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Old 02-15-2013, 12:54 AM   #1 (permalink)
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Default Holley Carb

Hi,

I have a 1970 Mustang Boss 302 with a Holley 780CFM. I having a little problem, I'm running rich on fuel mixture. The spark plugs are coming totally black and fouled. I bought the carb like 15+ years ago and was new in box. I did the idle mixture adjustments with a vacuum gauge and I got a max vacuum at 18-19in/mg. I also checked the float levels after the engine was off and they were ok. Those are the third set of spark plugs that I buy. Also I did the test of closing one of the idle screws and the engine died so the power valve is supposed to be good too. What can be the problem here? Here is a photo of one of the spark plugs.

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Old 02-15-2013, 01:07 AM   #2 (permalink)
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I had a holley 650dp. I used to set my idle screws by turning them in until the car started to die then back out 1/2 turn.repeat on other side.
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Old 02-15-2013, 07:06 AM   #3 (permalink)
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idle mixture screws only effect the engine when the throttle blades are almost closed (i guess technically a really small impact when they are opened, more and more).

your carb also has main jets, idle jets and power valve channel restrictors that effect how rich the carb is.

i guess it could be a number of things. is that carb supposed to be the stock replacement for your car?
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Old 02-15-2013, 07:19 AM   #4 (permalink)
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Install smaller main and secondary jets until the problem goes away. Best way to jet a car is to take it to a chassis dyno and have them run your air/fuel ratio. Other then that its just guessing.
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Old 02-15-2013, 08:54 AM   #5 (permalink)
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Quote:
Originally Posted by 6t9mach1 View Post
Install smaller main and secondary jets until the problem goes away. Best way to jet a car is to take it to a chassis dyno and have them run your air/fuel ratio. Other then that its just guessing.

good advise if you have always had this issue. But if the issue is a new one, then the car HAD been running fine with the current sized jetting, so something must have changed.

If you are certain that the power valve is OK, and that the air cleaner is not plugged up, then the next thing I would do is to take the carb off and check for a jet that has become unscrewed. Your jet(s) might be sitting in the bottom of the float bowl metering nothing.

Z.
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Old 02-15-2013, 09:25 AM   #6 (permalink)
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Quote:
good advise if you have always had this issue. But if the issue is a new one, then the car HAD been running fine with the current sized jetting, so something must have changed.
+1.
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Old 02-15-2013, 09:46 AM   #7 (permalink)
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One other thing, I've had numerous Holley carbs that ran rich with the stock float bowl settings. Sometimes you just have to lower the float for the engine to run right. But I would check for loose jets, etc. first if the problem is a new one.

Z
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Old 02-15-2013, 09:51 AM   #8 (permalink)
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You may have a blown power valve. It may be adding fuel even when its not needed.
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Old 02-15-2013, 10:36 AM   #9 (permalink)
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Quote:
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One other thing, I've had numerous Holley carbs that ran rich with the stock float bowl settings. Sometimes you just have to lower the float for the engine to run right. But I would check for loose jets, etc. first if the problem is a new one.

Z
I've also always set my float levels lower then the recommended Holley settings; never have had an issue with fuel starvation either.
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Old 02-15-2013, 10:41 AM   #10 (permalink)
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Float level needs to be checked/set with the engine running.

Make sure fuel pressure is between 5 and 7 psi.

Make sure choke is open and stays open. (added on edit)

Check condition of power valve gasket.

Make sure air bleeds are unobstructed.

How many turns out are your idle screws?

What is your initial timing?

What size primary main jet is in it now?

What type of engine time produced that plug color? Was it just idling or did you drive it?
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Old 02-15-2013, 01:26 PM   #11 (permalink)
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Welcome aboard VMF

nice car dude.

Id bet you have a weak igntion. replace the coil, check the points, wires, cap and rotor and make sure you have 7-9 volts to the (+) of the coil with the motor running
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Old 02-15-2013, 01:49 PM   #12 (permalink)
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what are the plugs gapped at? ive gapped plugs too wide before and my plugs came out just as black.

im with the others on the PV issue.
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Old 02-16-2013, 02:13 AM   #13 (permalink)
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Thanks for all your help! I will try to check that list during the weekend. The car is not in driving condition yet because is under restoration. Here is a video of the engine:

Thank you!
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Old 02-16-2013, 09:39 AM   #14 (permalink)
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Yes, please clarify. Did the engine run fine, then start running rich? Or has it run rich since you installed that carb?

It appears to me your primary jets are much too large. But, as mentioned, there could be other things going on. How high are you above sea level? If you live at significan't altitude, you'll have to run much smaller jets.
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Old 02-17-2013, 06:57 AM   #15 (permalink)
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Get the choke flap open all the way.

How many turns out are the idle screws?

What is your initial timing?
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