Super Mini Mad
Posted 11 October 2023 - 03:40 AM
I recently replaced my brake servo with a different used one, as my existing servo didn't seem to be working. Now everything works fine (brake is a little softer than Id like but whatever), however when i pump the brakes rapidly, the engine rpm goes up a little. do I need to lean my fuel mixture or something?
Edited by BaronVonchesto, 11 October 2023 - 03:40 AM.
Moved Into The Garage
Posted 11 October 2023 - 04:15 AM
Sounds to me like that at least the Vacuum Chamber and Valves in your new Servo are working perfectly.
What you are experiencing is normal.
Super Mini Mad
Posted 11 October 2023 - 04:25 AM
thanks Spider. So i'd say leave it as is?
why does the rpm increase though? does it somehow let more air into the engine?
Moved Into The Garage
Posted 11 October 2023 - 04:53 AM
The Vacuum that's in the chamber of the Servo is released to atmosphere (or in reality, atmosphere is let in), so when it next evacuated (builds up vacuum in the chamber), that's a small amount of air that's going straight in to the manifold. Yes, this does lean the mixture for that brief moment, but it's not normally an issue and it doesn't require any carburettor adjustments.
When running radical (wild) cams though as the manifold vacuum can be poor at idle and low engine speeds, that's when either an auxiliary vacuum chamber can be fitted and / or an auxiliary vacuum pump.
Speeding Along Now
Posted 11 October 2023 - 09:23 AM
The increase in RPM can happen when the brake pedal is depressed, but it is not that common.
One thing I would check though is to depress the brake pedal, hold it down, and listen to see if you can hear any "vacuum" sound at the atmosphere intake of the servo, there should be none, this is to check the integrity of the servo vacuum diaphragm, if you can hear anything then the diaphragm is suspect and this fault will have to rectified.
Super Mini Mad
Posted 11 October 2023 - 11:03 AM
The increase in RPM can happen when the brake pedal is depressed, but it is not that common.
One thing I would check though is to depress the brake pedal, hold it down, and listen to see if you can hear any "vacuum" sound at the atmosphere intake of the servo, there should be none, this is to check the integrity of the servo vacuum diaphragm, if you can hear anything then the diaphragm is suspect and this fault will have to rectified.
nop no such vaccuum sound is heard. if i press and hold the brake pedal it doesn't cause the engine rpm to go up. only if i pump the brakes rapidly.
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