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Discussion Starter #1
MTS 1200. Can someone enlighten me about the airbleed screws? One per throttle body? Where exactly? I assume these are to provide ‘idle air’ when the butterflies are closed? Can’t find mention in the workshop manual (but I never can find anything there!). How to adjust? I’m guessing you need to O2 probe to do it properly. Thanks
 

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You will see them on the throttle bodies - the front cylinder is easiest to identify on the bottom side, a biggish dark brass flat slotted + head screw in a shrouded hole x 1

Used for air bleed when the butterfly is closed, adjustment will stop the idle stall or backfire problems. From memory the front TB one is all the way closed as standard, this may cause the slowing down to a stop and idle stall problem.

Both my front and rear are at 1/2 out now, no more random stall coming to a stop..

This wont adjust idle speed, the ECU does that
 

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Discussion Starter #3
Thanks Les. It must have been a silly question as no one else seems to want to offer an opinion. I did read somewhere that was a significant difference between the front and the back...and somewhhere else that you needed to adjust using a vacuum gauge...anyone know what the procedure is?
Interesting you say that the idle speed is only 'adjusted' by the ECU. As the ECU is not 'hackable'...how does one actually adjust the idle? Thanks. Nick
 

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Regarding vacuum gauges: I think you might be getting confused with "balancing the throttle bodies".

The air-bleed screws don't do that job and merely allow a little more or less air to flow when the throttles are closed (or nearly closed).

Of course they have a marginal effect when the throttles are open, but the proportions are probably insignificant.

Many service people close them both up to keep things as rich as possible down low, although I am starting to wonder if a smoother low RPM could be achieved with "correct" mixture rather than "as rich as possible".

My 2c
 

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There is an interesting difference between systems with IACs (Idle Air Control) and those without.
The IAC system is a single air bypassscrew operated by a stepper motor. This 'T's into what amounts to a balance tube between the two throttle bodies.
In this type of system the amount of air at idle is always the same whether the bypass screws are open or shut. On the MTS1200 the idle air/fuel ratio is adjusted by the O2 sensors.
The exception is when the IAC is either open to max or closed to min, at which point the ECU can't control engine speed and it will display a check engine light. This can occur when the throttle stop is either open too much or closed too much for the IAC to control the idle speed. I kind of doubt that you could close the throttles enough that the IAC (which is a fairly large port) couldn't suupply enough air to idle, but I haven't tried that out.

In a system without an IAC, the air-bypass screws adjust the amount of air at idle. Without an O2 sensor, this also has a large effect on AFR for each cylinder, but it also affects the idle speed.

Ultimately, the correct way to set one up is to close the air bypass screws and disconnect the IAC hoses and synch the throttles by vacuum (I don't recommend disconnecting the IAC hoses because they can tear and that causes more problems.) The second problem is that you have to hold the throttle open to get it to idle because you have shut off the air needed to idle.
The second problem is that you have made the idle mixture rich by cutting off the air and now you have hold the throttle off idle to synch the throttle blades.
With the MTS1200 the sensors will adjust the AFR to 14.6:1 so that's not an issue.
After synchronizing the throttle blades, both of the air bypass screws should be opened to the same number of turns. And, of course the IAC should be reconnected.
At this point, in a system without O2 sensors in closed loop, the air bypass screws have a large effect on the AFR of each cylinder. At high vacuum, a small difference in vacuum makes a large difference in air-flow. A large difference of air-flow makes a large difference in AFR.
Therefore, the bypass screws are the devices used to equalize the cyl-to-cyl AFR at idle.

With the Mitsubishi closed-loop systems that difference can be seen by looking at the live Adaptive Fuel Value. This shows how much the system has to skew to get to 14.6:1.
A small tweak of the air bypass screws will get the AFVs closer to one another. These AFVs affect the closed-loop operation in the rest of the closed-loop operating range.

Doug
 

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Discussion Starter #6
That's all good to know. With my O2 sensors now out of the loop, (CJS ECU reflash and PCV) it makes it a little different. Play and learn time again! Thanks for taking the time to write that. Nick
 

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Hi Nick, did you manage to figure this out yet?
I am still not sure if the TB can and should be adjusted on the MTS1200.
Any suggestions, anybody?

Thanks
 

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Discussion Starter #8
Lets see what some of the others say...then I will tell you where I finished up! (nowhere very special!)
 

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That's all good to know. With my O2 sensors now out of the loop, (CJS ECU reflash and PCV) it makes it a little different. Play and learn time again! Thanks for taking the time to write that. Nick
Hi Nick
If you had the reflash from CJS Chris then the screws should be turned in all the way! He expressly said this to me, and warned me that dealers will adjust these during services.

Indeed this is exactly what happened! Went for the 7k mile service and when I got the bike back it was poping and backfiring like mad! Turned the screws in and it was normal again!

My understanding of why this is (laymans terms) is because the system is closed loop ie no O2 lambda sensors, so the AFR is set on the dyno at idle levels too!

Wish you all the best

Fritz :)
 

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Discussion Starter #10
Hi all.
Well reading Doug's comments (Moperfserv..earlier in this thread) one can see that the issue is quite complex and changes depending on if you have gone open loop. Having gone open loop you are contolling the AFR at 1000 to 1500 rpm from the (in my case) CJS's table. I also have the PCV 'on top' so have direct access to adjust myself. That all said...I have (and do) play with the screws to get a 'smooth' tick-over...and I did have the AFR checked at tick-over to make sure it was not way out. So in closed loop...who only knows...as the complexity of that is going on is beyond most of us! In open loop the AFR is controlled by the fuel tables BUT you will change it by playing with the screws so you may make the tick-over a bit smoother / faster / slower. I'm old fashioned enough to believe if it's running smooth...and not 'smelling' or 'hunting'...and not popping away....then the AFR is probably not far out. But I also know what I don't know...so a quick check of the AFR's is a good check. (I have to add for those who take a 'pop' at me sometime....this is my experience...on my bike. I am BY NO MEANS an expert in this field).
 

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Hi Nick
If you had the reflash from CJS Chris then the screws should be turned in all the way! He expressly said this to me, and warned me that dealers will adjust these during services.

Indeed this is exactly what happened! Went for the 7k mile service and when I got the bike back it was poping and backfiring like mad! Turned the screws in and it was normal again!

My understanding of why this is (laymans terms) is because the system is closed loop ie no O2 lambda sensors, so the AFR is set on the dyno at idle levels too!

Wish you all the best

Fritz :)
Does anyone know if I need to screw in both of these when using Tuneboy?
 

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Well, shit... definately didn't get the memo on this when i had my reflash done.

I started my bike this morning and once it had warmed up I screwed them in all the way (front one was already closed, back one was out about 1 turn). Idle immediately smoothed out, cured the hunting and surging I had at 2k rpm and below, and got rid of the decel popping I had at 2k rpm and below.

This is great info! Thanks Moperfserv and Multi Rider for your replies, and thanks Nick for starting the thread!
 

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Discussion Starter #14
As I sais in earlier posts here....play with them and see is the best answer!!
 

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Hi! I'm bringing this thread up to ask what was the final position of your bypass screw... Especially for those who have Tuneboy or flashed ECU but anyone who played with them can post here the result :smile2:

Thank you!
 

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Thank you. No stalling issues? Opening the throttle from a steady speed (low-mid RPM, around 4k) cause no problems?
 

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My '13 is bone stock, and both air bleeds are closed. I assume this is proper.
 
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