Author Topic: Is it normal to use main jet without needle jet?  (Read 311 times)

Offline user11235813

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Is it normal to use main jet without needle jet?
« on: January 13, 2018, 08:17:30 AM »
I found it difficult to title this post that's the best I can do!

I'm in the process of understanding the white plastic valves on the top of the carbs with 3 jets. This is not really a PAIR system as far as I understand it, it doesn't have anything to do with exhaust gasses. Also the Australian bikes while they have all the plumbing they do not have the extra bit of stuff attached to the crossbar under the tank like the CA models do.

From what I am understanding so far there is vacuum from the right carbs which goes to the petcock and is alway open. Then there is a T section that comes off that which goes into and out of a solenoid, and then goes to the white plastic valves. So I guess the solenoid either allows or doesn't allow vacuum along that tube.

Between the carbs is a pipe and a T section with a foam filter that allows atmospheric pressure into the carbs whether that atmospheric pressure goes into the carbs depends on the white plastic valves. If they receive vacuum from the solenoid then the valves open and the jet needle slide gets pushed down.

Forgetting about when that solenoid actually triggers the effect is to shut the needle jet. I am unable to see how this has anything to do with emissions control. If you're going fast enough and therefore drawing enough vacuum to operate the needle jet and the main jet, what would be the point of shutting down the needle jet, because the main jet will still be drawing petrol. Is this how it is meant to work?

Also I was thinking that if all the maze of vacuum lines are left in place, and one merely blocked off the T section between the carbs that allows atmospheric pressure in, wouldn't that be enough to completely disable this system, because even if the white valves detect vacuum and open, they will not be able to allow any air in anyway so the slide will still be sucked in it's open position. Is this a reasonable way to test the effect of this system before removing all the plumbing or am I missing something.


Offline TGTwin

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Re: Is it normal to use main jet without needle jet?
« Reply #1 on: January 15, 2018, 08:17:29 AM »
I was trying to understand the same thing (Aussie GS500, spaghetti mess of vacuum lines at the carb) and in the stickied "Carburetors and their various parts" post a legend had a link to this video:
https://youtu.be/FyA-L5XIYpo
which goes through exactly what you are after.
From what I understand, the vacuum relay thingy is activated by the "computer" which allows vacuum (or atmo, I can't recall which) to affect how much the needles are able to travel. I'm sure it's some emissions thing.
I took it all off and just capped off the white plastic things on the carb top. Didn't really run better afterwards, but also didn't run worse and definitely makes it easier to get the carbs off or work around them.

Offline Kookas

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Re: Is it normal to use main jet without needle jet?
« Reply #2 on: January 15, 2018, 07:57:58 PM »
I was trying to understand the same thing (Aussie GS500, spaghetti mess of vacuum lines at the carb) and in the stickied "Carburetors and their various parts" post a legend had a link to this video:
https://youtu.be/FyA-L5XIYpo
which goes through exactly what you are after.
From what I understand, the vacuum relay thingy is activated by the "computer" which allows vacuum (or atmo, I can't recall which) to affect how much the needles are able to travel. I'm sure it's some emissions thing.
I took it all off and just capped off the white plastic things on the carb top. Didn't really run better afterwards, but also didn't run worse and definitely makes it easier to get the carbs off or work around them.

Absolutely. There's far more 'emissions-control' related hoses and wires around the carbs than actual necessary stuff and it just makes it such a nuisance.

Offline penguin7316

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Re: Is it normal to use main jet without needle jet?
« Reply #3 on: January 20, 2018, 09:52:04 PM »
Did any body actually find out what these white vacuum operated carburettor things actually do. The video is good but does not say what the vacuum things do. They must do something other wise why have them on the carbs at all. Some one said that the vacuum solenoid that they are linked to is operated by the computer so perhaps if the motorcycle over revs the computer operates the solenoid to let air into the carb to let the main needle down so the bike can not rev any higher.
« Last Edit: January 20, 2018, 10:26:13 PM by penguin7316 »

Offline TGTwin

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Re: Is it normal to use main jet without needle jet?
« Reply #4 on: January 21, 2018, 06:11:23 AM »
I doubt it would be a rev-limiting feature, as it would be far easier to just cut spark at a high rev condition.
I'd say these lines make the bike run rich/lean under certain conditions, maybe deceleration. I'm still putting my money on an emissions control system rather than a safety/performance addition.

Offline Endopotential

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Re: Is it normal to use main jet without needle jet?
« Reply #5 on: January 21, 2018, 07:30:19 PM »
They are indeed for emission control.  Great write-up here
http://gstwins.com/gsboard/index.php?topic=57529.0

What would we do without the GS Encyclopedia?  Cheers to all who contribute
http://gstwins.com/gsboard/index.php?topic=70953.0

2007 GS500F Cafe Fighter - cut off the tail, K&N lunchbox, short exhaust, 20/60/147.5 jets, R6 shock, all sorts of other random bits...

Offline Endopotential

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Re: Is it normal to use main jet without needle jet?
« Reply #6 on: January 23, 2018, 05:49:15 PM »
Here's another good bit about the PAIR system.  Thanks for digging it up Josh.

http://gstwins.com/gsboard/index.php?topic=40142.0
http://gstwins.com/gsboard/index.php?topic=70953.0

2007 GS500F Cafe Fighter - cut off the tail, K&N lunchbox, short exhaust, 20/60/147.5 jets, R6 shock, all sorts of other random bits...

Offline user11235813

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Re: Is it normal to use main jet without needle jet?
« Reply #7 on: January 24, 2018, 03:28:24 AM »
I notice this from one of the links...

When the engine rotating speed is low, the solenoid valve 55 is closed to conduct intake pipe negative pressure to the lift control valve 41, and the lift control valve 41 conducts the atmospheric air to the atmospheric air introducing passage 40.


This looks like it's saying that the vacuum release that drops the slides only happens at low engine speed?

At any rate I blocked up the tube that allows atmospheric pressure in and it made no difference. I guess that the same effect could be achieved by simply unplugging the solenoid.

A vacuum leak that was stopped by putting a new plug on the carburettor vacuum port has started up again but not so bad. So I suppose that a complete removal of all this plumbing is in order. I note that this model in Australia does not that the charcoal box nor does it have the PAIR device attached to the left side of the crossbar under the tank.


Offline Kookas

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Re: Is it normal to use main jet without needle jet?
« Reply #8 on: January 24, 2018, 06:42:18 PM »
I'm glad all the emissions rubbish is inconsequential, because I'm pretty sure my tank is crushing the 'atmospheric air introducing passage' right now. All the extra hoses and especially the L-shaped thing with the second air filter on it make the carbs thirty times harder to remove and reinstall.