No vacuum pressure at idle

[stomper]

My rover has been running O.K, but there is a slight hesistation or skip under load. I purchased a vacuum gauge (I've never used one before) and hooked it up to the vacuum line off the webber 34in carb. I appears to sit at zero at idle. if I bump up the trottle a little, the vacuum pressure rises steady, and the guage does not flutter. I checked the intake manifold where it comes into the engine block with carb cleaner, as well as the rubber mounting block to the carb, and I can't get it to studder. I am at a loss at this point. Any ideas?

Perhaps I have an unseen crack in the intake manifold? I just want some thoughts from the forum before I start just buying and replacing parts.

[mongoswede]

see if you have a port on the actual manifold that you can attach the gauge directly to and then double check your reading. Its possible the port you are attaching to on the carb is not directly manifold vacuum

[NRutterbush]

I'm not familiar with your particular carb, but most carbs have multiple vacuum connection points.

Manifold vacuum (below the throttle plates) should be highest at idle, and drop as the throttle plates open.

Port vacuum (above the throttle plates) should be low or zero at idle, and rise as the throttle plates open.

When I used to do a lot of chevy truck stuff, you had to be really careful not to mix up the two when swapping carbs or distributors. Some distributors advance using a vacuum pot connected to port vacuum, others use manifold vacuum to "hold back" the advance, so that the advance happens when the plates open and the vacuum drops.

Look around on your carb, and see if there are any vacuum plugs, or threaded plugs that might be blocking another port.

Ultimately, if it used to run, and now doesn't, your vacuum problem is probably from a failed gasket or throttle plate bushing. The vacuum guage won't tell you much unless it is fluctuating.

Instead, take a section of 3/8 hose, and use it like a stethescope. Putting one end to your ear, and very carefully moving the other end around near the gaskets, throttle bushes, etc... will help you hear a loud roaring sound that is indicative of a vacuum leak. Be sure not to stick your hand in any moving parts while you are paying attention to the sound.

Alternately, put some water in a clean spray bottle (windex bottle or similar) and spritz the carb as it runs just off of idle. When the water gets sucked into the leak, you should hear the engine stumble a bit, letting you know where to look further.

Good luck.

-Nate

[stomper]

Thanks guys, I looked around, and pulled the plug out of the intake manifold, where the brake servo vacuum would attach (if I had one). Now at idle, it rapidly fluxuates between 18 and 22. It is fluttering so quickly that you can't see the needle at any given spot.

The manual I have that came with the vacuum guage doesn't list what this might indicate. The truck runs quite good, and has decent power. I just can't smooth out the idle perfectly. It gives a light popping sound out of the exhaust very intermittently. It also seems to occur more frequently when the vehicle is under load, and the RPMs are low. you can feel the skip when it does this.

[albersj51]

Are you absolutely sure you don't have a leak in the intake or exhaust? Also, have you confirmed your timing is spot on? I had that popping/backfiring (albeit much worse) when my timing was off. Also had an exhaust leak so no idea which is the cause but it's one or both. Good luck!

[Nium]

http://moodle.student.cnwl.ac.uk/moodledata_shared/cdx%20etextbook/dswmedia/toolsEquip/hpt/measuring/vacgauge.html

readings and symptoms at bottom of page

[stomper]

Thanks Nium, looks like weak valve springs. The P.O. had the engine rebuilt, and put very few miles on it prior to selling it. Guess they used the old springs.

[SafeAirOne]

I always liked this page on interpreting vacuum guage readings: Nice little animations for each of the 15 different scenarios listed at the bottom of the page:

http://www.secondchancegarage.com/public/186.cfm

[stomper]

That link is going in the bookmarked section! Thanks for sharing Mark!

[stomper]

Just an update, Today I adjusted the tappet clearances, and there was very little lash to some of the valves. Once it was adjusted properly, the vacuum guage settled right down, and now reads 21 inches Hg fairly smoothly.

Thought others might want to know, in case they get similar readings.