The effectiveness of this system is evident as the carbs are adjusted: small adjustments can be seen on the gauges as the carbs are adjusted. Table of Heating Oil Suction Line Vacuum Readings Heating Oil Piping System Maximum Running Vacuum Read Comments Single stage fuel unit, one-pipe system 6 inches of mercury Hg vacuum gauge Single stage fuel unit, two-pipe system 12 inches Hg Two-stage fuel unit, two pipe system 17 inches Hg Source: Heating Oil Pump Routine Maintenance, Suntec Pumps 2014 Unfortunately in a two-pipe system we don't install and cannot as easily use a similar gauge on that second line to check for leaks. Stop the motor and tighten the distributor hold-down bolt. Once the idle mixture is set, set the idle speed set up. Bearing this in mind do you still believe I must have a valve timing issue? High vacuum +++ low pressure. Cars that stall or race when warm may need the warm idle speed adjusted, while cars that stall or race when cold may need the cold idle speed adjusted. Setting timing Now that the idle mixture and speed have been set, it is time to set the timing.
The starting point to evaluate engine vacuum is the intake manifold. Make sure the transmission is in Park if its an automatic or Neutral if its a manual. Once you have established your diagnostic, you may want to further test those systems or components pointed out by the reading chart, based on your results. Our photo above shows a vacuum gauge installedon a tee between the oil filters and the oil burner. If the pointer has a slow floating motion of 4 to 5 inches - you should check the fuel control. For instance, a small block V-8 develops its best power under load at about 36-37 crankshaft degrees, and the best fuel economy at light throttle cruise happens at around 42 to 46 degrees of advance.
It should be noted that as the carbs are brought back into balance, the rpm will increase. Many times what is perceived as a transmission problem is in actuality an engine problem. Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next test to be sure. About the Author Robert Tomashek is an automobile technician and educator with more than 15 years of experience. If the engine passes that test then snap the throttle open and closed again. Within the last few years, vacuum gauges have slowly been replaced by a number of specialized and expensive tools. Check your as this condition often indicates too narrow of a gap.
On a 74 you should have a provision to attach a vacuum gauge. I have been known to cheat and pull off the other end of that hose and cap the vacuum source instead. Under some engine load conditions, ported vacuum may equal manifold vacuum, but it can never exceed it. A vacuum gauge helps you diagnose many car mechanical problems without using expensive tools or taking apart the engine to analyze each component. These become porous over the years, take on fuel, gain weight and sink. They can pop-back on accel also. When in doubt, get the pricier but safer gasoline-safe hoses.
Fine-tuning the carb in this manner will improve part-throttle operation and should eliminate that annoying off-idle stumble. The only way to spot this problem is to remove the carburetor and see if the throttle blades uncover more than 0. Your best analysis based on vacuum readings will come from your own experience, however. I did learn what the gauge could do but not how to hook it up. If all the idle-mixture screws are not adjusted the same, do that now. Low vacuum also can be an immediate clue to a plugged exhaust. As you can see, theres much more to setting idle mixture properly than just turning a couple of screws.
If rpm and vacuum are uneven, the cylinders aren't pumping equally. Some recent Holley and all Demon carburetors use four idle-mixture adjustment points. A low vacuum reading means you have poor piston rings or oil rings. Be sure to adjust the idle-mixture screws equally so that they are balanced. While you still have your vacuum gauge hooked up, it is a good time to set the idle mixture needles on the carburetor. I've never tried it using a vacuum gauge before, alsways used a timing light.
Again, experience will be your best guide. Where should the vacuum gauge be installed, how is it used, and what vacuum readings mean on the oil line. Vacuum should hold fairly steady between 19 and 21 in-Hg. He has also partnered with mainstream and trade publications on a wide range of editorial projects. It's usually a difference of 5 to 8 crankshaft degrees, sometimes as much as 10 degrees on some distributors.
In these cases the hose can be pinched a little to dampen the plenum pressure pulses. That prevents detonation from over-advanced ignition timing when the engine is in a loaded situation. Open the side gauge valve. I have mounted them in the dash. Weak valve springs if you suspect weak valve springs, rev the engine to say 4000 rpm and see if the needle swings back and forth rapidly. All of these seem to end up, on my vehicle anyway, at about 14-15deg initial advance. Steady idle Steady 14 to 16 inches Incorrect ignition timing.