My '67 F350 has been having an erratic vauum gauge reading at idle. It'll be in between 10" and 15" then will got to 0" to 30" very quickly (because of vacuum advance maybe?). I initially thought its was a vacuum leak, so I rebuilt the carb and put a new mounting and spacer gasket on, but it didnt fix the issue. Currently I think it might be just worn out valve springs but Im not sure. The vacuum steadies out at about 17" when I give it gas which is still low, so I was also thinking it might just be really retarded timing, as it seems the mechanical advance kicks in and steadies the vacuum?

The truck was an old farm truck that wasnt taken the best of care of, and has been sitting for several years yet was still running when I bought it a couple months ago. I dont know if its just an age thing and have to deal with it unless I want to rebuild the whole motor or if theres a solution to it that can be fixed without a major overhaul.

 

I'll try to post a video tomorrow as its late and I dont have a light outside for you guys to see.

 

There are at least 3 to 5 really good charts on the intertubes, all slightly different, on vacuum gauge indications, be sure to download and study them. The answer is there if you know what you're looking at. This can take a little time to figure out. There are a lot of diagnostics that can be done with these. Maybe 20-25 different? Plus, checking fuel pump pressure output. Pretty amazing tool really for something so simple and inexpensive.

Vacuum advance itself won't factor into gauge readings. Disconnect and plug while making the vacuum test. Test the vacuum advance canister for movement in the distributor and that it holds vacuum. Be sure the gauge is connected to a source of constant manifold vacuum. This is always somewhere below the throttle plates, not necessarily to the manifold itself. If you live at higher elevations there is a correction factor, lower numbers are OK at altitude but at as a rule at sea level a stock engine in mechanically good condition and in good tune should pull 18" to 20" Hg with a steady needle.

Generally a bouncing needle is not a good sign. An erratic needle usually means a problem that affects a single cylinder, a faster rhythmic needle bounce in time with the RPM might be something that affects all cylinders equally. Might need to speed up RPM and hold to pick out weak valve springs. Could try advancing/retarding distributor for highest steady needle, make sure to verify TDC is correct on balancer. Check spark plugs for indication of excessive oil or coolant leaks.

 

My guage is also a fuel pressure gauge so when i set the idle slightly too low the vacuum will swing between 0" 30" and then positive pressure.

 

Does the '67 have one of those funky load-o-matic distributors? They had no mechanical advance (centrifugal weights) and depended on two different sources of venturi vacuum from the carburetor.

I'm not sure but those gauge symptoms you describe don't really make sense. Every mechanic's vauum gauge will test fuel pump pressure.

A cylinder compression test or leakdown would be a good idea. Verify true TDC on the balancer using a piston stop, it is very common on Fords for the balancer ring to slip, screwing things up badly when somebody tries to set the timing. What do the plugs look like?

 

To me it sounds like a burnt valve on one cylinder. Does it have exhaust leaks at the manifold?

 

I wondered about that, or maybe head gasket between two cylinders?

 

After looking up symptoms of burnt valves, i think thats what it is, and i hear its somewhat common on the old FE motors. Looks like i get to take the heads off and might as well redo whole valve train stuff.

Thanks for the help.

 

When you pull the intake remove the lifters, do not be surprised if the bottom of them are cupped and not flat. You may also need a new camshaft. The non zink lubricants are wearing out a lot of cams. Shell Rotella and similar brands or zink additives must be used with all flat tappet cams.

 

Before you pull it apart do a compression test then you will know what cylinders to look at.

 

Yeah I'd want to do a couple more tests first, before slitting the ole' wrists.

 

I could do compression test to check. Im lik99% its burnt valve tho because ive got the shaking motor at ilde, noisy valve train, exhaust is sucking back in a little bit, ive been getting really bad misfires and back fires through carb.

How else should i check just to be sure?

 

Remove the valve covers and check that all rocker arms are moving the same distance. What you are describing to me appears to be camshaft lobe wear.

 

Try to find a vacuum leak somewhere. Including the carb base that you replaced or an old cracked vacuum hose.
I'd let someone else give you a technique for locating the leak.