Pilot jet rubber plug

Hi!

I'm restoring my mikuni B-28 carbs and I've run into a problem. On some pictures I've seen of mikuni rebuilds there's a rubber plug covering the pilot jet in the "carb bowl cavity". My bike has been running pretty good without these plugs so I'm wondering if the BS-28 should have them installed? What's the purpose of these plugs anyway?

 

The purpose of the Plugs is two fold:

1) Anti-tamper. So that the Pilot Air Screw isn't moved from Factory setting.

2) Keep the area below the Plug free from exposure to contamination and seizure.

I use the Erasures from New #2 Pencils.
Break the metal holder and use the whole erasure.
Trim the excess flush with razor.
Seal and Lock with a smear of Bathtub Caulk.

Future removal of the Rubber Erasure is a piece of cake.
Simply start a sharp pointed Drywall Screw in the Erasure and pull it out.

 

They're in the float bowl? :? The only rubber plugs I'm aware of are the ones Rick described, that cover the pilot SCREW not the pilot JET...

Could you add a link to the pictures you've seen please? I have heard of things like this in Honda Keihin carbs but not sure what they did...

 

Mikuni BS28 off a North American market 550, broken down: http://xjbikes.com/Forums/viewtopic/t=31061.html

 

Check out page 36 in this rebuild:

http://www.mtsac.edu/~cliff/storage/gs/ ... torial.pdf

These carbs are from a Suzuki, and the rebuild refers to BS 32-36, but internally they look pretty much identical to my BS28-carbs, hence my question.

I also found these plugs on mikesxs.com (for the BS34 carb):

http://www.mikesxs.net/product/48-5002.html

"Rubber pilot jet Passage Plug (Pk./2) Fits: XS650G/H/SH/SJ/SK/SL (1980-84) Stock Mikuni BS34 CV Carburetors. Used inside float bowl to seal passage to pilot Jet - 1 used per carb"

I should probably mention that the bike I'm working on is a XJ550 european model (4v8)

 

Strange thing is, they look like they block off the pilot jet & would stop it from functioning. :?

I'd say if they weren't there to start with, don't put them in now. Possibly the 32-36 sizes have a different fuel passage to feed the pilot jet?

 

They must.

The 4V8 550 and the 4U8 (USA/North American) 550s are quite similar, and I've NEVER seen that plug in any of mine.

The carb that's torn apart in my pictorial is a brand new NOS BS28 for a 550 Maxim (5K5) but it is virtually identical to the carbs on my 550 Secas (and my spare sets.)

I do know that the 550 Yamaha BS28s were different from other BS28s even, but the placement sure looks like they would completely defeat the pilot jets.

There's no mention of them in the Suzuki rebuild article, but they sure are there in the pics. I wonder if they're used to block off the pilot jets while setting the fuel levels, then removed?

I would NOT install them.

 

From another forum (http://69.46.28.106/forum/3-carburetor/ ... t-jet-plug)

".... When i bought the bike the plugs were not there, I made plugs from a lead sheet and it runs oh so much better. Inside the hole whre the main jet goes there is a hole that feeds the pilot jet, that is how gas gets to the pilot jet, with out the plugs the gas gets sucked through that hole which is above the main jet & floods the engine on anything above half throttle. Hope this helps, just thought i would post with my personal experience of the same situation."

Maybe another construction on the BS 34 carbs?


Anyway, I'll go ahead and put em together without the plugs. Thanks a lot for your input guys!

 

looking at page 36 of that tutorial, it looks like the pilot jet gets it's fuel through the emulsion tube.

CN

 

There's the difference. I'm pretty sure there isn't such a passage in our particular flavor of BS28s. I'll take apart the brand-new carb when I have a second and look though, now I'm curious.

 

Hmm strange design. I'm sure the XJ550's BS28s can't have this hole since the problem described on the other forum related to fuel being drawn back through that hole the "wrong way" (i.e. up where the plug would have been & into the emulsion tube area) and since it was above the main jet it caused the engine to run way rich.

Would be interesting to know why mikuni (and indeed keihin) felt the need to draw the pilot fuel through the main jet before the pilot jet instead of the "normal" way of doing things. :?