What's up guys? I recently put a vacuum gauge on my motor to try and tune my new carb a little bit and I got some weird readings. I found a port on the manifold to hook up to and started her up. No matter where I put the gauge on the manifold it will read about 5 inches at idle and then when I accelerate the motor the vacuum will climb to about 15-17 inches. Now this is the exact opposite of what I've read that is supposed to happen. Out of curiosity I hooked up to the PCV on the carb and got the same results, as well as with the brake boost. Anyone have any clue as to what is going on here? Thanks.

 

If the engine is running just half OK, then the gauge you're using is messed up somehow, as the reading you're getting makes no sense at all.

 

Thanks for the input Jim. The motor doesn't run bad. It's got this valve train knock, but other than that she runs like a beast. I used 2 more gauges and hooked them up to the same places and I got the same readings. It's a pretty fresh motor too, bout 50 miles or so. I'm pretty much stumped.

 

Try unpluging the brake booster and cap the line at the carb to see if your losing vaccum. Somethings going on

 

Thanks, I just tried that like you said and it did the same thing. I plugged off the pcv too. I looked for carb leaks by spraying carb cleaner around the carb and the intake. No difference. I guess this is mechanical, maybe it has something to do with that knock.

 

A steady reading below 5 indicates a leaky manifold, manifold gasket or carb gasket. Also check the heat riser. A reading of 8 to 14 generally indicates incorrect valve timing.

 

A plugged PCV can cause a knock just throwing that out there

 

Ok ill check out the pcv and see if it's clear. @ oldsrallye350 I've read the same thing. But seeing as how the vacuum builds I dont think it's a leak. I could be wrong.

 

All motors will build vacuum as the volume of air increases through the manifold.
Generally, a vacuum gauge is only usefull at idle.
Manifold vacuum is often highest when you are cruising at speed, as the throttle plates are mostly closed.
Also, make sure you are using a full time manifold vacuum source and not a timed vacuum port as a timed port will read little or no vacuum at idle.

 

Thanks, what you say goes along with what I have been seeing. The port I'm using is straight off the manifold. It's right over the top of a plenum tunnel going to the head. I even went as far as taking off the Edelbrock 750 I was running and replaced it with a Q-jet. This was to eliminate the carb adapter and eliminate a vacuum leak at the carb. So from what you say the motor is acting right?