I have a 76 F-150 460 CI with 57,000 miles on it.The carb was replaced everything else is original. It has been running very good but suddenly has a weak idle. It has plenty of power on acceleration and at cruise.This almost feels like the distributor is being moved retarded during idle, it has that low RPM shivery idle.
I have adjusted Idle mix and idle speed I can tell it is compensating A/F ratio and not correcting by how far out I have to turn the screws. Is there anything common on these engines I should look at beyond normal vacuum leak checks and fouled plugs?

 

If by "shivery" you mean its fluctuating/bouncing a few hundred rpm you more than likely have a vacuum leak somewhere, which would also be a culprit for relatively or near completely non responsive idle mix screws.

 

It comes and goes it will either idle good at around 750-800 rpm or rough and fluctuating around 450-500 RPM and it will shake..it runs perfect one minute and runs bad for the longer next minute. So far: I have leaned mixture out to test by using carb cleaner around all Intake gaskets,carb base,throttle shaft, brake booster, EGR block off plate. Nothing of any significance in RPM gain or change. returned mixture and idle to normal and it would still come and go.



I also pulled plugs out they are NGK plugs not sure I would be a fan of these plugs but they were all equally light brown ,fairly clean and even looked new on the outside shell. Plug wires do appear to be aftermarket and in good condition.

 

Check the torque of your intake manifold bolts. Plug all vacuum lines to eliminate any of those as a culprit. If that doesn't help it might be a carb problem.

 

Are all ignition components in good shape? Are you sure it's not a fuel filter/dirty carb issue? My truck ran great but idled on maybe 3 cylinders, until I realized that the carb had a big piece of debris in it.

 

Centrifugal advance might be sticking.

 

OK I checked those and they are torqued none were loose or uneven..no change in symptoms. Rechecked for vacuum leaks there, none present on intake top side at least.

 

I checked dist cap/rotor, secondary wires and vacuum advance movements. Centrifugal has movement when I turn the shaft..still could be a possibility. And I thought back ..this started within hours after I fueled the vehicle. I switched to front tank which was fuel from a different station..still did it anyway..was thinking water in rear tank.but that idea was eliminated..Now this has an edelbrock 750. I cannot find an external fuel filter inline..Does the 750 have one at the fuel line inlet?

 

I was on the same thought because this comes and goes from throttle changes when it happens..I might need to check that with a light..I do get a noticeable surge at light throttle anywhere from 25-45 MPH This is where I have discovered the Dist is frozen to the block..not going to chance breaking a housing.

 

Did this start right after replacing the carb? If so then it is likely the problem is related to the carb. Are all vac lines attached / capped? Mounting bolts tight.

 

The carb has been on it for some time..I bought the truck with it already on it. Did check top and mounting bolts right away ..also checked base gasket for leaks and throttle shaft seals for leaks it's stumping me.

 

Well I took the carb off and found scum and debris in the float bowl, cleaned all passage ways out and it runs good. presently It has however suddenly changed idle speed it has gone from 1,000 down to 600 just like a flip of a switch. some times it jumps back up to 1,000 I am thinking may have centrifugal weight issues/distributor problems as well.

 

If you have an EGR plate under the carb it can burn out internally and cause a variable vaccum leak you can't find.

 

The plate has been removed or it did not have one originally..I agree and I did experience that in my 1983 RV that has the 460 with EGR and a Holley.

 

Problem completely solved: Carb was dirty and some passages were plugged, I had found manifold vacuum port on the carb was partially plugged and was improperly working making the vacuum advance on full advance sporadically this making idle speed and quality vary. Once I switched the vac adv to ported vacuum like it should have been.. it ran perfect, rebuilt carb just for reassurance.