Im not a Mazda mechanic, but im learning.

[Wain]

Hello, firstly i have a mx5 mk2 engine in my roadster, but no brake servo. On the mx5 it has an air pipe from the brake servo to the engine. As i do not have a brake servo, is it ok to just cap it off or put a small filter on it?
When i was speaking to a chap at Stoneleigh show at the weekend about it, he was saying that it is very important for the 2 to be connected for both the flow of petrol to be correct when braking and the air pressure in the servo to be correct.

Secondly, on the carb i have a small silicon pipe on it which has been cut when the engine was taken out of the car. What is it for and where should i connect it to?

[skov]

You need to cap off the brake servo hole on the manifold.
If it's left open unfiltered/unmetered air will get sucked into it and it'll run like crap.

The little pipe on the top of the throttle body goes to the valve for the charcoal canister. Assuming you've junked the canister (most people do!) you need to cap that hole off too.

[alga]

Originally Posted by skov

You need to cap off the brake servo hole on the manifold.
If it's left open unfiltered/unmetered air will get sucked into it and it'll run like crap.

The technical term for this condition is "vacuum leak". Brake servo pipe is quite a thick one, you'd get idle speeds of 2500 rpm or so.

[Stot]

Originally Posted by Wain

When i was speaking to a chap at Stoneleigh show at the weekend about it, he was saying that it is very important for the 2 to be connected for both the flow of petrol to be correct when braking and the air pressure in the servo to be correct.

You wont have your foot on the accelerator during normal braking and the throttle will be closed so theres no servo/manifold fuel juggling going on that I'm aware of.

Maybe its confusion about the clutch switch that tells the ECU to enrichen the fuel when changing gear.

Cheers
Stot

[Wain]

Originally Posted by Stot

You wont have your foot on the accelerator during normal braking and the throttle will be closed so theres no servo/manifold fuel juggling going on that I'm aware of.

Maybe its confusion about the clutch switch that tells the ECU to enrichen the fuel when changing gear.

Cheers
Stot

The chap was saying that when accelerating- the manifold pushes air into the brake servo, increasing pressure in the servo ready for braking. Then when you brake, the air pressure is blown back into the manifold, thus reducing the air flow and help drop the revs.


Anyway, thank you for the feed back, i will get these capped off at weekend.

But my next query is petrol tank related.
I was reading in Complete Kit Car magazine and there is an excellent piece on setting up the fuel system, which made me realise i need a fuel pump! but it then read on about setting the fuel pressure provided to the engine, what pressure have you guys got yours set to??
Lastly the breather valve- is it ok to just have a small bit of pipe coming from the outlet, to a breather valve, then a bit more pipe just running up the back panel- maybe with an air filter on the end to ensure only clean air is sucked in??

[Davidbolam]

Originally Posted by Wain

The chap was saying that when accelerating- the manifold pushes air into the brake servo, increasing pressure in the servo ready for braking. Then when you brake, the air pressure is blown back into the manifold, thus reducing the air flow and help drop the revs.


Anyway, thank you for the feed back, i will get these capped off at weekend.

But my next query is petrol tank related.
I was reading in Complete Kit Car magazine and there is an excellent piece on setting up the fuel system, which made me realise i need a fuel pump! but it then read on about setting the fuel pressure provided to the engine, what pressure have you guys got yours set to??
Lastly the breather valve- is it ok to just have a small bit of pipe coming from the outlet, to a breather valve, then a bit more pipe just running up the back panel- maybe with an air filter on the end to ensure only clean air is sucked in??

Most of us are using tanks (beautifully made by k4kev) and are using the in tank pump from the mx5.if fits in with the loom etc.

A few people are using the mx5 breather. I am going to use one of the breathers available from car builder solutions. This will run up to near the filler with a bit of tube.

I agree with skov about the pipes. You can get nice silicone 8mm end caps on ebay fairly cheaply.

[Stot]

Originally Posted by Wain

The chap was saying that when accelerating- the manifold pushes air into the brake servo, increasing pressure in the servo ready for braking. Then when you brake, the air pressure is blown back into the manifold, thus reducing the air flow and help drop the revs.

The manifold never pushes air it always sucks. When the servo is primed/under vacuum the manifold cant suck on it any more so it acts like a bung really as if the pipe were blocked. When you brake the servo uses the stored vacuum to aid in braking. When you lift off the brake the vacuum needs creating again meaning the manifold can suck air from the servo again, causing a very slightly leaner mixture for a short time. More of a by-product of the system than a deliberate engine slowing thing.

Block em off everybody happy.

Cheers
Stot

[Wain]

I have bought a fuel tank and i was thinking of just fitting the fuel hose to the swirl pot with a after market pump and filter in the line. But thinking about it, ill need the level sensor as well to get an accurate fuel level reading. I may be better off then getting the pump/level sensor unit off eBay and then just fitting an after market filter in the line.
I assume this is what you guys are doing?

[deezee]

Originally Posted by Wain

The chap was saying that when accelerating- the manifold pushes air into the brake servo, increasing pressure in the servo ready for braking. Then when you brake, the air pressure is blown back into the manifold, thus reducing the air flow and help drop the revs.

Wow thats totally backwards. , some people have weird ideas. This is why the IVA is so important. Stops mentalists like this designing things.