Throttle Body Synchronisation
27 Jul 2017


The idea for the following vacuum synchronisation instrument came from Hugh Kenny, who wrote about it in March 2002. He got the idea from Tom Rowe. You can see the original article at and probably a dozen other sites. It was conceived to tune a boxer twin, so only used one tube. Basically, the tube is half filled with a liquid, bent into a 'U', where the liquid sits, and each end is connected to a vacuum port. The levels will be the same height, if the vacuum pressures are equal, but if they are not, the difference in pressure will raise the level on one side, and drop it on the other.
That works fine for a twin, but for a triple, you need 2 tubes. One to measure the differential between 2 & 1, and the other to measure that between 2 & 3. Cylinder 2 has a direct connection to the throttle, & 1 and 2 are adjusted in relation to it.

How to Build a Differential Manometer for the Tre

3 into 4 Manifold
Differential Manometer

You can see the finished device above. 3 pvc tubes bring vacuum from each throat, to a manifold, which splits #2 into 2, but just transfers #1 to 1 & #3 to 3. Another tube from 1 then runs down a wooden backing plate, through a retainer, then back to 2. This is repeated for 2 to 3.
The backing plate is about 1600mm long & 45mm wide.
Each down tube is 2500mm long with a bore of 4mm. The actual bore size is not important, but the folded length should be a meter, or more.
The vacuum ports on the throttle bodies are 8mm OD, belled to 9mm at the tips. I used some rubber hose that fit snugly over the pvc tube, to connect to these ports.
The 3 tubes that run back to the throttle bodies are about 3000mm long, but again, it depends on where you can hang the manometer, and how far you are be from your bike.
I filled the downtubes with automatic transission fluid (ATF), to about half way. Purging the air bubbles was done by repeatedly pressurising one side of a downtube. The bubbles will rise faster than the oil level. I did end up with a small bubble in one tube, that settled at the bottom. I assume that it's surface tension was too high to allow it to rise properly. Which probably means that a larger bore would have helped. But it made no difference to the way the manometer worked, so I left it there.
The downtubes were held in place at the bottom by feeding them through an aluminium block, with holes barely large enough to take them.

What I haven't shown are the tee pieces at the end of each 3000 mm tube. Originally I had placed a bleed hole in each tube and completely bypassed the stepper, but for a very long time now, I just tap each leg of the manometer into the line with the corresponding stepper vacuum tube. That way the ECU can still control the idle speed. Once set though, it doesn't move.

As an aside, ATF works well because it's red & easy to see. Also, if the vacuum pressures are way off, it may be sucked into the engine, where it will be burnt. A 4mm tube, if filled to 1250mm, will hold about 15ml of oil. Or just about nothing.
But it may happen, so you'll have to make sure that the Tre isn't WAY out of sync when you start.

The Procedure, using the Differential Manometer

Top view of connection to throttle bodies
Side view of connection to throttle bodies

  1. Remove all of your panels and the airbox.
  2. Remove the air temp. sensor from the airbox, and connect it back to the loom.
  3. Bypass the tip over switch, by connecting pins 2 & 3 on the associated connector.
  4. Support the fuel tank just above the frame, so that you can access the throttle linkages, and the fuel hose isn't unduly strained.
  5. Connect the fuel line & fuel pump harness.
  6. Hook up your analyser (Axione or TuneECU) so that you can read the stepper position.
  7. (Here's where colintornado's input kicks in)
  8. Screw out the two outer stops so that only the centre one holds all 3. Better yet, remove them completely.
  9. Use paper feelers - cut strips of printer paper (0.1mm) or better yet thermal printer paper (0.06 mm) - to insert between the centre of the butterflies and body.
  10. Open and shut the throttle, and gradually close the centre stop till it grabs a feeler, testing each one in turn. You may need to open 1 or 3 if either bottom out before 2. Once 2 is correct concentrate on 1 & 3 by turning the adjusting screws on the shaft which move 1 or 3 in relation to 2.
  11. When all are grabbing the same, screw the centre stop back to closed, as much as possible. At this stage the sync will be close.
  12. If you have moved #1 in relation to #2, reset the TPS.
  13. Warm up the engine to running temperature, then turn it off.
  14. Remove the 3 hoses from the stepper to the base of the throttle bodies.
  15. Connect Manometer hose #1 to #1, #2 to #2 and #3 to #3, then connect the corresponding stepper hoses to the other end of the Tee's
  16. Screw the bypass screws out one turn each. This will allow you to reduce one or two if necessary.
  17. Start the engine again
  18. Adjust the bypass screws until the manometer levels are even. Remember, closing a bypass screw increases the vacuum and raises the level in the tube connected to that throttle.
  19. Once level, close each bypass screw in turn, keeping the manometer level, until one screw bottoms out
  20. The stepper should be between 30 to 40 when finished. Lower than 30 and the engine will have flat spots and be prone to stalling. If outside this range, you'll need to use the analyser to adjust the "Manual Adaptation of The Stepper" up or down until the stepper sits between 30 and 40 at idle.
  21. All done. Switch off, remove the manometer and analyser, and put her back together.

From Johnny O (11/20/06 3:49 am)

"set the bi-pass screws at 1 turn out at the minimum. I tend to find that it eliminates the off-idle lurch that fuel injected bikes tend to suffer from. With the throttle plates synch'd properly, I can get the engine to tick-over smoothly as low as 700 rpm!"

Ed. The downside of this method will be that the ECU can not fully control the idle speed, since the lower limit is set by the opened bypass screws. If it can't do this properly, the mixture at idle will not be adjustable and you are likely to get spit-back due to an excessively lean idle mixture.
The actual idle speed is set in the ECU map.

The Benefits of Synchronising the Throttle Bodies?

My Tre had been getting about 130-140 km in city riding, before the fuel light came on. When I first checked the sync, the manometer levels were about 300 mm different on the worst pair. After synchronising to within 10 mm, the engine ran noticeably smoother, and the fuel light didn't come on until 155 km. I've only put one tank of fuel through her, but I'm pretty much convinced. She's back to where she was a year ago.

Another Method of Synchronising the Throttle Bodies

The standard method of synchronising carbureted 4 & 6 cylinder Alfa's & Fiat's, is to short each plug out in turn, and listen for the amount of idle rev drop. The further it drops, the more that cylinder was pulling. If they all drop to the same pitch (rev), they are synchronised.

This method is more accurate than measuring vacuum, because it takes the entire engine into account, not just it's ability to pull a vacuum. If the compression is down on one cylinder, it won't be able to raise a vacuum as well as the other cylinders. However, it is still possible to adjust it to pull the same as the others, by using this method

Now for the hard part. If the ECU can be told to "forget about" injecting, and igniting, a particular cylinder (many ECUs do have this facility), then checking the synchronisation could be as simple as plugging in your Tuneboy/TuneECU! You've just saved yourself an hour of work, dismantling, checking, then reassembling your Nelli. Even better, on the next major service, tell your mechanic not to do the synch job, 'cos it's fine. Save yourself some cash!
Unfortunately, Sagem haven't provided this facility, so it can't be done quite so easily. An injector or spark interrupter would be needed. Perhaps someone has already built such a thing, but I haven't found it, so in it's absence, I'll have to build it myself.