Carb / Throttle Body Sync Tool

Discussion in 'Technical & Maintenance' started by fortech, Apr 29, 2019.

  1. fortech

    fortech Active Member

    can anyone recommend a good but reasonably priced set of gauges for syncing carbs/throttle bodies?

    Better yet, anyone have some they wouldn’t mind loaning for a day?
  2. RossKean

    RossKean Active Member

    I made one (see pic). PVC tubing, 2 "Y" connectors, a bunch of zip ties, a couple boards and a fill with transmission fluid. Put restrictors in the tubes that run to the sync ports to keep levels from jumping too much.

    Suitable for relative (differential) vacuum but not absolute measurement. Some people make these with large (empty) reservoirs in case the channels are badly out of balance causing fluid to be sucked into one of the ports. I hook mine up to all four cylinders and put clamps on three. I remove clamps one at a time and balance as I go. No issues yet.

    Or have a look at this one with the reservoirs to accommodate a severe mis-balance as well as helping to dampen "noise". Post #10

    In terms of a commercial instrument, the Morgan Carbtune is generally well thought of. Reasonable pricing and no messy fluids.

    Attached Files:

    Last edited: Apr 29, 2019
  3. fortech

    fortech Active Member

    FB7E8FA6-92F0-4018-BC44-4D4F18A461D5.jpeg I’ve cobbled up a homemade rig as illustrated below. I needed to install a “T” for the left cylinder as the vac pressure sensor needed to be included on the Super Tenere as well. The “T” fitting is of much smaller inside diameter than the tubing, therefore I’m wondering if this is acting as a restrictor and skewing the results due to the right having no “T” or restrictor in place.

    To keep this simple, if I restrict the left line with the smaller diameter T fitting, would i have to restrict the right line with an equal sized fitting as well?
  4. fortech

    fortech Active Member

    2A0033C8-C8B9-40F0-A355-DADBC7788524.jpeg To clarify,

    The syncing process requires a T on the left throttle body to incorporate the vac pressure sensor.

    See image and note the smaller inside diameter of the T. I assume this would act as a restrictor and slew results as the right line does not have a similar fitting in place?
  5. RossKean

    RossKean Active Member

    I don't think it would matter. Once it is at equilibrium at a specific engine rpm, there is no net movement of air one way or the other. A finer restriction on one side will make that side slower to settle but should get there eventually. While Yamaha has a vacuum spec for the throttle body sync on my FJR, nobody ever pays much attention to the absolute vacuum - more important to get them all the same. You may need a restrictor in the line going to your vacuum pressure sensor - pressure oscillates rapidly as the engine is running.

    Note: the one I made is VERY sensitive (perhaps too sensitive) and can be used for two or four channels by simply plugging two of the lines. Compared to the old mercury sticks, the oil has about one fifteenth the density so one cm of mercury is actually 15 cm differential for oil.
  6. fortech

    fortech Active Member

    I don’t have any restrictors and I’m going to,assume that I won’t find any around town very easily. They will have to be ordered from the Interweb for sure.

    I will probably leave the settings as is for now and give it a test run when the weather improves. Would you expect my findings to change significantly once installing the restrictors?
  7. RossKean

    RossKean Active Member

    Don't think there would be much difference . Restrictors are more to stabilize the readings. Your setup with the large volume headspace above the liquid should effectively damp the oscillation. On my setup I have very little dead volume above the liquid but I have fairly narrow ID black tubing coming off the top and inserted 1" pieces of very narrow tubing (1/2 mm?) in that to create flow resistance. I think yours will be OK.

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