Mysterious Vacuum Hose— Advice Welcomed

[npgibson85]

Hello all,

This is my first post on this forum. I’ve found the information here very useful, so big thanks to this community!

Brief background—
About a year and a half ago I bought a 2007 ST1300 with 68k miles. It now has 73k miles. I’m at least the third owner. I’m currently doing some maintenance with the fairings off— coolant change, replacing a few coolant hoses, brake bleed, clutch fluid bleed.

I removed the air box top and bottom to gain access to the coolant hoses, so the bike is pretty well stripped down at this point.

In the process of removing the right side fairing, I noticed a loose vacuum hose with a screw in the end (please see photo). Later on, after removing the air box, I found that this hose connects to a T valve leading to the MAP sensor (please see photo— red arrow indicates the hose, red circle is where it exits the air compartment).

***My question is***: does anyone know where this hose is supposed to connect? The idle is very low for the first few minutes of operation. Sounds like it might die, but never does. After it warms up the idle seems fine. Otherwise it seems to run fine (though I don’t have any other ST experience to compare it to). Does seem to be a moderate level of vibration at higher RPMs.

Thanks so much for any thoughts you may have.
*** Also, if there are any suggestions for other maintenance jobs to perform with the plastics and air box removed, I’d be happy to hear those. I did already clean the 5 way T valve last year though.

Thanks,
Nick

 

[Buckeyechris]

Hi Nick,

The object your arrow points to is a check valve. My guess is that a prior owner (PO) had installed an aftermarket, vacuum assist cruise control. Instead of disconnecting the hose to the servo unit, the PO just cut the vacuum line and elected to seal it with a small screw.

If you can get under the MAP sensor, you'll likely find a hose from the 5-way Tee connected to a 3-way Tee. At one end of the Tee is your mystery vacuum hose. The other likely goes to the MAP sensor. I'd disconnect the 3-way from the 5-way and run a single hose from the 5-way to the MAP sensor.

If you're not familiar with the 5-Way Tee, check out this article on throttle body synchronization. There are some pics of the 5-way Tee.

Article [13] - ST1300 - Throttle Body Synchronization | ST1300 Articles

This is my first attempt at a How to article so bear with me. Starter Valve synch for ST1300. Special thanks to Blrfl for letting me borrow his Carbtune manometer. Remove seat. Move seat bracket to maintenance position. Loosen 10mm bolt holding rear of tank down. Remove two 8mm bolts...

www.st-owners.com

Welcome to the forum.

Chris

 

[npgibson85]

Hello Chris,

Thanks for your reply and link to the throttle body synch.

You are correct— there is a 3 way valve between the MAP sensor and the 5 way valve, and the mystery hose with the screw in it is in between, connected to the 3 way.
Please confirm I’m understanding correctly— remove the 3 way valve and connect the 5 way directly to the MAP sensor with a bit of hose? I imagine this would restore it to stock (pre-cruise control modification made prior to my ownership).

Hmm… I wonder if this will improve the low idle when cold (?)

Thanks,
Nick

 

[GitSum]

There is a small Phillips screw that fastens the MAP Sensor to a bracket on the throttle body. Remove the screw and you should be able to pull the sensor straight up to check for a T for that vacuum

 

[npgibson85]

There is a small Phillips screw that fastens the MAP Sensor to a bracket on the throttle body. Remove the screw and you should be able to pull the sensor straight up to check for a T for that vacuum

Excellent Gitsum, thanks for that photo. Super clear. And those are some snazzy looking blue vacuum lines… and where’s the dust?

 

[npgibson85]

Alright I went ahead and performed the suggested change with the vacuum lines. Will be curious to see if it idles/runs differently. Can’t start it until I get the radiator and water hoses back on though. Will report back.
Thanks again

 

[Buckeyechris]

Hello Chris,

Thanks for your reply and link to the throttle body synch.

You are correct— there is a 3 way valve between the MAP sensor and the 5 way valve, and the mystery hose with the screw in it is in between, connected to the 3 way.
Please confirm I’m understanding correctly— remove the 3 way valve and connect the 5 way directly to the MAP sensor with a bit of hose? I imagine this would restore it to stock (pre-cruise control modification made prior to my ownership).

Hmm… I wonder if this will improve the low idle when cold (?)

Thanks,
Nick

Nick,

I would remove the 3-way, and run a single hose to Map Sensor. It might improve your idle. If the screw didn't completely it would reduce vacuum to the MAP sensor. However, you may also need to sync your throttle bodies.

Chris

 

[Mr.E]

You have the other pipe circled in your photo... if you didn't know, that one goes to the underside of your Air Filter box (bottom tray).

 

[billo]

I had the hose to the MAP sensor clog. The five way was fine. Might be worth a look.

 

[SMSW]

Alright I went ahead and performed the suggested change with the vacuum lines. Will be curious to see if it idles/runs differently. Can’t start it until I get the radiator and water hoses back on though. Will report back.

I hope I'm not to late with this suggestion. A number of guys have suggested (in other posts) lengthening the hoses to the 5-way-T and positioning it above the throttle bodies. This gives you easier access to the T should it clog in the future.

 

[RobbieAG]

While you're in there, you should check the 5 way T and hoses for any blockages. You can run a pipe cleaner through the T to see if there's any dirt or carbon buildup.

 

[npgibson85]

I hope I'm not to late with this suggestion. A number of guys have suggested (in other posts) lengthening the hoses to the 5-way-T and positioning it above the throttle bodies. This gives you easier access to the T should it clog in the future.

An excellent suggestion. I believe I’ve read about that modification on this forum before. Getting to that 5 way is a real knuckle buster. Perhaps a job for the future.

 

[ST Gui]

There is a small Phillips screw

That's most likely a JIS screw. If it turns easily a Phillips driver will probably do no harm. If it's something that might be tight or stuck then there's a risk of stripping the screw. JIS drivers are available at Amazon and McMaster-Carr and a lot of other places if you don't already have one or two.

The screw in the hose looks like a Torx but it's hard to tell from this angle.

 

[GitSum]

Your right, it is a JIS screw (has the dot).

I only use a couple of Vessel screwdrivers when I work on my ST, and just take it for granted...



This is a nice upgrade

 

[npgibson85]

UPDATE:

I've performed the change to the vacuum hose between the 5-way and the map sensor, per the advice of Buckeyechris. I used OEM Honda vacuum hose. I also cleaned and lubed the (what appears to be the) idle air control valves, which were very dirty and stiff. Cold started it up-- perfect idle!! It used to idle very low until warm-- now it seems perfect.

With the plastics removed I also: removed and cleaned the radiator, replaced 4 coolant hoses/clamps, replaced the thermostat and O-ring, flushed and replaced the coolant (Honda type 2 silicate free), bled the brakes and clutch, adjusted the counter balancer shafts (a hot point of contention on this forum regarding efficacy, I understand), and lubed the adjustable windscreen tracks.

Planning to get the plastics reinstalled soon, minus the panels to access the radiator cap and counter balancer adjusters for a ride or two.

Thanks to everyone on this forum-- it has been an excellent source of information.

-Nick

 

[GitSum]

Sounds like we both were on a similar journey. I just finished up with my list, on my 'new to me' 2012 I bought in January