Loud tapping- not little end or valve gear! Traced to servo NR valve -fixed!

Ever since I have restored this motor there has been an obvious tapping whilst it runs. Being the type of person I am I immediately assumed the worst and imagined another major overhaul. However, it didn't seem to be engine speed related, the noise comes on starting and acceleration, vanishes on overrun- obviously the motor is still turning then so this at least  suggests its not motor-originated. My next suspicion was that it was the speedo cable (I've had these tap on bikes) or something rattling on the pedal box which I have also changed. However these were eliminated and poking the stethoscope around the rear of the dash I found that the noise was detectable on the master cylinder but loudest on servo body. Finally I identified it to my surprise as coming from the servo non return valve. On close inspection I could actually see through the nylon cover and see the valve opening and closing. Its a ball bearing and when it closes it makes a metallic "tap". Research shows this is by no means unusual and experience could have saved me a 30 min search.

I have a short video of this phenomenon... needed  a bit of editing so sorry for the enforced freeware logos. 

Since manifold vacuum is related to engine speed the NR valve is needed to preserve vacuum in the servo plenum chamber. You wouldn't want servo assistance to vanish with engine speed would you? Consequently the valve opens when manifold pressure is lower than pressure inside the servo and allows air to be sucked out of the servo into manifold producing the vacuum inside the servo to assist the brakes. If manifold vacuum falls the valve shuts preventing air from flowing  from the manifold into the servo and so preserving vacuum assistance.  In other words the valve allows air out of the servo but doesn't let it flow in the other direction.

The most likely cause of this tapping behaviour seemed to me to be an air leak making it hard to maintain servo vacuum- either in the servo, the NR valve itself, or possibly even the manifold. However, given the rapidity of this valve movement, I would expect the mother of all air leaks as the cause. In fact I can detect no hissing from servo, valve or heater actuators so for the time being I'm going to ignore leaks as a possible cause.

One odd possibility is that this constant valve cycling is caused by pressure pulses along the vacuum hose. Ideally you would want the servo valve to respond to average pressure in the intake manifold, but every time cylinder 4 descends on intake there will be a transitory increase in vacuum in the manifold above no 4 intake valve... in other words just where the vacuum hose connects.  I am aware that this effect was a complication in later cars that used a vacuum driven ignition advance system for the distributor, and a restrictor was needed to even out these pressure pulses- the simple metal elbow of my car being replaced in later versions by a right-angled plastic junction with a narrow bore.

Whilst a restrictor would probably solve this problem, there is no evidence that such an item was ever fitted to my car and yet valve cycling wasn't a problem. There must be another solution. Checking the parts lists showed that Lotus specified a 711mm hose between the rear manifold take-off and the servo for early cars like mine that have no restrictor. It was so long it had to have a separate P clip to hold it out of the way on the bulkhead. I don't think Lotus would specify such an inconvenient length (or a length at all) unless it were important.  Looking at my car it seems I have a shorter hose at 670mm

My vacuum hose... 67cms only

Can these 5 cms really make a difference? I cut an extension of 10 cm and fitted it with a plastic hose joiner.

So splice in an extra length-I started at 10cm

I didn't expect anything- and indeed on starting I did get the usual tap tap tap... but after a couple of seconds this vanished. This behaviour seems reproducible- some tapping whilst vacuum is established immediately after starting and then none!

I am truly amazed that such a simple cure could solve the problem! Anyway I tidied up the hose, fitting jubilee clips to all joins and trimming it down a little so it fitted better. Currently my hose is around 72cms long and at this length it slips easily through the bulkhead P clip without rubbing on the fuel pipe- and I still have no tapping!

Looking vertically down the bulkhead at the hose routed through the P clip(its the black vacuum hose below PVC fuel pipe)

Incidentally if you do need to change the non return valve the original Girling kits are now hard to find and expensive.  Unlike the originals,  new valves such as those from Sjsportscars don't come with the sealing grommet.  You can manage with a normal grommet which will seal G10R270-12-210 although it's not shaped correctly internally. A better replacement can be obtained from Rimmer Bros where it's sold for the Mk1 Ford escort
https://rimmerbros.com/Item--i-BAU1019 .