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Re: [turbobike] inlet plenum pop off valve

Tim Drew (timdrew@cix.co.uk)
Fri, 27 Aug 1999 19:24 +0100 (BST)

Stuart,

> US gsxr's seem to have one conveniently placed in
> the rubber boot between the right carb and the head,

UK spec 11's also have a vacuum take-off on the #4 carb which is used to 
provide a signal to the fuel cock. I'm now successfully feeding my dump 
valve from this point via a short length of 8mm hose.

If you also intend to use this point for an *un-damped* boost gauge you 
will have to severely restrict the hose to the gauge, otherwise the 
vibration at low revs will shake the pointer off! I use a cheap leak down 
valve (The type you can buy at any motor accessory shop) as a damper. What 
I've done is seal the adjuster thread with PTFE tape, then I placed the 
valve in the pipe between the inlet rubber and the dump valve, connecting 
the boost gauge to the point where the valve would normally leak air. You 
can then adjust the valve to damp out the vibration on the boost gauge.

> dont.The only other place I can think of is from one of the plastic carb
> balancing stubbs on the carb top,but I'm not sure if the small opening 
> will see enough vacuum to operate the valve quickly?

I originally made a manifold which connected to all the carb tops and used 
this to provide manifold pressure for the dump valve / boost gauge etc. 
This worked well but was untidy so I opted for using the take-off on the 
#4 carb rubber.

Another point you may find of interest. I originally supplied my boost 
retard switch (which was set to about 3 psi) with manifold pressure. I 
found that when steady state high speed crusing with about 3 psi on the 
gauge the bike surge quite violently. What seemed to be happening was the 
pressure switch was clicking in and out so the bike would accelerate for 
about a second, then decelerate for a second. This was bad enough to be 
quite frightening on a high speed bend which was invariably where it 
happened! What I did was re-locate the switch so it's now looking at 
plenum pressure rather than manifold pressure and the problem's gone.


Regards

Tim Drew
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