Question about the rubber "pilot jet" plug

[Road_Clam]

Calling on the Mikuni CV gurus. How eactly does the pilot circut draw fuel when there is a rubber plug covering the passage way?? I have never seen this before. Thanks for the input.

[DimitriT]

The plug is there to cover up the access hole of the pilot jet. The hole is not for drawing fuel. It's just for getting the pilot jet in and out. Fuel is drawn through a channel which connects the pilot jet channel to the main jet. You can see this channel if you look down the pilot jet channel.

[earlfor]

The pilot draws through a channel to the main jet. Keep the rubber plugs installed.

Earl

[Road_Clam]

OK, I see it now (with a flashlight, and sticking my eyeball right down in the hole). Now I can see how the jet draws fuel. Thanks,

[rphillips]

Another one of those little passages that sometimes may be missed when cleaning. Also a bike will idle fine & rev up, when in neutral, when these little plugs are left out, but put the bike in gear & try to ride away, & the bike will die, deader than 4 o'clock.

[DaveDanger]

rphillips, I'll hafta disagree with you a tiny bit about leaving those rubber plugs out. When I disassembled my carbs for cleaning back when I first obtained it, (3 or 4 months ago?).. Those rubber plugs had either dissolved into a goo from some previous cleaner of some sort, or they were left out at the previous rebuild and replaced with a dollop of silicone RTV. Either way, when I got done with my rebuild, I had no plugs to install, so decided I'd order them later and go ahead with my reassembly for the time being. I have ridden the bike quite a bit since then and haven't had any suggestion of a problem with power. It pulls quite well at anything above idle. I'm not claiming that this is a good way to go, simply that mine has demonstrated zero problems with the rubber plugs left out. I WILL have them installed before I begin any serious riding.

[KEITH KRAUSE]

I think the plugs only purpose is an attempt to stop tampering.

[Road_Clam]

I think the plugs only purpose is an attempt to stop tampering.

After thinking this over, the only logical conclusion I can see for the purpose pf the rubber plugs is to disallow the intrusion of bowl sedimant being drawn into the pilot jet. The alternate "port" probably has a "clean" source of fuel, as it's not being drawn directly from the bowl. If I remember right, the # stamped on Mikuni jets refers to the amount of fuel in CC's the jet can flow in one min. of time. Those pilot jets have a TINY passage, and it doesn't take much to clog the jet. The #1 carb issue I see time and time again is "poor ideling" and it ALWAYS stems from clogged pilot jets.

[KEITH KRAUSE]

I think the plugs only purpose is an attempt to stop tampering.

After thinking this over, the only logical conclusion I can see for the purpose pf the rubber plugs is to disallow the intrusion of bowl sedimant being drawn into the pilot jet.

Then all carbs would have them. The main would have to have a plug too, since it feeds the pilot jet (CV) and any dirt would still wind up clogging the pilot jet.

[tfb]

Hmmm... they can't be to stop tampering, because they're so eaaasssy to remove. In fact if you brush against them accidentally when you have the bowls off, they almost fall out.

From memory there was a bloke on the Katana Central forum last year, who had all sorts of hassles with his bike's carburettion. Then he discovered the plugs were missing, bought a set of newies, and all was well.



Mike.

[Road_Clam]

Bottom line is Mikuni would not waste time installing them in each carb unless there was a very specific reason. Even though I don't quite fully understand their purpose I have a new replacement set going back in. BTW the new plugs fit REAL tight too.
Rich

[rphillips]

I apologise for the bad imfo. A buddy of mine, had his 1980 GS 1000 E carbs cleaned at the local bike shop (hang out). When finished, the mechanic started & reved the bike a few times & all was well. My buddy got on it & started to pull out into traffic, for a test run, & the bike just bogged down & wouldn't pull. The mechanic, standing there with us said "aw shucks, I left those little plugs out. We went back in the shop, and there they were, laying on the work bench. He pulled the carbs back off, replaced the plugs, and the thing then ran like it should. That was the only time I'd seen carbs put together without them & that's what happened. From that, I thought any bike with these carbs would do the same thing if those little plugs were left out. Thanks for straightening me out.

[Jethro]

People, people, calm down. The purpose of the plugs is to force the pilot jet to draw from the lowest point in the bowl, so the tiny bit of pressure there isn't quite as affected by extreme lean angles.






I have no idea if that's true some old stoner told me that once! I've also ran without the plugs for some time with no ill effects, but I can't say how extreme I was leaning.

[KEITH KRAUSE]

Hmmm... they can't be to stop tampering, because they're so eaaasssy to remove. In fact if you brush against them accidentally when you have the bowls off, they almost fall out.
Mike.

How easy they are removed has nothing to do with it.
This is an emissions issue. If told to, Suzuki needs only to make an attempt.
Look at the mixture screws and fuel/air screws on various carburetors, also an emissions issue. All the factory does is wipe some sealant on the screw and that's all they have to do to show compliance.

[KEITH KRAUSE]

And as for performance changing if the rubber plugs are removed over the pilot jets, I'm not going to argue with people who say they've seen it with their own eyes, but on this CV carburetor, it simply makes no sense. Not with the bike straight up or on the sidestand.
The pilot jet draws fuel through the main jet, then goes up a passage to the pilot jet. The main jet is emersed deeper in the fuel. It will still be emersed regardless of bowl level, as long as the floats are set correctly. The pilot jet access hole is also emersed in fuel. The pilot jet will still regulate the same amount of fuel if through the main or with a "secondary" passage because of a missing plug. Vaccuum at the pilot jet doesn't change. Fuel flow through the pilot jet doesn't change. Performance can't change.
Again, not to argue, but if a mechanic left these out and the bike ran wrong, then he did something besides leave the plugs out. Many others here say the plugs didn't effect their performance. So who's right?
All I will add is the pilot jet will not contribute to such a serious performance problem when ridden as described earlier simply because the pilot circuits design only regulates fuel at closed throttle and nearly closed throttle. If a bike has problems pulling away, that's a jet needle issue or less common, a cut-away issue but not on these carbs.
So I won't go with who's right and their personal experience because they conflict. I'll go with the design of the carbs and knowledge of how the vacuum draws fuel through a jet. Look at an illustration of these carbs and you'll see what I mean.