1990 ST 1100 Carb Sync

[OBONE1]

Hello fellow ST owners. I had work done on my st 1100 last fall, the O rings under the carb were leaking.
The bike now seems to be making a different noise , not engine I suspect the carbs need to be synched. The idle varies occasionally, when throttle applied the noise goes away.
I'm leaning towards the carberators have to be synched. I understand that #4 carb is the base carb, however I'm at a loss where the vacuum lines are to be connected to the gauges?.
Appreciate all the feedback

 

[newtonhubcap]

Just went through this after installing all the coolant hose on my 1994 ST1100 which at the same time I went through the carbs (cleaned and rebuilt carbs) were in dire need of it. Once all the hoses were re-installed and coolant filled (went with ICE Coolant), carbs back in. Went through and synched the carbs. You are correct the #4 cylinder is the base carb. the vacuum ports are located between the valve cover and each carb insulator. On the left you will see the two vacuum tubes teed together. Separate them at the tee be sure to block off the tee pipe ends. On the right side, same thing but you will see a single vacuum tube on the #1 cylinder with a cap and the number #3 cylinder the vacuum tube attaches to the fuel vacuum valve. Once have those pulled out you are good to go. For synching purchase I used the DSynch2 software. Super simple and efficient in time.

If you can I would replace the vacuum tubes personally. Buy it from your Honda dealer. You will need 3.5mm diameter.

Best of luck

 

[dwalby]

here's a clip from the service manual showing the vacuum hose locations.

Your description of the symptoms is a bit vague, but varying idle speed is more likely a vacuum leak issue and not carb sync related. And, if you have a vacuum leak, then there's probably more of a chance that you'll mis-adjust them while trying to do a sync with an active leak.

 

[OBONE1]

Thanks for all the feedback I'll start with a sync first? And if they don't come to tolerance then I will begin replacing vacuum lines

 

[dwalby]

when the work under the carbs was performed did they put new carb boots on as part of the job?

If not, then that's probably where your problem lies, since old brittle boots won't seal very well and the vacuum leak would cause your reported symptom. Or, maybe they put on new boots but didn't tighten all the clamps properly. I'd do the carb sync LAST, not first, since that's really not going to have anything to do with your symptom.

Its also possible they disturbed one of the vacuum lines in the process.

 

[Oldbikefixr]

Hope this isn't too late to help. Photos are from my refurbishment two years back.

Last time I was into our 1991 I replaced all the water, fuel, air and vacuum tubes and hoses 'cause I didn't want to have to remove the carbs again any time soon to fix leaks on 31 year old hoses. I put a 't' into the vac hose going to the fuel shutoff valve and ran the extra hose up. Made all four extra length, enough that I could clamp them to the frame member on each side of the carbs. The two photos show the right side carbs...the fuel valve is unbolted so I could photo the 't'. Second photo shows the finished location of the lengthened hoses....no more fishing for them next time the carbs need synching.
Just a note, two of the vac hose fittings on the intake manifolds were loose, one extremely loose.


This shows the location of the vac hoses before I replaced them and the carb boots.

See the whole thing I documented as 'ST1100 Refurbishment'. Use the Search function top right of the page. Many of the rubber parts were cracked, hardened, leaking.

 

[OBONE1]

Thanks again for all the great feed back. I've gone ahead and purchased the sync gauges and 3.5mm. vacuum hose. Ill check the carb boots, when the mechanic changed the O rings underneath the carbs he mentioned the boots were in good condition.
The layout drawing is very helpful, is there any reference to the adjustment screws locations?
I'm going to dive in on the weekend for this fix.

 

[Roger]

You will see two screw heads side-by-side in the dark and murky depths between the carbs on the right side at the bottom of this pic. Only it's too dark! - . On the left side there is only one screw. Do the left side first, then fiddle with the right pair until everything matches. .....It's very sensitive, so be careful. Hopefully your tickover and noises wll be fine afterwards.....
Thanks Oldbikefixr for the image.....

 

[dwalby]

when the mechanic changed the O rings underneath the carbs he mentioned the boots were in good condition.

that's a red flag, unless you know they were changed sometime prior to the O-ring work (I suspect they weren't)

They may have looked good, but are not as flexible as new ones, won't seal properly, and would explain the symptom you're having.

 

[OBONE1]

Tks I'll take your advice. I'm not that good with carbs will have to find out the difficulty level of replacing them. Maybe take the bike to a shop.

 

[ChriSTian_64]

when the mechanic changed the O rings underneath the carbs he mentioned the boots were in good condition.

Sometimes, a mechanic would say he didn't changed one part, because that part seems to still be OK.

I have notice that they will sometime declare a part to be OK, when they are unable to get a replacement part.

 

[newtonhubcap]

Hope this isn't too late to help. Photos are from my refurbishment two years back.

Last time I was into our 1991 I replaced all the water, fuel, air and vacuum tubes and hoses 'cause I didn't want to have to remove the carbs again any time soon to fix leaks on 31 year old hoses. I put a 't' into the vac hose going to the fuel shutoff valve and ran the extra hose up. Made all four extra length, enough that I could clamp them to the frame member on each side of the carbs. The two photos show the right side carbs...the fuel valve is unbolted so I could photo the 't'. Second photo shows the finished location of the lengthened hoses....no more fishing for them next time the carbs need synching.
Just a note, two of the vac hose fittings on the intake manifolds were loose, one extremely loose.


This shows the location of the vac hoses before I replaced them and the carb boots.

See the whole thing I documented as 'ST1100 Refurbishment'. Use the Search function top right of the page. Many of the rubber parts were cracked, hardened, leaking.

I noticed the silicone coolant hoses. Have to ask if those are pre-formed but more importantly who did you purchase them through and what size? Thank you!

 

[Oldbikefixr]

I noticed the silicone coolant hoses. Have to ask if those are pre-formed but more importantly who did you purchase them through and what size? Thank you!

I ordered a set of hoses thru Ebay right after I bought the bike nine years ago as 'just in case' spares. They were supposed to fit. When I finally got to replacing old hoses, NONE of the preformed hoses fit. So, I took the old hoses to a local auto part shop and they allowed me to match the factory bends with their stock hoses. Some are still available from on line part sellers. I'm thinking that most of the large diameter hoses were from Toyota and Honda cars, of course there was a little trimming of the ends required. I picked up the bulk hoses from another local automotive shop as the first didn't have what I wanted...they were all very thick walled and equally stiff.

I ended up with three ID silicone hoses, 3.5, 4.5 and 7.5 mm ID for water and 5.5 mm ID black hose for fuel line and cut them to shape. This is according to my notes.

You should probably look at an online manual for the ST to verify those are correct but they fit well, not too loose/not too snug. Also, don't toss your old hoses until you have matched the lengths with bulk hose.

Three years, now according to my log, and no drips, leaks, splits, or so forth.