Increasing vacuum?
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Junior User
Joined: Jun 2003
Posts: 92
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From: Montana
Increasing vacuum?
Can anyone tell me what would cause the vacuum on my truck to increase when I increase the RPM's? Should'nt it be just the opposite? I picked up a new vacuum guage and I am pretty new at diagnosing vacuum problems.
Thanks
Thanks
Senior User
Joined: Aug 2000
Posts: 407
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youngguy,
Think of the vacuum as suction being created as the motor is pulling in air. With the throttle plates nearly closed, as in the case of the motor idling, then it is like a sucking on a straw with the end almost completely plugged. If you sucked harder the vacuum would go up – even if you did slightly remove more of the plug on the other end. Eventually, as the carb fully opens, the motor does not have anything to pull against.
Do this test. With the motor idling snap the throttle open. While doing so watch vacuum gauge and you will see it drop very low momentarily and then climb back up. Then do it again and hold it open a bit. Again, as the carb is quickly opened, but the motor has not yet increased in speed, you will see the vacuum drop way down. As the engine speed increases the vacuum will climb too. Be careful to not hold the carb open too far as the unloaded motor will rev freely and you could over rev it.
I hope that helps,
Mark
Think of the vacuum as suction being created as the motor is pulling in air. With the throttle plates nearly closed, as in the case of the motor idling, then it is like a sucking on a straw with the end almost completely plugged. If you sucked harder the vacuum would go up – even if you did slightly remove more of the plug on the other end. Eventually, as the carb fully opens, the motor does not have anything to pull against.
Do this test. With the motor idling snap the throttle open. While doing so watch vacuum gauge and you will see it drop very low momentarily and then climb back up. Then do it again and hold it open a bit. Again, as the carb is quickly opened, but the motor has not yet increased in speed, you will see the vacuum drop way down. As the engine speed increases the vacuum will climb too. Be careful to not hold the carb open too far as the unloaded motor will rev freely and you could over rev it.
I hope that helps,
Mark
