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carb rich problem
Hello to all ! I have a problem that my Dyno tune guy doesnt have an answer to. I would appreciate any input. pardon me but I will explain it as best as I understand it.
My car is running rich while cruising between 2000 -3500 rpm's. My car runs at about 14.2 at idle , 12 between 2000-3500 and back to 13.2 under full load(when the secondaries kick in) I have a 428 motor, 2 - 600 vacum secondary Holley carbs, factory type progressive linkage , High performance carter fuel pump (9 psi and drops to 7 psi under load), CJ heads that are reworked, 524 lift cam, 248 duration, MSD 6al system, 10.5 to 1 pistons. My Dyno guy said the carbs are out of the idle circuit but have not hit the power circuit in my condition. So the jets (66) have not come into use yet. My car pulls 9-12 psi vaccum while idling. I hope this makes sense. Thanks for any input. olaf herrick |
I would back off the linkage to the second (front ) carb a little at a time till you get to what you want, just my $.02
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Sorry but I do not get it. The "Power" band comes in when the vacuum drops low enough for the "Power" valve to open. Have you run the engine with a exhaust analyzer hooked up so you can get a true reading of what is going on? I used this same set up on a 427 Medium Riser Homan and Moody engine we did for a Cobra kit we did years ago and had no trouble with the stock carbs out of the box. Never had to change the jets or do to much in the way of adjustments.
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we hooked up an engine analyzer , inserted the O2 sensors behind the headers . The readings are correct , I was told that eventually this condition will foul plugs, provide worse gas milage (not sure if thats possible) and a loss of some power . I know my car will make you cry from the rich mixture if your in a confined space. The factory power valves are set at 6.5 psi which appears to be an OK setting . It just seems to me that if I can get the carbs in the power circuit sooner my problem wouldnt exsist. I'm going to try and adjust the linkage as Viking suggested. Olaf
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Olaf,
Setting up a set of dual carbs on a BB Ford can sometimes be tricky, can you describe to us (in detail) the exact set up of your carbs (ie: squirter sizes, jets front and rear, primary and secondary, etc). What about the progressive linkage and how it is set up? This may be a case of mix matched carbs, a secondary adjustment of the carbs and their own linkages, or perhaps even something as simple as a float level issue. Bill S. PS: Have you looked at the timing from a vacuum standpoint???? |
Olaf, It does not sound like a linkage problem. If the engine is running rich at the RPM you are talking about I would go to a smaller jet size. Anything off of idle goes through the jets. If you are running a vacuum secondary system like the OEM set up the back carb only add very little to the mix till they kick in. Does the mix problem clear up at WOT or when the rear barrels start to kick in?
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My carbs are a new out of the box set of 4160 , 600cfm Holley carbs, manual choke , vaccum secondary. I'm told this carb has a size 66 front jet and a metering plate equal to a size 64 jet. The linkage is a Branda unit that turns around the carbs (putting the choke on the drivers side).I'm not sure of the squirter size but they are stock units.
My Dyno guy said that my motor is not running at 2000rpm off the jets. He seems to know what he is talking about but its a little over my head. I know the idle circuits are predrilled and I thought that as soon as the throttle is applied the jets are used . He said that there is a spot between these two circuits that my motor is in before WOT . Apperantly when I'm cruising (normal around town driving) my car is in this intermediate circuit. The Dyno guy said some larger carbs actually have a provision for this but mine doesnt. I hope I did a OK job describing my delemia |
Olaf, when the carb butterfly gets past the idle air bleed slots it start to use the jets. So anything of fast idle will use the jets. The power system does not come in till the engine vacuum drops below the 6.5 of your power valve. Try going to a smaller jet it should help. I have set up lots of multiple carb engines and most are over jetted as the stock carb is set up as a single so they are jetted to take the whole load of the engine. If you are running two carbs you have to cut back on the jet size. Try it jets are cheap.
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My dual four barrel Ford 427 Hi Riser intake is split into TWO separate front and back "chambers". The rear carb feeds ONLY the back four cylinders and the front does, well,,, the front four.
Therefore I cannot "lean" or "richen" one carb over the other, they HAVE to work together. Not sure what kind of manifold you got Olaf, but check that before you get the two carbs out of synch in any way. |
My manifold is much like yours Excaliber , although there is a chamber that connects the two carbs in the middle but it is rather small . I think it just equalizes the two carbs. Olaf
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Excaliber is correct. The carb setup is dependent on the manifold type, single plane or dual plane. In the Tunnel Wedge design which is a single plane, each cylinder has acess to both carbs. In the dual plane design which is the most common dual quad manifold the carbs are split and each cylinder only has access to half of each carb. Because of this design a dual plane usually can run a bigger carb that a single plane.
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