Starter (choke) plungers

SWest's video is no longer available on YouTube. I've tried to clean up this thread. There's plenty of good info below.
Bill/KZQ
 

Is there anybody making new rubber plungers that I can replace the old rubber with?

Send them to zed1015 (Rob Meggitt) in the UK. Cheap renewal service.
kzrider.com/forum/profile/14135-zed1015

Sending your plungers to zed1015 is the obvious answer.  That said, I've replaced plunger seals myself before, and it's not horribly difficult.  The motivation to try myself came from a thread here, where someone posted photos of a tool they made to punch out the rubber donuts.  I copied the tool, but struggled to make consistent donuts.  In the end, I changed the process, and have had good success since.

First off, you need some 2mm thick viton sheet rubber.  I use grade A sheet, but B is slightly better, since it has even more chemical resistance.  I'm pretty certain that the A sheet is superior to whatever Mikuni used previous, though.  I punched a small hole in the viton, using a leather punch.  Then a piece of sharpened tubing can be used to punch out the outside shape.  Try your best to keep the holes centered, but as long as the OD is oversize, don't worry.  

I stack half a dozen donuts on a finishing nail, then chuck them up in a drill motor.  Then spin, and sand down the OD, truing up the concentricy, and achieving the proper OD.  VM carbs need a 7.0mm diameter rubber donut, and BS/CV carbs need 7.1mm. 

To remove the old rubber, I drop the plungers in carb dip.  After a couple of days, the rubber softens, expands, and pooches out, so you can use a small pick and remove it.

Getting the new donut into the recessed cavity can be challenging.  I lube the rubber with silicone grease, and use a small, somewhat dull edge screwdriver to tuck in the rubber.  You can get about 75% installed this way.  The last part is the hardest; I took an old pick tool, curved the end into sort of a spoon shape, and use that to slowing work the rubber into the plunger recess.  I've torn a rubber or two this way, but after a while, I got the hang of it and the install came off no problem.  Worst case, remove the rubber and start again.  If I can do it, so can others.  Oh, and Clay has been replacing the rubbers for many years.  Actually, I think he simply stacks a small O-ring on top of the old seal these days, since his bag of viton donuts has gone missing.  I'll leave it up to him to finish the story, though.  

 

Only thing to add to Nessim's process is, be very gentle working next to the center pin of the plunger.   That pin must be precisely parallel to the axis of the shaft in order for the pin to move easily in its bore in the carb body.  The clearance is just a couple thou (inch) or so, if that. The metal of the pin is quite soft, and the pin can be bent easily.  Doesn't take much deformation to cause the pin to hang in its bore.

Good Ridin'
slmjim & Z1BEBE

We're researching alternative plungers.  Could someone with 29 smoothbores that have known original plungers measure the shaft OD indicated with the red arrows, please?

 

Looking for dimension to the second decimal place, i.e. 4.xx mm.

If you have a metric micrometer, that would be much appreciated.  If not, accurate metric vernier calipers will suffice.

Thanks!

Good Ridin'
slmjim & Z1BEBE  

slmjim+Z1BEBE wrote:

We're researching alternative plungers.  Could someone with 29 smoothbores that have known original plungers measure the shaft OD indicated with the red arrows, please?

 

Looking for dimension to the second decimal place, i.e. 4.xx mm.

If you have a metric micrometer, that would be much appreciated.  If not, accurate metric vernier calipers will suffice.

Thanks!

Good Ridin'
slmjim & Z1BEBE  

They are the same as the stock VM items.

Jim, all three versions of these choke plungers are right at 4.00mm. Chrome plated or not, all 4mm. Smoothbore is irrelevant because they used the identical ones for all the VM series. What does matter is the rubber cap if the plunger isn't returning, VM20-455 is the rubber cap Mikuni number. Is this the problem?  

Actually 4 different styles if you count the "ribless" early 28's.

wrote:

Jim, all three versions of these choke plungers are right at 4.00mm. Chrome plated or not, all 4mm. Smoothbore is irrelevant because they used the identical ones for all the VM series. What does matter is the rubber cap if the plunger isn't returning, VM20-455 is the rubber cap Mikuni number. Is this the problem?  
 

Seeing conflicting info elsewhere on the indicated shaft diameter, 29 smoothbore vs VM28SS plungers.

 It would be an easy measurement with no disassembly needed.  Raise the plunger & measure just above the rubber plunger seal.

Thanks for the responses everyone.

Good Ridin'
slmjim & Z1BEBE
 

asphalt900 wrote:

Actually 4 different styles if you count the "ribless" early 28's.
 

What's the difference in early ribless VM28 plungers vs later VM28 plungers?  Interchangeable?

Good Ridin'
slmjim & Z1BEBE

slmjim+Z1BEBE wrote:

asphalt900 wrote:

Actually 4 different styles if you count the "ribless" early 28's.


 

What's the difference in early ribless VM28 plungers vs later VM28 plungers?  Interchangeable?

Good Ridin'
slmjim & Z1BEBE

All these VM plungers are pretty much the same at the business end with an 8mm diameter plunger head and the only variation there being the length of the metering pin which doesn't affect their function.
All variations will physically fit and work in the body of any of the VM series carbs.
The main differences are shaft length and profile of the VM/SC versions ( 2 bolt top )  at the lifter end to accommodate the different lifter arm and bobbin types which can be accounted for with either mixing and matching or modifying lifter arms and bobbins.
All VM/SS plungers from the VM22,24,26,and 28mm ( 3 bolt top ) carb banks are the same except for the finish in either plated or plain brass.