Holley 750DP on 360 - Too Rich?

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craigibc

wishin' my Demon was done
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Hey Everyone,
So I have a pretty much bone stock '72 360 (8.5:1 comp) that has a weiand aluminum intake, hooker comp headers, and a Holley 750 DP that I put on it; per advice from a friend.

So I fired the car up this week and it seems to run INSANELY rich, black smoke everywhere. Is it even feasible to have this carb on a 360? I have about $200 into this carb, so I am stuck with it. If you have any advice for jetting it down or what not, please let me know.

Thanks,

- Jon
 
Jon,

Adjust the float levels, and see what it's pulling for vacuum. if the Power valve is above the vacuum it's pulling, you will have issues.

My suggestion, get the floats correct and put power valve plugs in it until you get the idle mixture correct.

BTW, Timing first, crank as much initial into the car that it will handle without kicking back on the starter or running on. I'd run in the 14-16 range as a start point.

Kyle's car has a 750dp on it and it runs good. 20+ mpg on the freeway.
 
Thats what I just picked up for Tara's dart at the recommendation of my engine builder/mechanic friend.

the 360 we are going to be running is a '71 (8.5:1) performer manifold, mildly ported J heads, comp 284-284 480-480 cam, msd 8mill wires, orange box, electronic dist, Hooker comp headers. we think it will be about 330hp, maybe if we are lucky.
It's not the best combo but we are putting it together for about 1k so it will have to do for now.
 
Like cracked says, get the timing right first as that will keep the primary throttle blades out of the transfer slots and make sure the float level and fuel pressure are correct. If the power valve is blown, it'll barf fuel out of the boosters at idle too.

Here's the kicker. If you run 14-16 degrees of advance on a stock or MP distributor, you'll have way, way too much total mechanical advance. If you have an MP distributor you can solve this problem. Check out my tutorial here: http://www.forabodiesonly.com/mopar/showthread.php?t=67882 You can also weld up the slots in the advance weights on a stock distributor, but you'll most likely need to make a advance spring change as the stock spring/s are too light.
 
Also a quick check to see if the power valve is blown is to turn you idel mixture screws all thew way in on booth sides if there is no responce that power valve is toast.Holley has a tech site with lots of tech help there.
 
Holley 750 d.p. is too much carb for a "stock" 360,and seeing as you paid $200.00 i'll assume its used 'first thing i'd suggest is a rebuilt kit to see what your dealing with and go from there,a vacuum secondary carb would be a much better choice too...
 
stroked 340 said:
Holley 750 d.p. is too much carb for a "stock" 360,and seeing as you paid $200.00 i'll assume its used 'first thing i'd suggest is a rebuilt kit to see what your dealing with and go from there,a vacuum secondary carb would be a much better choice too...
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The carb was rebuilt by me with help from a friend. I personally believe its too much for my currently stock 360. Its a mechanical secondary, and have a power valve plug in the rear metering block. Thanks for all the input guys, I'll check into what was said.

- Jon
 
Just take the time to get the jetting right.
 
The 750cfm DP is a bit much for that engine... I paid a lot more than $200 for mine, so first of all that seems like a good deal to me. Secondly, I would strongly second what rumblefish said. Holley makes a jet kit for that and I would strongly suggest picking one up. They tend to run about $70 I believe, but it has all the jets you could ever need. Its a great kit, especially if your kinda guessing at how far to step it down. But step down the jets for that engine, and when you beef up your motor a bit to be able to handle it, just bump the jets back up to the original size.
Hope that helps.
 
A Holley 750cfm is too much carb for a stock 360
 
It maybe over-sized, but it'll run. A carb only draws what air the pistons suck in. Fuel is drawn by that same air. As mentioned, jetting may be rich. The idle curcuit may need to be leaned, too. That is, the idle feeds, not the mixture screws. DP's are set up pretty rich from Holley, or at least they used to be.
As an aside, I think a lot of carbs are set up too rich. Think of an ox/ace torch. When you LEAN it out, it gets hotter. Lean is power, to a certain point.
 
Lean is mean but fat is where it's at for a saftey point of view. More so with N02 or supercharged engines. You can allways jet it down. The issue with big carbs is there harder to tune and get runing well since the air tends to move slower throught he venturis.

You can do it. Take your time. It'll be fine.
 
I ran a 750DP on a 9:1 318 with no issues....granted it was jetted 68/70...6.5PV, and MAN it sure did run good.
 
crackedback said:
My suggestion, get the floats correct and put power valve plugs in it until you get the idle mixture correct.
Click to expand...


If you do this don't forget to up the jetting 8-10 sizes and square it up/same jet size primary/secondary.HINT if it only has a primary PV and you remove it change/up only the primary jet.

And definitely check the float, adjusting it so that fuel just makes the bottom of the site hole.

Stock jetting should be 70-71 primary 80 secondary with 6.5 PV in primary only - no PV in secondary - no provision for it either.
 
1wild&crazyguy said:
If you do this don't forget to up the jetting 8-10 sizes and square it up/same jet size primary/secondary.HINT if it only has a primary PV and you remove it change/up only the primary jet.

And definitely check the float, adjusting it so that fuel just makes the bottom of the site hole.

Stock jetting should be 70-71 primary 80 secondary with 6.5 PV in primary only - no PV in secondary - no provision for it either.
Click to expand...

Yes, if you plan on running the PV plug you have to make up for the loss of fuel flowing through that metering by upping the jetting.

I only use the PV plug to get the idle squared away and grab a vacuum reading at idle and in gear. Once you get those two vacuum numbers, you select and install an appropriate power valve. The plug is only used an initial tuning tool to take one more variable out of the equation.

Plus, running an older carb with no PV protection, you don't smoke PV's with an accidental backfire.

I'm going to guess jetting will be in the 67-75 and 6.5-7.5 PV
 
SpeedThrills said:
It maybe over-sized, but it'll run. A carb only draws what air the pistons suck in. Fuel is drawn by that same air. As mentioned, jetting may be rich.
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The problem with an over sized carb is that the air velocity through it isn't high enough to properly draw fuel from the bowls. This results in throttle response and bogging issues. A 750 DP is quite a bit over sized for a stock 360, it can be made to work with careful tuning but it won't have the throttle response or driveability of something in the 600-650 range and won't give you any more power than the smaller carbs either.

Carbs and cams is definitely and area where bigger is not better, everything needs to be matched for the intended application.
 
dgc333 said:
The problem with an over sized carb is that the air velocity through it isn't high enough to properly draw fuel from the bowls. This results in throttle response and bogging issues. A 750 DP is quite a bit over sized for a stock 360, it can be made to work with careful tuning but it won't have the throttle response or driveability of something in the 600-650 range and won't give you any more power than the smaller carbs either.

Carbs and cams is definitely and area where bigger is not better, everything needs to be matched for the intended application.
Click to expand...

The statement ''won't give you any more power than the smaller carbs either''
Is bogatah.

Only a dyno or the driver of the car would know that, besides we know the bigger carb to an extent will most always increase the top end hp with a lil tq knocked off the lower rpm.imo

A 750 is 375cfm for each 2 barrels, 600= 300cfm per 2 barrel, so 75cfm diff or 1 11/16 opposed to 1 9/16.

But the point is, if you can't make 375 cfm initiate signal on a 360 then it must be 360 cubic millimeters.lol
So tune the primary first for take off/cruise then fiddle the secondary/your right foot to make the transistion as smooth as possible.

Something that can be KEY in using a carb on the big side is intake choice, A dual plane could come into play.

I think careful pump shot tuning will initiate enough signal for this app, and while maybe not Ideal with this combo, will work fine if jetting and pv selection are chosen by using the standard methods given in this thread.

good luck and ask away any questions you have there are enough people here to help.:cheers:
 
The problem with your statements is that an engine is simply an air pump and the amount of air it can ingest is a value that can be calculated knowing the displacement and rpm. A 360 turning 6000 rpm can ingest 625 cfm assuming 100% volumetric efficiency. But only highly sorted out race motors will approach 100% VE a more realistic number for a stock motor is 85% and at that value you are talking about 530 cfm. If the engine turns less than 6000 lets say 5000 it can only consume 442 cfm. You would need to be turning 8000 rpm to get over 700cfm.

Now it's not as simple as calculating the max air flow and picking an equivalent carb. The carbs are rated at a pressure drop across the venturi. More pressure drop and they flow more less and they flow less. For max power at WOT you want zero vacuum in the manifold so the carb will flow less than it's rating so you do need to go bigger to prevent the carb from becoming a restriction. But going bigger than what is necessary will not increase power but it will impact low rpm throttle response and driveability.

I have had 600, 670, 725 and 750 on my 360 which is quite a bit hotter than a stock motor and there is no difference in top end up to 6000 rpm between the 670, 725 and 750. But the low rpm throttle response and driveability is much worse on the 725 and 750 than the 600 and 670. There is a noticable but not big drop off in power above 4500 rpm with the 600.

As I previously stated the 750 DP can be made to work with careful tuning but it will likely require more than just main jet and idle mixture adjustments. Things like air bleeds, boosters, accelerator pump nozzles will likely need to be changed to make the car perform acceptably in normal driving and will never be as good as a smaller carb.
 
dgc333 said:
The problem with your statements is that an engine is simply an air pump and the amount of air it can ingest is a value that can be calculated knowing the displacement and rpm. A 360 turning 6000 rpm can ingest 625 cfm assuming 100% volumetric efficiency. But only highly sorted out race motors will approach 100% VE a more realistic number for a stock motor is 85% and at that value you are talking about 530 cfm. If the engine turns less than 6000 lets say 5000 it can only consume 442 cfm. You would need to be turning 8000 rpm to get over 700cfm.

Now it's not as simple as calculating the max air flow and picking an equivalent carb. The carbs are rated at a pressure drop across the venturi. More pressure drop and they flow more less and they flow less. For max power at WOT you want zero vacuum in the manifold so the carb will flow less than it's rating so you do need to go bigger to prevent the carb from becoming a restriction. But going bigger than what is necessary will not increase power but it will impact low rpm throttle response and driveability.

I have had 600, 670, 725 and 750 on my 360 which is quite a bit hotter than a stock motor and there is no difference in top end up to 6000 rpm between the 670, 725 and 750. But the low rpm throttle response and driveability is much worse on the 725 and 750 than the 600 and 670. There is a noticable but not big drop off in power above 4500 rpm with the 600.

As I previously stated the 750 DP can be made to work with careful tuning but it will likely require more than just main jet and idle mixture adjustments. Things like air bleeds, boosters, accelerator pump nozzles will likely need to be changed to make the car perform acceptably in normal driving and will never be as good as a smaller carb.
Click to expand...

It looks right on paper but there are a lot more variables to it, those same formulas say like 650 for my 340, which was tried and sucked with 'lil snappier bottom and nothing beyond 5000 rpm tq wise. just my case.

BTW thermoquads around 900-/+ cfm, 280cfm primary and like 600cfm secondary with no boosters just a air door that upon mechanical opening of the secondary corks the engine to get air speed up enough to not fall on it's face, but wait, thats a worse scenario than this?imo

I'll say it again if can't get 375cfm tuned to run on a stock 360 and simply control your right foot to ease into the secondarys, that btw have a booster in every barrel to mix/atomize the fuel! then there is a problem other than the carb.

Yes dave I want you to know I understand where you're coming from and thats a good general guide line, I just wanna keep the focus on fixing/helping in tuning his carb.

This can work though double pumpers are not the best choice for stock/automatic cars.

Justin.

BTW ran an 850 on a stock 340 in a 69 dart 4spd, yeah it blubber'd a lil but would toss yer head on to the back seat 3000rpm up.
 
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