Hitchcocks Forum

On my 350 Bullet there is an alloy spacer between the carburettor and the cylinder head which has a small bore nipple attached. What does the other end of the flexible small bore tube connect to?

It's supposed to go into the air filter box so that any air entering the manifold adapter is filtered. It seems unnecessary to me to filter the air going in it because I see no reason why at normal altitudes the valve should need to be open other than to confirm the "pilot" air screw setting when setting the idle, then closed off again.

It might possibly need to be left slightly open if riding the bike on a high mountain (to compensate the idle setting for lower air density) for a prolonged period, but I doubt it.

My alloy spacer (or manifold adapter?) does not have the needle valve, just the nipple for the tubing, so do I just block it off or leave it open?

That tube is part of the Pulse Air Valve system.
If you remove the PAV (good idea) then this needs sealing off.
Alternatively you can remove the spacer altogether.

Or hook it up to one of the pretty vacuum gauges...

http://www.speedograph-richfield.com/ht ... ffers.html

I thought what a waste of money..... But two of those gauges are cheaper than a pair of Davida gauges...

I would hazard a guess that they are no where near as accurate as davida, or even damped.

I've had a four set for years.

That's the problem with having a single, I can't balance the carbs

John

Pulse air valve? The one my 350 sounds the same as this and didn't have anything connected to it, only the plain, clear plastic tubing to the air box.

The PAV was a requirement for any export models needing to be Euro 2 compliant, though as yours was an Indian market model that might not have been required on its home turf, my home market parts book doesn't show it as listed against the 350 Electra, only the 500 Bullet and the Lightning models.

The factory might have intended to fit one to the old-style 350s later on, I notice that the very latest 350 cylinder heads came with an extra lump on the casting near the exhaust port, which looks like it was intended to be machined to take an air injection pipe union, similar to what the AVL Electra-X heads have.

On later classic models, eg the Sixty-5, the valve sat in one of the toolboxes with an air feed into the top of the exhaust pipe.

https://www.hitchcocksmotorcycles.com/p ... pages/2418

I've just checked the workshop manual for my 350, edition 3/04 (Watsonian Squire) which I received with the bike.

The carb tuning section shows diagrams as per my bike with what is described as the "auxiliary air screw" which controls the aperture for the nipple where the clear pipe to the air box is connected.

The auxiliary air screw is used to confirm the setting of the (pilot jet) air screw on the carb. In short, having initially set the idle, the auxiliary screw is opened to see if the idle speed increases. If it does, the idle mixture is still a bit too rich so further adjustment of the carb air screw is needed. Once the idle is set, the auxiliary screw is closed, job done.

There is no extra lump on the cylinder head casting on my bike for a PAV valve. It's a 2004 bike.

If the OP's bike doesn't have an auxiliary air screw but only an open "nipple" on the manifold adapter, then it is indeed different to my bike.

Having said the above, I removed the rather clumsy and over complicated air box on my bike and fitted a washable "pod" type air filter directly on the carb. I also removed the clear plastic pipe because it served no real purpose (even when originally connected to the air box).

The auxiliary air screw still fulfils it's original purpose when I set the idle. I kept the adapter because the extra length should help provide extra torque.