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Think my carb is too big
#445900
08/22/09 11:22 AM
08/22/09 11:22 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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1969 Dodge Dart GT 360 out of a 78 chrysler new stock cast crank, stock rods and bolts , speed pro hyper 4 valve relief pistons, comp xe284 hyd cam, mopar ductile rockers studed with crane adjusters (dont ask why I had to upgrade), ede heads mild port, ede air gap, 750 street hp dbl pumper, carter high flow mech pump, headman bomber headers, MSD PRO BILLET dist 13 base 32 total, msd 6al box, 727 2800 midwest converter, b&m mega shifter, 8 3/4 3.91 richmond
I have only been able to get 4-6 mpg. Plugs look golden brown but while idling in gear at a stop sign it starts to load up. While cruising in town lower rpm it pops out the exhaust.
It has no hesitation or stumble at wot
I have a 600 dp laying around wondering if its worth a try and what jets I should start with.
4150hp 750 dp I curently am using has 68's and 74's with a 4.5 pv
Last edited by skep419; 08/23/09 11:59 AM.
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Re: Think my carb is too big
[Re: skep419]
#445901
08/22/09 11:27 AM
08/22/09 11:27 AM
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Joined: Sep 2007
Posts: 14,889
up yours
Supercuda
About to go away
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About to go away
Joined: Sep 2007
Posts: 14,889
up yours
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IMO, a double pumper is a waste on a streetable car. YOur cam isn't really big enough to require one.
You have to tune the idle circuit and I am not talking about the idle mixture screws here. There are innumerable books out there on how to tune a Holley, get one, read it, ask us questions till you understand it then buy a wideband O2 sensor and tune the Holley.
You have to understand that ANY double pumper is going to have a substantially richer out of the box idle circuit than a vacuum secondary Holley, it's how they are built. When you use a double pumper in it's factory intended role it works. When you use it in your application it doesn't work so great.
You are going to have to tune the idle feed restriction and idle air bleeds to fix this.
Oh, a 750 might be a bit too big for a 360, but it isn't so far off that it isn't useable with some tuning. you might get a bit better mileage and a hair crisper low speed out of a 600-650, but the 750 will likely improve your topend. Depends on what you want.
They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
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Re: Think my carb is too big
[Re: skep419]
#445905
08/22/09 01:30 PM
08/22/09 01:30 PM
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Joined: Jan 2003
Posts: 864
St Paul MN
73swinger
super stock
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super stock
Joined: Jan 2003
Posts: 864
St Paul MN
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Skep419, is the powervalve the right one? is it opening at in gear idle? that would relly kill mileage. Just for comparison, my 750 DP equipped 340 dart with 727 & 3:91 gears gets at least 10 mpg.
Chris Schwartz
73 Swinger 340 12.451 @ 108.78
73 Gold Duster, needs a plan.
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Re: Think my carb is too big
[Re: skep419]
#445908
08/22/09 02:22 PM
08/22/09 02:22 PM
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Joined: Feb 2003
Posts: 10,214
Someplace you aren't
SomeCarGuy
I Live Here
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I Live Here
Joined: Feb 2003
Posts: 10,214
Someplace you aren't
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Bump the timing up some. 32 total leaves you some room to go up in the initial and stil be OK on the total.
a 750 DP comes with 72/80 jetting. You are way under that now. I would like to see what it does with the stock jets as I bet you have the idle mix screws way out right now, and might even have the idle speed set high. Resetting the timing should help that some.
Do you have the vac advance hooked up?
I want my fair share
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Re: Think my carb is too big
[Re: forphorty]
#445909
08/23/09 11:47 AM
08/23/09 11:47 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
4-6 mpg is not due to having a 750 dp. You have other problems. Check the float level. Sounds to me like it is trying to flood.
float level good.
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Re: Think my carb is too big
[Re: 73swinger]
#445910
08/23/09 11:49 AM
08/23/09 11:49 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
Skep419, is the powervalve the right one? is it opening at in gear idle? that would relly kill mileage. Just for comparison, my 750 DP equipped 340 dart with 727 & 3:91 gears gets at least 10 mpg.
The vacuum jumps from around 3-5 at idle. But I watched the holley video and they said to have the car in gear while you read the gauge. So the 4.5 I'm running could be wrong.
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Re: Think my carb is too big
[Re: Pacnorthcuda]
#445911
08/23/09 11:54 AM
08/23/09 11:54 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
Have you checked for a blown PV? You really need to rule that out. Your jetting doesn't sound that fat for the carb, although a 750DP isn't ideal for your car. A reduction in jetting might help the problem but I think its a partial bandaid for whatever else might be wrong. Might you have 50cc accelerator pumps on it? (instead of 30's, at least on the primary)
Do you drive with a reasonably steady foot?--DP's can kill MPG if the driver is a constant pedal modulator!!!
Thats a pretty big cam if you don't have say at least 9:1 compression to match.
Is your 4-6 mpg number accurate or might it really be 6-8? Is your odometer accurate or does it read 'slow'?
just random thoughts
I have 30's all the way around. I normally don't have a steady foot but I cruised a full tank out on the highway and the mpg didnt change.
The compression figures out to 9.3
My speedo is slow by 10mph never thought of that 
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Re: Think my carb is too big
[Re: RobX4406]
#445912
08/23/09 11:56 AM
08/23/09 11:56 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
13* initial with a xe284 isn't going to cut it. Add at least 5* and limit mechanical to get your total.
My guess, it's loading up because the throttle plates are open too far at idle.
Got more than one 360 with a xe268H running 750dp's with no problems. One gets 20mpg on the highway with 2.76-2.9x gears
Throttle plates are not open to far at idle. Primarys have 1/8" breathers
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Re: Think my carb is too big
[Re: SomeCarGuy]
#445913
08/23/09 11:59 AM
08/23/09 11:59 AM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
Bump the timing up some. 32 total leaves you some room to go up in the initial and stil be OK on the total.
a 750 DP comes with 72/80 jetting. You are way under that now. I would like to see what it does with the stock jets as I bet you have the idle mix screws way out right now, and might even have the idle speed set high. Resetting the timing should help that some.
Do you have the vac advance hooked up?
Idle mixture screws are all just under 1 1/2 turns.(best vacuum reading)
I forgot my dis took a dive a few months ago. The MP is now a MSD pro billit.
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Re: Think my carb is too big
[Re: skep419]
#445914
08/23/09 01:06 PM
08/23/09 01:06 PM
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Joined: May 2008
Posts: 5,399
Aurora, Colorado
451Mopar
master
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master
Joined: May 2008
Posts: 5,399
Aurora, Colorado
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After checking for fuel leaks, check float levels, needle/seat sealing, make sure float does not have a pin hole or crack letting fuel in and sinking the float, and power valves (a leaking/blown power valve will cause problems.) If you are using a mechanical fuel pump check fuel levels with the engine at a higher than idle RPM. I watched a Chevelle on the chassis dyno that seemed to run/idle fine, then under acceleration it was blowing fuel out the carb vents.
Anyhow a 750 carb is a good size for performance, but a lower RPM street driving you will be in the transition circuit area alot. This area can be adjusted by changing the idle restriction in the metering block, but a smaller carb will put the carb into the main metering circuit sooner because it will take a larger throttle angle to cruse at the same speed.
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Re: Think my carb is too big
[Re: 451Mopar]
#445915
08/23/09 01:16 PM
08/23/09 01:16 PM
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Joined: Jan 2003
Posts: 27,421
Balt. Md
383man
Too Many Posts
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Too Many Posts
Joined: Jan 2003
Posts: 27,421
Balt. Md
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The 750 DP is a good carb and will work fine for you. I have used them on stock 383's and Eddy headed 440's. First if the speedo is 10 mph slow that a big factor right there. Heck years ago a tech used to put smaller speedo gears in to let the cust think they got more miles to the gallon. Fix the speedo first. How does it run while driving it. You can fine tune everything but most DP carbs are not that far off on a combo like yours. Ron
Last edited by 383man; 08/23/09 01:17 PM.
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Re: Think my carb is too big
[Re: superb70]
#445918
08/24/09 01:58 PM
08/24/09 01:58 PM
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Joined: Apr 2007
Posts: 782
Minnesota
skep419
OP
super stock
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OP
super stock
Joined: Apr 2007
Posts: 782
Minnesota
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Quote:
Bump the base timing to 16 deg. which will then increase your vacuum reading quite a bit. Then set idle speed to 850rpm then fine tune the carbs idle mixture screws for best vacuum.(you may have to re adj. idle speed a bit) After you are happy with the vacuum reading you can lightly tap the primary accelerator pump arm to squirt a little of fuel into the primary bore,if there is no change in rpm your set, if rpm raises your a bit lean if the rpm drops your a bit rich. 
Thanks for the info.
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