Too Much Vacuum?
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Too Much Vacuum?
A little background: I have an 85 F-250 with a 460 and 770cfm holley carb. It hesitates when I go from complete stop or very low rpms (not all the time though). It gets worse the warmer the engine gets. I have previously been given suggestions from this forum to check the power valve and check for vacuum leaks. I changed power valve and nothing happened. Then, looking for vacuum leaks, I thought that my incredably leaky brake booster might be the source of the problem. When I put in the new brake booster (which removed the constant loud hissing), the hesitation got worse. I readjusted the distributor, fuel/air screws, and idle rpms to get it the best I could, but it hesitates more than it did. Is there something I missed readjusting? Is my problem unrelated to the vacuum? Or has there always been too much vacuum? Any help would be very appreciated! Thanks.
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I think the problem is the adjustments from an unsealed vacuum system to the new sealed vacuum system.
One has to make sure all mechanical devices are within spec before fine tuning can take place.
What condition is the vacuum advance on the dist in ?
One has to make sure all mechanical devices are within spec before fine tuning can take place.
What condition is the vacuum advance on the dist in ?
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The distributor and the advance are only two months old. In trying to solve the hesitation problem, I have put in a new intake, carb has been rebuilt even though it has maybe 10,000 miles on it, and all fuel system components are new.
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From: Virginia
I would take the aircleaner off, and with the engine off, look down the carb throat, and push the throttle ever so slightly. You should see gas squirt out and down the carb throat. Even the slightest movement should create some fuel dribbling into the carb.
If you can move the throttle some before any fuel squirts out, then your accelerator pump may be out of adjustment like Dennis said. Any time the throttle blades open suddenly, a little fuel has to be added by the pump till the engine catches up, or you will get a hesitation(lean stumble is what they call it).
It could be a couple other things, but this would be easiest to check.
If you can move the throttle some before any fuel squirts out, then your accelerator pump may be out of adjustment like Dennis said. Any time the throttle blades open suddenly, a little fuel has to be added by the pump till the engine catches up, or you will get a hesitation(lean stumble is what they call it).
It could be a couple other things, but this would be easiest to check.
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From: lino lakes, mn
770 seems kinda big these trucks usually came with a 600 cfm vacumn secondary carb.Accelerator pump is really important when going to a bigger carb if the engine can't use it it's like flushing a toilet. To much air not enough fuel and not enough velocity to pull the fuel in.
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Franklin2, I think your on to something. The throttle seems like it moves quite a ways before gas comes out the nozzles. Could I also need a bigger plunger as well?
P.S. Is a 770cfm carb too large for a 460?
P.S. Is a 770cfm carb too large for a 460?
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From: Virginia
Everyone believes you have to go by the formulas for carb sizes, but by design, a vacuum secondary carb will be forgiving and size itself to what the engine requires. Remember 90% of the time you are only using 1/2 of the carb anyway. I don't think it's too big myself.
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