What does the Pilot Jet Plug do?

[Pharkmeh]

for a bloke who knows bugger all about carbs this is gr8 thread with even gr8er replies....


special kudos to Keith for the most comprehensive and well worded techy reply Ive seen.... =D>


ffs - by the end of it I thought I almost understood something about carbs.... but nah - Im still mikanikly tarded...](*,)


keep up the good work8)

[DimitriT]

I'm not sure what you mean here. Fuel can't go "around" the main jet and across the connecting port. Do you mean by-passing the main jet and entering through the unplugged tower? If so, the pilot doesn't care if it gets it fuel from the submerged main jet or the submerged unplugged tower. Both main jet and tower "holes" are far larger than the pilot jet hole, so the pilot jet will still be the regulator regardless of where the fuel comes from. Either through a missing plug or the main jet, the fuel still comes from the bowl. Removing a plug won't flood the pilot circuit.

Thanks for a great explanation on how the idle and pilot work at throttle. I'm really fascinated by carbs and the CV carbs are amazing designs IMHO.

When I said that fuel may bypass the main jet I meant that if you look at the carb diagram, the little channel which connects the idle "tower" with the main "tower" joins the two above the main jet but below the idle jet. I'm thinking once the needle is raised and the main jet is now the restricting orifice, is it possible for fuel to go up the unplugged hole in the idle tower and come around across the channel (in the reverse direction that it would go when the hole is plugged) and go up the main tower. I'm guessing this would only happen at WOT.

[denydog]

DIMITRIT, I think you just may have explained the reason for the plug. Sounds like a reasonable theory.

[focus frenzy]

Thanks for a great explanation on how the idle and pilot work at throttle. I'm really fascinated by carbs and the CV carbs are amazing designs IMHO.

When I said that fuel may bypass the main jet I meant that if you look at the carb diagram, the little channel which connects the idle "tower" with the main "tower" joins the two above the main jet but below the idle jet. I'm thinking once the needle is raised and the main jet is now the restricting orifice, is it possible for fuel to go up the unplugged hole in the idle tower and come around across the channel (in the reverse direction that it would go when the hole is plugged) and go up the main tower. I'm guessing this would only happen at WOT.

actually it will suck gas bypassing the main jet at anything above a slight cracking of the throttle, not just WOT.
without that plug in place all the bike would do is idle, any attempt to open the throttle would be rewarded with a severe flooding problem.

if it was a anti-tamper plug wouldn't it be something that didn't just fall out when the bowl was removed??
say like a cast/ pressed in aluminum plug like that found over the pilot screw?

[DimitriT]

actually it will suck gas bypassing the main jet at anything above a slight cracking of the throttle, not just WOT.
without that plug in place all the bike would do is idle, any attempt to open the throttle would be rewarded with a severe flooding problem.

if it was a anti-tamper plug wouldn't it be something that didn't just fall out when the bowl was removed??
say like a cast/ pressed in aluminum plug like that found over the pilot screw?

It's not for anti tamper. You need access to the idle jet so you can remove it/clean it. Someone said that there's a riser on the bowl which presses up against the plug to hold it in place. I've never verified this but I do have the rack off the 550 right now so I'll check before they go back on.

[8trackmind]

Someone said that there's a riser on the bowl which presses up against the plug to hold it in place.

Yes, there is one.

[KEITH KRAUSE]

actually it will suck gas bypassing the main jet at anything above a slight cracking of the throttle, not just WOT.

Not so. The CV carb is designed so significant vacuum doesn't act on the main circuit until approx' 1/5 throttle (in my opinion) though Mikuni says it's closer to 1/4 throttle position. Until that throttle position is reached, there's not enough vacuum present to draw up the fuel through the main jet hole and around the jet needle/needle jet spacing. This same vacuum would be even less effective trying to draw fuel that originates from the larger tower hole. The bike would still be running through the pilot circuit.

without that plug in place all the bike would do is idle, any attempt to open the throttle would be rewarded with a severe flooding problem.

Also not so. Even if fuel DOES get past the main jet, there wouldn't be any flooding when attempting to open the throttle because the smallest passage in the main circuit would be the space between the jet needle and needle jet. The jet needle would simply do its job until 3/4 throttle. In this case, the smallest passage will regulate flow, not an unplugged tower.

[KEITH KRAUSE]

When I said that fuel may bypass the main jet I meant that if you look at the carb diagram, the little channel which connects the idle "tower" with the main "tower" joins the two above the main jet but below the idle jet. I'm thinking once the needle is raised and the main jet is now the restricting orifice, is it possible for fuel to go up the unplugged hole in the idle tower and come around across the channel (in the reverse direction that it would go when the hole is plugged) and go up the main tower. I'm guessing this would only happen at WOT.

I already replied to this in my previous reply (see reply to focus) because he went a step further about what throttle positions could be effected.
I do see the possibility at WOT though.
Without talking to Mr. Mikuni or being a little molecule of fuel, I can't say 100% either way if the available vacuum at WOT can draw the fuel through the main circuit if that fuel originates from the tower hole. Can the vacuum overcome two approx' 130 degree turns without weakening it? Would there be enough vacuum to draw fuel from the larger tower hole? You KNOW the vacuum would be less trying to act on a larger hole, let alone a larger hole AND the main jet combined. How much less? I would think the flow would be compromised and you may not get enough fuel. But could it ADD to the main jet flow and result in a too rich mixture at WOT(?), I can't say it's not possible. The amount of vacuum though is limited in its ability to draw fuel and even the added resistance of say, 2mm lower bowl fuel level, will cause a lean condition, not flooding.
Another thing I find interesting is the past comments from owners of CV carb equipped bikes.
In a recent thread about the plugs, at least two owners said their bike ran worse, at idle only, if the plugs were out. Another two said there was no difference at idle or any other throttle position. I don't remember all the owners but I believe one that said there was no difference was Jethro, who's played around with his carbs/jetting quite a bit and sounds reliable. If the plugs do/do not effect performance, why the range of comments/results? How could the bikes run differently if the plugs are vital? Maybe other problems were involved to cloud up the situation and the answer, I don't know.
Another important thing to bring up, for me anyway, is why NOT install the plugs regardless of their true reason for being there? If we were talking about expensive parts that have a questionable existence or it took major work to reach them, I could see the gain in not installing them. But they're cheap to buy and simple to install. If I rebuilt some CV's, I would order new plugs along with all the other parts needed for a rebuild.

PS: I was delayed in sending this post, so if I'm "out of synch" with any others who posted during that time...sorry.