Build your own manometer

The abrupt throttle response has bought much criticism of the current model VFR.  I trawled through many forum threads dedicated to why the off-on throttle transition would cause the bike to lurch forward.  Older models of the bike do not demonstrate this trait and even some current models don’t.  One common theory is that the “starter valves” are not balanced on the bike and when badly configured would cause issues in the initial throttle travel.

To balance the starter valves requires measuring the vacuum each of the four cylinders generates.  Typically his is done with a set of vacuum gauges.  Adjusting the starter valves is a fiddly process.  When you get any vehicle serviced, you can equate “fiddly” to “expensive”.  Likewise, buying a set of vacuum gauges to allow you to measure all four intake vacuum pressures is not “cheap”.  (Well, if you know in advance that this will fix the problem, it’s money well spent. Given the circumstances, that was a pretty big “if”)

One forum member came up with the cheap alternative: Build your own manometer.

“Basically, a manometer measures pressure (Mano- is the Greek term for “gas” or “vapor”). Many expensive manometers (like the Motion Pro or Carbtune) allow you to measure the vacuum of each cylinder in cmHg (centimeter of mercury). Since the starter valves on the VFR VTEC are supposed to be set equal to each other, then there’s no need to measure the vacuum of each cylinder. All we need to know is what the vacuum in the cylinders are, relative to the other cylinders.”

I decided that a home-made manometer was around the budget that I was prepared to spend “on a hunch” and thus I built my own.  Unlike “Darth Bling”, I was unable to source the adapters to use wider pipes, so I went with 4mm clear plastic tubing throughout.  I’m not presumptuous enough to claim that this is a better set up – in fact, I’m almost certain that it is worse, but it was one that was dictated by the limitations of local hardware shops.

The first manometer I built consisted of four tubes of around 2m in length, connected by two T-pieces.   On the other ends (the bit that connects to the intake vacuum hoses) I put 4mm “barbed” connectors.  This allowed for a fairly easy connection to the bike.   The good thing about using 4mm tubing, is that common garden irrigation/sprinkler systems use 4mm piping as well.  That means T-pieces etc are common.  The bad thing about using narrower pipes is there is a lower volume of oil.  I used some 4-stroke lawn mower oil, because (a) it was a pretty red colour, and (b) because I had some “lying around”. 

T-piece connections  Mower OilFillingVersion 1.0

Unfortunately, the vacuum pressure difference on my bike was such that all the oil was sucked out of a couple of the hoses when it was connected up. – I hit the kill switch before it had a chance to suck oil into the engine!  Once a tube empties, air gets introduced into the pipes and puts bubbles into the oil.  This makes the manometer ineffective as the oil levels will never be level in the hoses.  The quickest way to “reset” the manometer and remove the air-bubbles is to disconnect one of the hoses from the T-piece and drain all the oil from the system.

Lessons I learnt from making my first manometer:

  1. I had about 1.2 metres of hose above the level of the oil.  For my bike, this appears to be sufficient to mean a watchful eye will prevent oil being drawn into the engine.  (That is, you should have sufficient time to panic and hit the engine kill switch if something goes wrong)

  2. I needed more oil in the pipes.  A higher volume would mean less chance of all oil being drained from one pipe and thus less chance of introducing air bubbles into the system.  The additional weight of the extra oil should also reduce the effectiveness of the vacuum and make the oil levels change at a slower rate.

  3. A way to “bleed” air and oil from the system would make life easier.  (Or so I thought…)

  4. Knowing approximately how much oil your manometer will need makes life simpler when filling.  Do the calculation – it’s not hard! 

Volume = π xx h

(Of course, remember that the equation uses the radius, not the diameter and yet tubing you buy invariably is measured by diameter! – Something I’m embarrassed to admit I forgot. – Still, this blog entry is more about learning from my mistakes so hopefully you won’t make the same ones…   Also, apologies to all those stuck with the imperial measuring system…  Measuring things in metric makes life easier here:  If you specify all measurements in cm, you end up with cubic cm for the volume, where 1cc = 1mL. )

So… The second manometer I made used piping of around 3.5m per tube.  This was longer than intended, helped by the generosity of the local hardware shop that sold me 15m of tubing for the price of 12m.  For this device, I used three T-pieces.  The “middle” T-piece connected to a “tap” that could open and close, allowing me to bleed the system if necessary.

Each tube has just over a metre of oil in it, which equated to a measly 13mL of oil in each tube.   This time, the oil didn’t race straight up the pipe at an alarming speed and things were looking good… until…  After around twenty seconds, the levels of all four pipes started to rise!  Two seconds later, I managed to stop looking bewildered and noticed that air was being drawn back into the bottom of the manometer via the tap on my bleed hose!  So it turned out that I had introduced a double-edged sword.  Whilst the bleed line was a great way of removing air from the system, it was also a great way for putting air into the system!

So I hereby declare my first working version of the manometer to be Version 2.1.  It’s not what you would call a pretty manometer, as the excessively long tubing sits in an un-kept mess at the bottom…   But it works, doesn’t leak oil (or air) and thus serves the purpose of a more expensive piece of equipment. 

Manometer Version 2.1

Note:  I had just moved the manometer when I took this photo, and as the pipes were not tightly held in place, they possibly moved – thus explaining the differing levels of oil.  Given enough time, they would “re-adjust” such that all oil levels were… err… level!

Here’s the breakdown of the parts used in the construction (not including the tubing discarded along the way)

15 metres of clear 4mm tubing… $0.63 x 12m = $7.56

 3 Micro Joiner 4mm barbed T-piece adapters… $0.50 x  3  = $1.50

 5 Mircro Joiner 4mm barbed connectors… $0.31 x  5  = $1.55

~55 mL 10W40 Mower motor oil – sourced from existing supplies.

 8 cable ties – sourced from existing supplies

 1 piece of MDF board – sourced from existing supplies

 1 shoelace (to hang the manometer from) – sourced from existing supplies

Total: $10.61. 

In all honesty, if I factored in the prices for parts I already had, I might have bumped the price up to $20 – $25.  Also, for V2.1, I only strictly needed 4 connectors and two T-pieces, but you get the general idea…  By the way, a set of four vacuum gauges will likely set you back between $100 and $200.

As for how the balancing of the starter valves went and whether it improved the bike, well, that’s a story for another day…


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