these DaVinci air bleeds? That’s why I was asking if anyone had ever tuned on these before. I did a Google search and can’t find anything regarding tuning these carbs, or what correlates to a stock setting.
 
middleagecrisis said:
My concern is the air bleeds and if there set for fat jetting with no power valves. A 4779 Holley doesn’t have adjustable air bleeds and how do I adjust these DaVinci air bleeds? That’s why I was asking if anyone had ever tuned on these before. I did a Google search and can’t find anything regarding tuning these carbs, or what correlates to a stock setting.
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Look up Holley list numbers for the 750.

https://documents.holley.com/techlibrary_carb_numerical_listing.pdf

Buy a jet kit for that carb.
Buy an air bleed kit.
Buy a set of power valves

Start with a MAB of size of .028 IAB .068 from memory.
PMJ 70
SMJ 80
IFR's .31
Primary Sq .28
Secondary Sq .31
Power Valve 6.5

That should be enough to get you started and tune from there.
 
middleagecrisis said:
My concern is the air bleeds and if there set for fat jetting with no power valves. A 4779 Holley doesn’t have adjustable air bleeds and how do I adjust these DaVinci air bleeds? That’s why I was asking if anyone had ever tuned on these before. I did a Google search and can’t find anything regarding tuning these carbs, or what correlates to a stock setting.
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It’s easy with replaceable bleeds like that.

Off the top of my head I’d start with a .028 main air bleed, a .070 idle air bleed and no bigger than an .030 idle feed restriction.

Put the power valve back in and drop the primary main jets 6 sizes.

While you are in there make sure there are only two emulsion holes open, and they should be no bigger than .028.

That should get you a decent start.

If you post pictures we can walk you through it. It would be nice to see the guts of it.
 
What you need to understand is you need a reasonable baseline from which to work from and what was posted will provide that. Then as you work the Idle and Transfer slot then you move to mains making small changes as you go and pull out the plugs and see what they look like and to see if they correspond with the changes you make. Heres a tip. If you buy an O2 meter you can watch the changes in AFR as you change jetting to make sure it corresponds to what your engine is actually seeing.

All of this has to happen with the advance curve so make sure the ignition is up to the task and the advance curve is suitable.

There is a wealth of knowledge on this forum with some very knowledgeable people willing to help you on your new carb journey so utilize it.

Its actually better to buy a basket case and make it right so as to learn through the experience that will make you a better tuner in the end.
 
Reminds me of this ole antique i run on our iron head mill. Early 90s swap meet purchase. Bleeds are drilled on this one where yours is “fancy” with screw in style. It’s always ran great on a number of engines it’s been on. Btw I HIGHLY recommend the Superperformance gasket kits.

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IMG_1495.jpeg


IMG_1496.jpeg
 

middleagecrisis said:
Here's the requested pics. Power valves blocked off, 84 Primaries & 85 secondaries. Like I said in my original post, this was previously used in a 1/8 mile drag car. I've spoken with Dan and he's a great guy, I just don't know if my budget will allow his complete rebuild price. If too many things need to be recalibrated to get this 750 streetable for my 360 build, then I may just have to save up.

View attachment 1716366299

View attachment 1716366300

View attachment 1716366301
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Looks like a Proform main body to me.
 
Newbomb Turk said:
It’s easy with replaceable bleeds like that.

Off the top of my head I’d start with a .028 main air bleed, a .070 idle air bleed and no bigger than an .030 idle feed restriction.

Put the power valve back in and drop the primary main jets 6 sizes.

While you are in there make sure there are only two emulsion holes open, and they should be no bigger than .028.

That should get you a decent start.

If you post pictures we can walk you through it. It would be nice to see the guts of it.
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Agree with this but I’d drop the main back 10 sizes when I put the power valve back in it. Really It would depend on the pvcr size. Wonder if the op can tell us what that is.
 
You guys are clearly more knowledgeable on Holley tuning that I am. Where are the PVCRs located and what is a typical size or how are they marked? I’ll go out in my shop and look in the morning, as long as I know what I’m looking for.
 
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