engine has low vacuum, cant find the problem

[Angelo M.]

I have a 350 small block which was rebuilt many years ago. It ran well then but now its showing 8lbs. vacuum, should be about 15lbs. Engine rebuilt with stock parts and running 350hp cam. Showing 159-171 compression. Parts I replaced: rebuilt carb., rebuilt dist., new plugs, new plug wires, new GM coil, new fuel pump, new gas tank. Car is running pointless ignition. Timing is set after 8BTDC. Used spray to check for leaks around the manifold and could find no leaks. Car runs for 10 minutes and when you turn off the engine and turn it back on it will not start. After waiting a few hours it will turn back on. I suspect due to the low vacuum carb. is flooding out. Any ideas on what I should do next?

[Roger S.]

Re: engine has low vacuum, cant find the problem

My 350 was running about 8"Hg vacuum when I got it. After many hours of checking things, rebushing the carburetor throttle rods included, I found I had 8"Hg vacuum at the oil dipstick, ie the intake manifold gasket to engine block/engine head was not sealing. Vacuum should not be in the crankcase. Anyway I took the intake manifold off and regasketed it and reinstalled it. No vacuum at the dipstick tube and engine vacuum was up to almost 14"Hg.

Roger

[Angelo M.]

Re: engine has low vacuum, cant find the problem

Thanks for the info. It makes sense, I went back to the car and from the carb I heard exhaust noises. At first I thought it was coming from the headers but I have changed my exhaust to OEM stock. I still hear the noise coming from the carb. The engine is having a tough time idling as well. I will put the vacuum gauge and see if I am getting vacuum from the oil pan. I will keep you posted.

[Terry R.]

Re: engine has low vacuum, cant find the problem

Sounds like you've tried the obvious but a restricted exhaust will cause low vacumn. Plugged Catalytic convertor,muffler, tailpipe, etc. Good Luck

[Louis T.]

Re: engine has low vacuum, cant find the problem

Hi Angelo,

You mentioned checking for vacuum leaks by spraying - I assume you used a carburetor cleaner. For a more precise and neater approach - probably safer as well - I would suggest using a propane torch. You probably already have one around for plumbing duties; if not, a kit (bottle and nozzle) should be under $20. Attach at least three feet of vacuum hose to the nozzle, and affix a ball inflating needle to the other end of the hose. (They usually have the hole on the side of the tip, so you should probably cut off a bit of the tip so that its opening is at its point - a Dremel with a cutoff wheel is ideal.)



Good luck!

Louis

[Brad K.]

Re: engine has low vacuum, cant find the problem

I agree with Terry above. Corvettes sometimes are started and stopped too frequently and are left sitting with moisture inside the mufflers. The thin, rusting plates can very rapidly cause some cheap mufflers to disintegtate internally and cause them to plug the hole exiting the rear of the muffler.

A couple years ago I had the same experience in a motor home that wouldn't pull itself over hills. This was very frustrating until I got pi**ed off and got under the vehicle and pounded on the muffler with a mallet. It solved the problem....until it plugged up again....It at least got me home....

A quick check.....Feel the air coming out of the exhaust. If the flow feels restricted....maybe you've got a problem.