Question about the rubber plugs on pilot jet passage.

[Squishy]

Hello,

So I've been working on a GSX600F katana which has the same carbs as the GSF400 (BST33SS).
Like my bandit, it has the 4 rubber plugs on the idle jet, and it will draw fuel via the passage from the main jet.
The plugs on this set have shrunken and don't fit properly anymore. They won't fall out because the float mechanism holds them in place, but they don't fully close the pilot jet tube.

Now I was thinking about this and did some googling but many people disagree with eachother.
Does the plug actually affect the mixture (when it's leaking or removed) or will the pilot jet always be the limiting factor?

Or in other words: did they design the carbs in such a way that the mixture is different when it goes via the main jet, or could you even run without the plugs as the pilot jet will always be what limits the fuel flow.

Looking for some opinions 

[greg737]

Now I was thinking about this and did some googling but many people disagree with eachother.
Does the plug actually affect the mixture (when it's leaking or removed) or will the pilot jet always be the limiting factor?

Or in other words: did they design the carbs in such a way that the mixture is different when it goes via the main jet, or could you even run without the plugs as the pilot jet will always be what limits the fuel flow.

Interesting question...

I can't see what the purpose of the Rubber Plugs is.

But I can talk through the Pilot Jet a bit. Maybe that will help.

The Pilot Jet is shaped like a tiny combined version of the Main Jet + Emulsion Tube combination, so it's logical to assume it functions in a very similar fashion.

So the Pilot Jet has a small orifice at its bottom just like the Main Jet does.  This means that just as with the Main Jet there has to be fuel sitting in the tube that the Pilot Jet is mounted in and that tube has to be full of fuel right up to the bottom of the Pilot Jet all the time, waiting to be sucked up and used.

But the design of the carb has the Pilot Jet screwed into the carb in a position just below the main intake bore, which puts its base (where its little fuel intake port is located) above the fuel level in the float bowls.  This can only mean that when the bike isn't running the fuel level will sit below the bottom of the Pilot Jet.

I guess that means that when you start cranking the bike there might be a moment or two when the Pilot Jet doesn't actually work because it hasn't managed to suck the fuel upward to its little intake port.

Then there's the other half of the Pilot Jet that looks like a little miniature emulsion tube.  The Suzuki Service Manual drawings don't show it but I understand that the Pilot Jet's mini-emulsion tube is actually connected to the Main Air Jet by a little tube (port, orifice, whatever you want to call it) that links it to the Main Air Jet by way of a connection to the area of the top of the Emulsion Tube, which means it is ultimately connected to the Main Air Jet.

Here's an illustration somebody did:

(it's a nice drawing even though it mis-identifies what is being supplied by the little connection port between the main Emulsion Tube and the Pilot Jet.  The drawing labels the dashed blue line as being "intake fuel from main jet" which is wrong.  It is obviously air for the Pilot Jet's "miniature emulsion tube" not fuel, which obviously comes up through the "miniature main jet" orifice at the bottom of the Pilot Jet.  If this wasn't the case then why would the Pilot Jet have an opening in its bottom?)

So, I still don't know what the Rubber Plugs are for... 

But I've begun to wonder what would happen if that connector port between the Emulsion Tube and the Pilot Jet mini-emulsion tube got plugged up.  If that connector couldn't feed air to the Pilot Jet's mini-emulsion tube then the engine vacuum would force the Pilot Jet to feed pure liquid fuel into the engine, making it run very rich.

Further, if that connector port air was even reduced at all by a partial blockage it would reduce the fuel emulsification in the Pilot Jet's mini-emulsion tube by some amount and have the effect of causing the engine to go toward a rich condition.

[Squishy]

Here's an illustration somebody did:
[snap]
(it's a nice drawing even though it mis-identifies what is being supplied by the little connection port between the main Emulsion Tube and the Pilot Jet.  The drawing labels the dashed blue line as being "intake fuel from main jet" which is wrong.  It is obviously air for the Pilot Jet's "miniature emulsion tube" not fuel, which obviously comes up through the "miniature main jet" orifice at the bottom of the Pilot Jet.  If this wasn't the case then why would the Pilot Jet have an opening in its bottom?)

So, I still don't know what the Rubber Plugs are for... 

But I've begun to wonder what would happen if that connector port between the Emulsion Tube and the Pilot Jet mini-emulsion tube got plugged up.  If that connector couldn't feed air to the Pilot Jet's mini-emulsion tube then the engine vacuum would force the Pilot Jet to feed pure liquid fuel into the engine, making it run very rich.

Further, if that connector port air was even reduced at all by a partial blockage it would reduce the fuel emulsification in the Pilot Jet's mini-emulsion tube by some amount and have the effect of causing the engine to go toward a rich condition.

Actually the drawing is not wrong. The blue dotted line is the passage way you mentioned previously, where the pilot jet draws in fuel via the passage-way (blue dotted line) through the main jet.
You can see the passage when you use a flashlight into the pilot jet port. The pilot jet port is actually plugged (is what I'm talking about) air-right so else it wouldn't be able to draw fuel from anywhere.

My question is whether it will change the amount of fuel when it can draw if the plug leaks.. or that the limation is set with the pilot jet.

[greg737]

My question is whether it will change the amount of fuel when it can draw if the plug leaks.. or that the limation is set with the pilot jet.

I'm not at home right now, if I were home I would go out to my garage and look at my spare set of BST32 carbs.  I have to admit I've never paid any attention to the Rubber Plug thing.

But if it is as you say, that the Rubber Plug is obviously designed to completely and totally plug the bottom of the Pilot Jet tube, I would say "yes" the rubber plug does change how much fuel the Pilot Jet gets because if it is only drawing from that passage to the Main Jet/Emulsion Tube area then it can only be drawing emulsified fuel (an air/fuel mix) rather than pure liquid fuel.

With the rubber plug tightly sealing the Pilot Jet tube: the engine's vacuum can only draw fuel into the Pilot Jet from the Main Jet/Emulsion Tube assembly (through the "blue dotted line passageway").  Because the rubber plug is blocking the base of the Pilot Jet tube the Pilot Jet will only get the emulsified air/fuel mix that is produced in and around the Emulsion Tube.

That's what I'm seeing when I imagine the BST33/BST32 operation.

Without: If the rubber plug is not installed (is missing) from the bottom of the Pilot Jet tube then under engine vacuum the Pilot Jet would be free to suck up pure liquid fuel directly from the Float Bowl.  But because the orifice in the bottom of the Pilot Jet is so small it would also still draw a substantial amount of air/fuel from the "blue dotted line passageway".  This version of Pilot Jet operation would deliver more fuel, resulting in RICHER engine operation.

It is odd isn't it?  If the Mikuni engineers knew they were going to completely block off/seal up the bottom of the Pilot Jet tube then why did they put a little hole/orifice in the bottom of the Pilot Jet itself?

Maybe the Mikuni engineers found that while the BST Carb Pilot Jet system worked well on larger displacement bikes it didn't work well on smaller bikes (our B4s and the GSXR400s).  Maybe the Pilot Jet system, as originally designed, was delivering a too-RICH fuel/air mixture to the tiny 100cc cylinders of the B4 and GSXR400, so by trial and error they figured out that if they did something as simple as completely plugging the intake shaft it corrected the situation.

[Squishy]

Hm.. are you sure the the mixture from the blue-dotted line is emulsified? Maybe via the main air screw, because from the top the emulsion tube is closed by the needle. From the bottom, it can only draw fuel (main jet).

[greg737]

yes, everything in and around the Emulsion Tube is subject to the mixing effect of the Main Air Jet.  It's by design. 

The Emulsion Tube has holes all around, from bottom to top and the Main Jet has only the one little hole right at its center.  If fuel is going to rise up the Emulsion tube it will be mixed with a lot of air.  That whole assembly gets a lot of turbulent flow so if you put a port (the "blue dotted line passageway") somewhere up near the top of the Emulsion Tube I think you're going to get a transfer of air/fuel mixture (emulsified) rather than straight, clean, pure air. 

Especially when the throttle is not opened at all.  Under that condition the engine vacuum isn't raising the CV slide so the Main Needle is down and blocking most of the Emulsion Tube flow which would make the "blue dotted line passageway" a proportionally larger path.  This would cause it to get a lot of the Emulsion Tube mixture.  Then, when the throttle is opened (and the vacuum raises the CV slide and with it the Main Needle) the majority of the Emulsion Tube flow would change to the new path-of-least-restriction which is right up past the perimeter of the now-raised-up Main Needle.

[greg737]

You really got me interested in this carb issue.  I had some time this afternoon so I did some interwebs searching.

The bottom-line truth of the matter appears to be this: If your Mikuni carbs show a rubber plug at the bottom of the Pilot Jet housing then you better make sure there's one on there.

But the problem is that Mikuni has designed and implemented several very different Pilot Jet systems.  They've built some very similar looking carbs around these very different Pilot Jet systems.  And various motorcycle manufacturers have caused further confusion by installed these varying systems on bikes of the same model families, swapping systems between year models, sometimes between neighboring year models of the same bike, and even between bikes of the same year as specified for delivery to different countries around the world.

So when I searched around the internet and read information and opinion I learned that there's a lot of confusion about these rubber plugs.

This might explain some of the Bandit 400 carb problems I've read about in the past.  And it would explain a bike suddenly becoming an over-rich, unrideable pig and ending up abandoned and ignored, pushed to the back of a garage.

[Squishy]

You really got me interested in this carb issue.  I had some time this afternoon so I did some interwebs searching.

The bottom-line truth of the matter appears to be this: If your Mikuni carbs show a rubber plug at the bottom of the Pilot Jet housing then you better make sure there's one on there.

But the problem is that Mikuni has designed and implemented several very different Pilot Jet systems.  They've built some very similar looking carbs around these very different Pilot Jet systems.  And various motorcycle manufacturers have caused further confusion by installed these varying systems on bikes of the same model families, swapping systems between year models, sometimes between neighboring year models of the same bike, and even between bikes of the same year as specified for delivery to different countries around the world.

So when I searched around the internet and read information and opinion I learned that there's a lot of confusion about these rubber plugs.

This might explain some of the Bandit 400 carb problems I've read about in the past.  And it would explain a bike suddenly becoming an over-rich, unrideable pig and ending up abandoned and ignored, pushed to the back of a garage.

Yeah I googled beforehand, and there was about 50-50 split on opinions.. some swore it didn't matter because both idle and main jet are covered in fuel and it doesn't matter whether it draws from the main jet or pilot jet.
I think I'm just gonna order 4 new ones ($4 /piece  ) and see the result.
The GSX600F is currently showing symptoms of being too rich (RPM drop before going back to idle RPM, hesitate on riding away etc. runs fine otherwise). Mixture screws are 1,75x out instead of 1.5x out, that shouldn't cause the symptoms in my experience (in fact a little richer than factory is almost always better). I'l leave them at 1.75x and see the if the plugs changes anything. Should be easily noticeably.

[ventYl]

I've also noticed that there are many different sorts of BST(32/33/34)SS carbs out there which are different in details (like these plugs, fuel valve housing brass / plastic, etc.).

For both of my Mikuni racks I'm pretty sure that the passage from main to pilot jet is below the lowest row of holes in emulsion tube. That made me to believe that there's only fuel in the passage and behind. Otherwise I can't imagine why separate air passage would be present for pilot circuit.

I expected that short stroke 398ccm engine would create too much turbulence compared to overall drag when dragging fuel through so wide hole in pilot jet passage when idling and/or in transition to main system so they isolated it and created narrower one which limits pulsing while not blocking flow at all.

Speculation: the combined effect of drag from pilot system and main system which are interconnected via the passage might create some non-linear richness characteristic. With throttle flaps closed drag from main system will be small but pilot system will have quite an huge vacuum while with flaps getting opened the difference will move towards main system which might change richness in some unimaginable way.

[Gouraami]

My Rubber plugs are also not sealing 100% anymore. I am planning on making some seals for it some or other time.

I was googled this a few months ago while rebuilding my carbs.

According to what I read greg is correct about the pilots being blocked for smaller displacement bikes (unless we read the same information). There is not enough air speed at idle to make them usable. 

I was going to post a similar question. The reason I was going to post is due to my bike having no power under 3000rpm, pulling away from a robot is very difficult at low revs. My reason for thinking the pilot jet rubbers are at fault, is that due to the rubbers not sealing, it is running rich off the pilot jets till the slide opens once more air flows. Does your bike also have no power under 3000rpm?

EDITED BY ME - Meant to type: My reason for thinking the pilot jet rubbers are at fault, is that due to the rubbers not sealing, it is running rich off the pilot jets until the slide opens and more air flows flows through, then running off the main jet solving the problem

[Squishy]

I was going to post a similar question. The reason I was going to post is due to my bike having no power under 3000rpm, pulling away from a robot is very difficult at low revs. My reason for thinking the pilot jet rubbers are at fault, is that due to the rubbers not sealing, it is running rich off the pilot jets till the slide opens once more air flows. Does your bike also have no power under 3000rpm?

This is exactly the problem I have with my GSX600F.  When pulling away I have to give way more throttle than normal, and then when it gets all the air it shoots forward and goes.
It also dips below idle RPM before returning. Typical symptons of running too ruch.

Yes your explanation is what I think too, however it would mean that the pilot jets are not at all limiting the fuel for the 'slow system'. Because if it did, it wouldn't matter where it got the fuel from or how much it has access to - it's limited by the pilot jet hole size.

[Gouraami]

I see I didn't type correctly, but I think you understood what I meant.

This is great, now you can replace them and tell me if it solved the problem  hahaha, only joking, it is another job I need to add to my list of things to do.

It led me to believe my bike was running rich as the problem isn't as bad when it is cold, but once warm it is a bitch! Rich symptoms...

Lets see who gets to this job first, I probably have to work Saturday, so it won't be me 

[greg737]

Yes your explanation is what I think too, however it would mean that the pilot jets are not at all limiting the fuel for the 'slow system'. Because if it did, it wouldn't matter where it got the fuel from or how much it has access to - it's limited by the pilot jet hole size.

I think you/we are zeroing in on an understanding of the dynamics of this situation.

I'm becoming convinced that the point at which "it all goes wrong" when you leave the Rubber Plugs off of the Pilot Jets is dictated by the engine vacuum. 

When the throttle plates are closed (or mostly closed) the engine vacuum is stronger at the Pilot Jet ports than it is at the Main Jet/Emulsion tube port.  Under this condition the Pilot Jet system will function properly with or without the Rubber Plug installed.

Under this condition the direction of flow in the connection tube between the Emulsion Tube area and the Pilot Jet is correct.

But... as the throttle plates open up the vacuum effect becomes much more pronounced at the much larger Main Jet/Emulsion Tube port.  As the engine's vacuum effect becomes stronger at the Emulsion Tube port than at the much smaller Pilot Jet ports. 

(Here's where the problem starts) Without the Rubber Plug installed it appears that as this stronger suction begins to develop it causes the direction of flow in the connection tube between the Emulsion Tube area and the Pilot Jet to reverse the direction of flow.

With the Rubber Plug installed this reversed flow can't happen.  Without the Rubber Plug this reversed flow can happen and it sucks fuel up the Pilot Jet tube and through the connection tube and into the Emulsion Tube.

This has the same effect that installing a Main Jet that's too big.

[Squishy]

Yeah, I get what you're saying.. but why would it suck fuel through the connection port and pilot jet tube when it can just suck more easily from the main jet?

[greg737]

but why would it suck fuel through the connection port and pilot jet tube when it can just suck more easily from the main jet?

The short answer is: Because it can. (it's the whole "path of least resistance" thing)

Think about this hypothetical situation: If you were to keep installing a bigger and bigger Main Jet (larger orifice) what would happen?  Well, for any given level of vacuum the bigger Main Jet would flow more fuel.  This would be true up to the point where the Main Jet orifice is so large it "defeats" the given amount of vacuum.

So, what I'm saying is this: Without the Rubber Plugs in place when the engine's vacuum is transferred away from the Pilot Jet ports by opening up the throttle plates there's a point where the force of the engine's vacuum (suction force) at the Emulsion Tube opening (a very large opening) will be greater than the suction at the Pilot Jet ports (which are very small) and this is the point at which the flow through the connection tube will reverse.   And this will cause fuel to come up the Pilot Jet tube, through the connector tube and into the Emulsion Tube. 

And I would bet that the situation is not only dependent on how far you open the throttle plates, but also on total engine vacuum being generated at the moment.  So at times when the throttle plates are open but the vacuum is relatively low the connector tube flow wouldn't reverse.  But when the throttle plates are open and the engine vacuum is high the connector tube from will reverse.