TOPIC:

Hello,

i'm actually checking the chokes plungers on a vm28ss.

Is there a way to test them for leaking?

Thank you

Check the rubber seals, check the plugs, black plugs = leaking.
 

Actually there is, a bench vacuum test thru the outlet choke hole in the venturi. I made up a piece of test equipment years ago but only because i do a lot of these, probably not practical since you are only doing a set. Ditto 73z1 on the sooty plugs when other parameters have been established. If you want a quick fix for cheap, find some 1.5mmCS x 1.5mmID x 4mmOD  o-rings. 4 required, one per plunger protrusion. I clean-off the crap on the existing pad and dab a bit of superglue to retain the oring, wipe excess off immediately and re-install per bore using the spring pressure provided till glue sets. What does this do? As long as that protruding round seat you see looking down the plunger casting bore isn't all pitted, you've now provided a seal to keep the fuel from being sucked-up during high-vacuum/closed throttle situations. The o-ring does push up the final plunger assembly a smidge, adjust lifter rod arms to suit if necessary. Have been doing this for years on bikes that Have been running for years and i'll set you back probably less then 10bucks. 

73z1 wrote:

Check the rubber seals, check the plugs, black plugs = leaking.


 

^ THAT ^
Rich running with correct jetting is the main suspect symptom.

We know of no way to test for leaks manually by allowing liquid to remain on / in the plunger seat area for long periods because of the design of the carb body.

If checking visually, use the very highest magnification you can obtain & good light.  A 'donut' depression around the center needle is normal where the synthetic rubber contacts the seat.  Radial cracks in the seal guarantee a leak. Be very careful handling the plungers, as the center needle is thin, soft brass & relatively easy to bend.

We experimented with soaking the seals in acetone for a short period to cause them to swell. We don't yet know how long afterward doing so will improve sealing.  A member on another forum suggested wintergreen oil; we haven't tried that yet.

KZR forum member ZED1015 offers a service to refurb OEM starter plungers with new seals.

Niche Cycle offers new VM29 smoothbore plungers that mic the same as the OEM plungers in VM28SS Z1/A/B carbs. Dimensions are on the order page. The shafts are raw brass instead of plated.  We're testing a set this season on a '74 Z1-A.  So far, so good.

Good Ridin'
slmjim & Z1BEBE