Dual plane manifold carb tuning - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 20 Old 04-12-2019, 06:31 PM Thread Starter
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Dual plane manifold carb tuning

I need some carb tuning advice here. I recently swapped out my Weiand 7515 intake manifold which is single plane, for a stealth that is duel plane. I am running a 302 w/p heads, N41 cam with full bolt ons and holley double pumper 650 w/ mechanical secondaries. Since the swap it runs lean.Timing is at 14 degrees. The car starts right up, idles at 850 at around 13.1 on the wideband. Starting off from a stop, with light throttle and load the wideband rolls right over to 14, 15, 16 as the throttle increases. Car obviosly starts bucking.

I have never had this much of an issue dialing in my carb. Is it normal to have to increase jet size or power valve for a dual plane?

Any advise would be great. I need to drive it before it gets much hotter.

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post #2 of 20 Old 04-12-2019, 07:13 PM
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Normally a dual plane manifold will be jetted leaner than a single plane intake. Are you sure that there are no vacuum leaks?

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post #3 of 20 Old 04-12-2019, 07:19 PM Thread Starter
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Yeah, i went over every fitting and sealed area with a can of carb spray. The dual plane is a lot taller
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post #4 of 20 Old 04-12-2019, 08:02 PM
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Bump......I'd like to hear more about this. Someone chime in

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post #5 of 20 Old 04-12-2019, 08:54 PM Thread Starter
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I was just thinking about it, the only I forgot was that I do have a 1" phenolic spacer, that is open. I have heard 2 different arguments about using it with this kind of manifold. I haven't heard of anyone else having these problems though.
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post #6 of 20 Old 04-12-2019, 09:46 PM
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dual plane intake pulls better vac. lean out by a jet or 2 sizes
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post #7 of 20 Old 04-12-2019, 10:19 PM Thread Starter
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I'm confused now. Wouldn't I up the jet size to eliminate the increase in vacuum causing the lean mixture?
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post #8 of 20 Old 04-12-2019, 11:43 PM
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If the car is idling at 13:! a/f ratio it isn't lean, it is rich. Fix that problem first. Always start with optimizing the steady state conditions before moving onto the transient conditions.

If the mixture moves in the direction of lean during acceleration, then the carb is not supplying enough fuel for these conditions. If it does this a little bit in reaction to small throttle inputs, it is going to be the main jets. If it goes in the direction of lean much faster when the rate of throttle change is increased, then the accelerator pump is not supplying enough fuel. Since a dual plane intake manifold has more runner length, it has more total surface area. This will always require more accelerator pump volume to compensate for the wetting of the increased area. If you are using a carb spacer, that increases the surface area even more.
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post #9 of 20 Old 04-14-2019, 04:57 PM Thread Starter
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I purposely run it a little fat on idle. I took 67 main jets out and put in 69s and a 4.5 power valve. With the cam I get a best of 10" of vacuum. I did re adjust the idle screws also, idles at around 14.5- 15 on the wideband. Just light throttle rev in the driveway, it leans out. I don't understand what the deal is. I have never had this much trouble with tuning this setup. The car ran awesome with the other intake. I had this one lying around and had heard good things about it. I'm sure where to go from here. Anyone else have any ideas here.
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post #10 of 20 Old 04-16-2019, 11:28 AM Thread Starter
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Bump Bump

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post #11 of 20 Old 04-16-2019, 01:53 PM
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Dual plane manifold carb tuning

I run this exact same setup in the race car, the only difference being ported AFR 165s and a .5Ē spacer. As Jack said, you need bigger jets.

My setup: 78 mains, 74 secondary, 40 main squirted, 31 secondary squirters. The main body has changeable air bleeds, not sure if yours does or not.

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post #12 of 20 Old 04-16-2019, 04:00 PM
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Are the accelerator pumps / squirters working properly? You rev it in neutral and its leaning out / stumbling?

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post #13 of 20 Old 04-16-2019, 04:14 PM Thread Starter
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Mustang415....I have larger jets, thank you i will add my 74 jets when i get home.

Curt C......I checked adjustment and operation and both look good. I removed the nozzles and there is no crap, and are squirting normally. In the driveway it a quick rev to about 3k it starts to lean out. If i adjust the idle screw out some it is fine until under load. I have gone over the entire manifold looking for vacuum leaks.
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post #14 of 20 Old 04-16-2019, 04:35 PM
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Quote:
Originally Posted by Jack Hidley View Post
If the car is idling at 13:! a/f ratio it isn't lean, it is rich. Fix that problem first. Always start with optimizing the steady state conditions before moving onto the transient conditions.

If the mixture moves in the direction of lean during acceleration, then the carb is not supplying enough fuel for these conditions. If it does this a little bit in reaction to small throttle inputs, it is going to be the main jets. If it goes in the direction of lean much faster when the rate of throttle change is increased, then the accelerator pump is not supplying enough fuel. Since a dual plane intake manifold has more runner length, it has more total surface area. This will always require more accelerator pump volume to compensate for the wetting of the increased area. If you are using a carb spacer, that increases the surface area even more.
I always read your posts 2 or 3 times. Your depth of knowledge is impressive and your delivery is very practical without sounding like a jerk. I almost always pick up something I didn't think of or that was not directly part of the question you are responding to from your posts too.

ie the wetted surface impact on accelerator pump sizing.
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post #15 of 20 Old 04-16-2019, 05:50 PM
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Quote:
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If i adjust the idle screw out some it is fine until under load.
Fattening the idle mixture can help crutch the job of the accelerator pump (transition between idle to throttle). I assume it goes lean again once you get under load even with a rich idle?

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post #16 of 20 Old 04-16-2019, 05:59 PM Thread Starter
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That is exactly what happens.
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post #17 of 20 Old 04-16-2019, 08:50 PM Thread Starter
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Thanks guys for all your advice. I got home and threw in a set of 74 primary jets. It is running rich now, at idle. Which is no big deal because I can dial that out. When given slow easy throttle, it will stay within a decent balance on the wideband. Although, when punch it quick it will lean it again. So, what do I need to do to the accelerator pump? It is properly adjusted. Should I upgrade to the 50cc pump or up the squirters? Thanks again, I do appreciate your advice.
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post #18 of 20 Old 04-16-2019, 09:09 PM
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Quote:
Originally Posted by Flfox81 View Post
Thanks guys for all your advice. I got home and threw in a set of 74 primary jets. It is running rich now, at idle. Which is no big deal because I can dial that out. When given slow easy throttle, it will stay within a decent balance on the wideband. Although, when punch it quick it will lean it again. So, what do I need to do to the accelerator pump? It is properly adjusted. Should I upgrade to the 50cc pump or up the squirters? Thanks again, I do appreciate your advice.
Up the accelerator pump nozzle size. You can fine tune it by changing the accelerator pump cam. For the cams, One end of the spectrum would be a big shot up front that ends fairly quickly. The other end of the spectrum is a smaller shot but over a longer period of time. See what the car likes with a bigger nozzle first (dont go past a 35 without upgrading to the 50cc diaphragm ), then fine tune with the different cam profiles if you need to.
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post #19 of 20 Old 04-16-2019, 09:31 PM
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Are you actually driving it or just punching it in neutral? Go drive it safely and advise how it acts. You canít really diagnose drivability and transition punching it in neutral. Make one change at a time or you will get lost in the forest real quick. Go slow.

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post #20 of 20 Old Yesterday, 11:23 PM
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Seems like if you changed the mains, and the idle mixture went rich, you've got another issue. The mains shouldn't affect the idle mixture unless the throttle is open a good bit at idle-i.e.-exposing the the majority of the transitions. Do you have 4 corner idle?

Am I right? Been a while since I messed with a Holley.

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