Hi All I am in the process of fitting a servo to the bus (RHD) and would be grateful for some advice regarding the vacuum take-off on the inlet manifold. At some point in the past the engine has been changed and is now a single port and as far as I can tell, without taking the manifold off, there is no take-off on the manifold. The question is, where is the best place to have a stub fitted? I was wondering if my best option would be to buy a new manifold, I assume that it would come with a blanking plug so all I would have to do would be to screw in a stub. If I do have to have a stub welded to the manifold I was wondering if it would be easier to have a spacer made to go between the top of the manifold and the carb, has anyone done that? Thanks. Phil
I've been looking into this for my twinport....which has a small 'flat,' cast into the left hand manifold at the cylinder end. On a single port i think you would need to tap into the main branch, fairly centrally, below the carb...if you route the hose slightly upwards at the hose barb end, or tap the barb at a slight angle, before running it up to the servo, it would allow any fuel mist to drain back into the base of the manifold....someone may have a better suggestion, but essentially, all you need is a relatively constant vacuum source, which isn't likely to disturb gas flow in the manifold too much, or draw atomized fuel into the hose.... another option would be to use a 90 degree elbow hose barb....
Heres where the servo takeoff is on a dual port manifold: A new singleport manifold won't have a boss to drill into to install the takeoff pipe as a servo wasn't an option when singleports were about.
I'd just drill and tap the manifold end. No need for any welding; as long as you use a tapered tap it'll seal lovely.
I'd try fitting it where Vanork suggested, it won't draw fuel into it, the engine is sucking air out of it.
Don't forget that there's nothing actually flowing anywhere. Once the servo's been suitably "sucked", there's no air or fuel flow at all...
my thoughts exactly....but when i got in touch with the guys at trigger wheels about the best place to fit a barb for a MAP sensor, he said any location where it's unlikely to draw atomized fuel into the hose...so presumably it must happen to a certain extent....who knows?
What exactly happens when you brake? The manifold draws on the back of the servo diaphragm to create a pressure differential...you put your foot on the brake....the vacuum's destroyed, and then immediately built-up again via the line, at the manifold end....
From when you start the engine, there is a vacuum both sides of the diaphram. When you brake, a valve is opened letting air in via the breather to one side and closing the passage between the two halves. When you take your foot back off the brake presumably it sucks a tiny bit to rebuild the equal vacuum each side of the diaphram.
http://www.lotuselan.net/forums/lotus-elan-f19/interesting-very-interesting-t20484.html maybe not an issue on buses?
Indeed. No continuous suckage, just a top-up from time to time. I'm thinking suck-with-finger-over-the-end-of-a-straw here...
It never sucks with the engine under load. Modern cars still have vacuum servo's because they're so cheap and effective and almost never go wrong.
Mine had a non return valve - but it didn't work any more - I've bought a new one from JK tho I think you can pick them up cheaper else where.. That would stop any push of air
It needs the non-return valve to prevent a backfire through the carb wrecking the servo. If it's in good order it also maintains the vacuum after you switch off for quite a while. Long enough to disconnect all the pipes from the M/C, let the fluid drain out and unbolt it! I guess I have no vacuum leaks.
Diesel T25s (and diesels in general?) have a servo pump run off the engine not a pipe connected to the manifold.
