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Discussion Starter #1
OK, I dynoed my car and it runs very strong at wide open throttle. The driveability of the car is not so great. When I am accelerating the car has a section within the primaries that bogs. When it was dynoed they told me that the air/fuel mixture is perfect and not to play with the jetting at all. I have played with the carb cams and changed the squirters to 31's instead of 28's. I have adjusted the accelerator pump so that it squirts gas as soon as I touch the throttle. I have bought a pack of carb cams but I dont even know what to look for which one to use?? This problem is most annoying when I am on the highway or just want to coast and I touch the gas a bit, it will hesitate before it goes. The problem was alot worse but then I adjusted the accelerator pump/throttle and it cleared up a little but there is still a problem there.

Please help!!
Thanks
 

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Put your accel pump and squirters back where you had the least hesitation. It sounds like you just need to adjust your idle air/fuel jets. If you are not confident in doing this by ear, buy a vacuum gauge. They do not cost much and will show you what you are doing. Hook the gauge up to a manifold vacuum source. It should have 12 to 18 inches at idle as opposed to none. Turn the jets until the engine changes speed stopping at the highest vacuum reading. Do the other side and then go back to the first. If the idle gets too high during or after the process adjust it back to normal with the idle speed screw.
Good luck,
 

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Where is the bog? Is it on throttle tip-in or once you're already moving? It sounds to be either the pump cam still being off or possibly the wrong power valve. If your a/f on the dyno was good your jetting should be ok.

What's idle vacuum?

Drop me an email here at work at [email protected] and I can send you an Excel spreadsheet that one of the carbedMustang Mailing List members put together. VERY helpful in picking pump cams. Plots the volumes and ramp profiles of the cams by color. You can simple look at the profile graph to pick based on needing less or more pump cam.

If it's bogs immediately, then goes you've got too much cam or squirter. Goes, bogs, then clears and goes again you've got too little cam. The bog there is it running out of pump shot and then the primaries finally come on and it goes again.

Don't change the squirters and cam at the same time. As it'll be hard to track what's working. The 31s are a good baseline.
 

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Discussion Starter #4
The bog isn't there when I take off from a stop. I feel it the most when I am at like 60MPH and put the gas down a quarter way down or so. I was always told to take the air/fuel screws all the way in and then bring them out 1.5 turns. I have a vacuum gauge and I will do as the post above states.

Does one side control the air and the other the fuel or do they both do the same thing?

I turned out the screw by half a turn more(so total of 2 turns out) and the car ran alot better with less of a bog. But it still did bog.

Thanks for the help
 

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Each screw sets it's own side. These are idle mixture screws and are for setting idle mix only.

Possibly the power valve. Once you get the idle mix set and get a idle vacuum reading, the power valve should be half what the idle vacuum is. Some use 2" less, I've had good luck so far w/ halving the value. For example.....if your idle vacuum is 12", use a 6.5" pv.

http://www.jason.fletcher.net/tech/carbtuning/carbtuning.htm is a good source for some basic tuning info w/ a link at the bottom for additional info at http://www.bob2000.com/carb.htm
 

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The idle circuit affect operation well up into the cruising rpm range, that's why when you fattened up the idle circuit it helped the drivability.

If you don't have it dialed in yet, go ahead and increase the primary jets 2 sizes, which is the minimum change that makes a difference (due to slight variations in flow from one jet at a given size to the next).

You may need to go up as much as 3 jet sizes, but try 2 first.

With a slightly fatter primary jet, you will be able to lean the idle back down a tad, and still get the cruisability you are looking for.

Nick
 

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Is it a double pump carb or a vac. secondary unit? If vac. sec, maybe a problem with the spring or diaphram. I would assume double pump since I have not heard of a 700 vac. sec carb.

Tom
 

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when adjusting the screws...remember if you have a dual plane the screws adjust the opposite side...lol...take a look at a dual plane and youll see what im talkin bout..definately use the vaccuum guage...they do have a 700 vac secondary however i dont think it would run on a single 28 squirter so im assuming its a double pumper...another question...you say it doesnt miss from dead stop...but are you spinning...it might be possible that your motor can take the back barrels banging open if it revs quickly (for example) free revving in neutral or spinning...but when under a load it wont for example when it dead hooks on slicks or from a 60 punch..if you have steep gears and an auto matic with no convertor sometimes a vac sec is better for the street...hope this helps..my brothers aod 355 geared coupe car wouldnt take it with a tight converter..we switched to a vac secondary..might check your floats/ free flowing fuel pressure/volume too:);)
 

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that's good advice but

I don't think you adjust the left screw for the right side etc. It's let for the left, right for the right. There's no crossover passes in a regualr Holley.

Nick
 

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Pipemaster.......the idle circuit does affect things up thru the cruising range as it, the transition circuit, and main circuits all contribute fuel thru that range. Agreed. I just don't want any confusion that they should be used to adjust anything but the idle mix tho.

I also don't recommend just increasing jets on the primaries. You should set the primary jets either on the dyno at part throttle cruise (reading a/f), by doing a part throttle shut-down and reading plugs, or jetting down until you get a lean surge and then back up approx 3 jet sizes. This will get you to a lean cruise jetting. Good for mileage at part throttle. Make sure the power valve is correct for the motor's vacuum readings. If you REALLY want to get into it the power valve channels can be resized to provide the correct additional fuel at WOT for the primary side. Most of us just fudge this with secondary jetting tho.

fastbird91......I understand what you're saying but I can see where Pipemaster is confused. You're speaking of what side cylinder bank wise is affected and normally you'd think as Pipemaster is.......what side of the carb is affected.
 

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heheh...sowwy..lol;) ..just like to argue....just kiddin'....just for future reference..i had a couple cylinders run lean on a dual plane carb setup once.....and kept adjusting the wrong corner:p....
 

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was a good point to make tho. It's not totally uncommon to jet the corners differently based on how the plugs are reading. Important thing to remember on a dual plane.
 

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Discussion Starter #14
I finally got a vacuum gauge and some time to play. I hooked up the vacuum gauge and the reading was 11psi. I played with the screws and got it up to 12psi. The car is still hesitating the same as before but a bit less. Should I have more vacuum than that? I backed out the screws alot but it made no difference in the reading. When I had the car at the dyno they did have the machine hooked up to read my exhaust for rich/lean. I was told by them not to touch the jets at all. Reading one of the posts somebody said to go with a bigger jet in the primary...do you guys still recommend it.
Thanks for the help!!!
 
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