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Dual Quads Idle High
I removed my single 4 manifold off my 428 and installed a Blue Thunder MR dual quad with a new set of BJ-BK's from Carls. My problem is I can't get the thing to idle down below 1000rpm. I have all the stops backed off, including the secondary stops, and still the same. Seems to run rich also. I used to be able to get a steady 600rpm with the single 750. I can choke it out by covering any of the primarys or secondarys with a rag, so I do not beleive I have any vac leaks.
Anybody else have this problem?? Thanks, Anthony |
are you running he factory dual quad linkage ??? and do you have an electric choke on the front carb...if so make sure the choke isn't binding...and your accelerator is adjusted...just to make sure disconnect the accelerator linkage to the carbs to see if it will idle...at that point turn your idle mixture screws to until it stalls...most likely something is not adjusted properly.
Tony |
Put the secondary idle stop back where it was (just crack them open if you don't have the spec), disconnect the linkage from both carbs and crank the idle screws in a half turn past initial contact...start it and adjust the idle to 750-800 or where you like it (if you have a choke make sure isn't acting on a throttle cam). Turn it off and set the linkage so it doesn't move the throttle arms when installed, should be a nice slip fit (make sure the choke doesn't hold things open as it cools). It sounds like linkage problems...post a photo of the linkage if you can.
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Also, split the idle between the two carbs. Don't let it idle off of one carb only.
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Except maybe when adjusting the mixture for one carb.
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Thanks for the quick replies.
I have the proper progressive linkage. I have done all that you guys have said and still the high idle. I have even run it without any linkage on either carb, and the choke is not holding it (manual choke). The only way I can get it to idle low is backing off the timing, then it gets real sluggish off idle. Keep em coming!! Anthony |
Do the BJ/BK carbs have PCV ports? If so, did you plug one and hook up the other? If not, the extra air could cause it to idle way too fast. My manifold and both 1860's each have a port, so the carb ports are plugged and the PCV is hooked to the rear of the manifold.
Also, if your manifold has a port, it should be used or plugged. |
What's the initial time set at? You positive you don't have a vacuum leak? A leaky manifold gasket could cause a high idle.
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There are no pcv ports. My initial timing is at 12 to get a decent idle, it really likes 18 to be crisp. Backing the timing down makes it sluggish and rich smelling...
I know the top of the intake is not leaking, don't know about the bottom side of the ports, however it does not pull in any oil, so I beleive I can rule out the leaky intake. This is frustrating. I might try taking one carb off and plating it off to see if the problem goes away, and switch it to the other one to see if it is one of the carbs. When I have the carbs on the bench and all the stop screws are backed out, you can still see a little light past the plates- I have even backed off the screws that hold the plates to the shafts and retightened them in the closed position to get a better seal. (They are peened well so they will not back out). Anthony |
Fast idle
I ran into a similar situation after installing BK and BJ carbs on my 427 about 2 years ago. Finally found that the choke mechanism inside the housing on the front carb was allowing the fast idle cam to move slightly after even slight acceleration, thus keeping the idle too high after releasing the accelerator.
Fix: Take the housing cover off, position the choke plate completely open, and then as you re-attach the internal spring to the choke arm, preload the spring to force the choke plate to remain open longer. In other words, back off the choke so there is no way the fast idle cam can effect the linkage. Of course you will have no choke when it is cold, but with engines like that you really don't need them anyway. Heck, they sound rough at idle all the time anyway...its part of their CHARM.:) If you want to check the fast idle cam before adjustment, you can carefully watch the linkage where the cam is located when you move the linkage to open the carb. You will probably see the cam rise just enough to keep the throttle plate from completely closing when released. Good luck, Bob |
Manual choke is not interfering, fast idle screw has plenty of play.
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I have the same carbs on my Boss and had to go back and forth between the idle mixture screws to get the highest vaccum reading(which increases speed) then backing off the idle speed set screw to get where I wanted.
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ACMJG is correct. I had exactly the same problem when I first used a pair of #8007 390 cfm Holleys. Follow his instructions, remove the carbs from the intake and then remove the base plate being careful not to damage the gasket. Back off the idle screw and slightly loosen the brass screws that retain the butterfly plates to the shaft and jiggle the plates for the best seal. Then replace the base plate. I did that to both carbs, all for butterfly plates each and it solved my problem. Also, while the base plate is off, check to be sure that the butterfly plates are installed with the correct side facing up. There is a top side and a bottom side to the butterfly plates. The edges of the plates are at a bevel, not cut perpendicular to the plane of the plate. The bevel allows a better seal when the plate is fully closed. If the butterfly plate is installed upside down, it won't seal correctly. I found one plate uinstalled upside down and had to correct it when fooling with my new Holley carbs.
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