Stock 82 F100 300 inline 6 - 92k miles

 

heat and AC controls are Vacuum actuated. Check the vacuum hose that passes thru the firewall. Upper passenger side between the heater core and the vent door vacuum motor.

 

You are right. It was broken at the firewall and now repaired. I have closed off all hoses and this is what I'm getting....7.5 in. Thanks... still looking for leaks

 

Make sure that the intake manifold bolts are tight. They are usually loose on these engines. Back when we worked on cars checking those and the carburetor base plate screws for tightness was the first thing to do during any tune up work.

 

I got to ask did you check the vacuum before you removed all the smog stuff? If so what was it?

So far all good things to check.
It sounds like you may have factory AC if you have a vacuum line going through the firewall above the HVAC box.
If so you would also have a vacuum can, metal?, on the right inner fender well. Pinch off the vacuum hose to it and see if that helps.
Mine had pin holes all cross the bottom that caused a vacuum leak.
I took it off, took bracket off and used fiberglass rosin to coat it. Painted it, put bracket back on and put back on the truck.

There is that vacuum tree down under to the rear on the intake manifold. There is 1 nipple that is hard to see and even to test for leaks that is capped from the factory.
This cap cracks and causes a vacuum leak so double check this tree.

A carb not adjusted right and / or the halves leaking as said can cause low vacuum.
Timing not set to at least factory, to far retarded, will have low vacuum. Bump the timing up to 10* BTDC as a start.
Last thing I can think of is do a compression test and / or leak down test as the motor may just be wore out.

I have no ideal how many miles my motor has or what the vacuum is as I have never checked it that I can remember
Dave ----

 

ok so i would take an organized approach to finding the leak. Disconnect the hose that leads to your heat and AC controls and plug it. Then check your vacuum again. If you still show 7.5 in” your leak is on the engine side of the fire wall, if it rises then you have at least one leak still in the cab. There are a lot of different ways to isolate and identify the leaks. I would use this form of bracketing to chase it down. And use the diagrams in the EVTM for your year at garysgaragemahal.com

 

Really appreciate the advice. I didn't check the vacuum before desmogging but this engine ran prefectly before and still does. Think i know what i did wrong...I was measuring vacuum at the distributor line. Checked at the Manifold on the intake and in getting around 18 in. Is this the proper place to check? Can someone tell me which small port on the bottom of the carb (carter 1 barrel) should be connected to the distributor? The short one on the passenger side rear or the longer one on the passenger side front? While I'm at it.... the idle mixture screw on the bottom front passenger side of carb was almost all the way out. I screwed it all the way in and it didn't kill the engine and seems to make no difference where is it set. The carb was recently rebuilt.
What could be going on here?
Thanks again!

 

Here we go again. This engine would not run fine if it had only 7.5 inches of vacuum at idle. Just like the factory gauges, do not go by one source or gauge and let it turn your world upside down. Get another gauge. Or you may be plugged in the wrong port.

 

What RPM is your idle set to? Did you check vacuum on the two nipples at idle? At increased RPM? Some may be ported and only open at higher RPM.

Initial setting on the idle mixture screws are 1.5 to two turns out from closed. Don't over tighten as the seat can be damaged. Who rebuilt your carb?

 

I would have to ask the same question, "what was the idle speed set to when you were checking the ports for vacuum? If the idle speed is too high you are no longer on the idle side of the carb.

Using your vacuum gauge and motor up to normal temp lower the idle speed to factory setting. You may need to adjust the idle mix screw in or out if it will not idle at the lower RPM.
Once you have the idle speed set using the vacuum gauge adjust the idle mix screw in and out to get the highest reading on the gauge.
You may need to go back and adjust the idle speed. Keep going between the mix & speed till you get them where they need to be.

Now when you check the timing, motor up to normal temp, idle speed set, vacuum disconnected and plugged and set it to 10* BTDC
Yes I know factory is like 6* or 8* BTDC but trust me you want to bump it up to 10* or even 12* BTDC as long as you dont get pinging or kick back on hot start.
You may need to go back and adjust the idle speed back down when you bump the timing up from factory.

I run 12* to 14* BTDC on my 300 and I do get a little pinging when hot out and pulling a 20' enclosed trailer but I found if I run high test, motor runs better and lot less pinging.
One day I will bump the timing back a little.
Dave ----

 

Really appreciate the help. I think I'm good now that I have read up on the vacuum subject. Understanding the difference between port and manifold vacuum it makes sense now. Then came timing... thought the big 1/4" notch had to be the timing mark but of course it didn't appear with the light so I thought my balancer had slipped. Kept looking and finally found the very small mark which you almost need a microscope to see. Thanks again

 

For some reason the 300 motor timing marks are so light you can hardly see them.
That is why it is best to get under the truck and sand the edge so you can see the marks and then use paint to hi light it.
Dave ----