Sunday, October 24, 2010


Today I built a manometer to find out if our motor creates enough vacuum to operate the spill plunger.  A manometer measures pressure or vacuum and is measured in inches of either mercury or water.  It's a simple device and I build one using the basic instructions shown at


To operate the manometer the PVC pipe was filled with colored water...

Then the ruler was positioned at the top of the water column.

 The tubing was attached to the rear of the fuel injection plenum.
 Then, with the coil disconnected, I cranked the motor over and it developed about  3.5" of vacuum.  This is with the PCV valve installed.
 I removed the PCV valve, plugged the hole then cranked the motor again.  This time we got about 4.0" of vacuum.  The PCV valve only took .5" of vacuum.  Not bad.

So what does this mean?

One of my favorite sites for fuel injection information is Ramjets that Run at  Jerry Bramlett talks about cold start cranking vacuum and states, "Only .5" to .7" water vacuum is needed at the main diaphragm during cranking to generate a good fuel stream at all nozzles."

So, in conclusion, since we only need .5" to .7" water vacuum and we have 3.5", I guess we need to look elsewhere for our cold starting problem.  Next we will investigate the high pressure fuel pump at low RPM.  Stay tuned. 


  1. Great manometer, I hope your ruler was calibrated :)

  2. The way the fluid bounced around during cranking I think I could have just used an old wooden yard stick. Oh wait, I did.

  3. Check the fuel lines from the gas tank all the way to the engine for something restricting the flow of fuel......including crimped bends.......also make sure the gas tank's cap is tight so vacuum can be obtained. Jim p.

  4. Jim, We unhooked the gas line at the fuel injection unit then turned the motor over. Plenty of gas is being pumped from the gas tank. As far as the gas cap goes, it's a vented cap which means there is no vacuum or pressure applied to the tank like they do in newer cars. I am starting to think there is something incorrectly installed, missing, or damaged around the spill plunger. Ty and I will investigate further this weekend. Keep noodling though. Thanks.