Carburator Synch Procedure

[NegoC8]

First Post. Have had my '91 ST1100 for a year. LOVE IT! Just had to replace a cylinder head coolant outlet nipple that cracked - replaced both sides and hoses while I was under the carbs (thank you Bike.Bandit!!). Now it runs like crap. Rechecked everything. Last night I pulled the 1&3 plug leads - NO CHANGE in idling. Separate 2&4 pulls killed it instantly. I'm running on two cylinders!!! Getting gas and spark on the plugs. Tearing out little remaining hair. All I can think now is that the fuel air is off on one 1&3. Will a synchronization fix this? How do you do it? my service manual and owners manual make references, but no proceudre. Love some help.

 

[Fred D]

Here's some info I've compiled over the years...until I used a flowmeter

Cheesy drawing of vacuum lines....

 

[BigTom]

I find it hard to belive fuel/air went that far south in such short order.

Did the motor have any heat to it at all? Does it pick up one or both of the cylinders after it warms a bit? Does the choke make any difference to 1 and/or 3? Those would be carb sync. questions...

I think maybe Fred is on the right track...look to see if you knocked any vaccuum lines off when you were under the carbs.

You are sure you have plug spark at the suspect cylinders? Maybe the HT leads off the coil? 1 & 3 share a coil.

Uhhh, I didn't read the PDF that Fred posted, just lept in here with my .02. Let us know, and come back with any further questions...

 

[NegoC8]

One explanation that has me on this path is the connecting link under the carb cluster between throttle valves was accidentally bent during the original problem solution. I straightened it out to the best of my ability in place, but now suspect it adversely affected the synchronization.

It does seem to pick up more power at higher RPMs. On a test drive I was able to get to ~85mph in relatively short order, although not as peppy as I remembered it prior to the coolant leak problem. Can't imagine that kind of performance on only 2 cylinders.

Checked, double checked, and triple checked all vaccuum lines under and around the carbs.

Definitely have spark.

Posted .pdf looks to be VERY helpful. There are some edits to it that call into question the "best" approach. I just bought a vacuum meter, vacuum hose, and "t"s while I was out at lunch. I will attempt the synch tuning tonight and/or tomorrow, then report back the results, and perhaps update the document to what I thought was the best of the best within it.

THANK YOU for your help and insights!!

 

[number9]

If you are going to try and use 1 vacuum gage connected to all 4 carbs at once with T's, you won't get a proper sync at all.

FWIW, the best gage to use on carb sync, IMO, is the SK Flowmeter. Since getting one and syncing the carbs with it I have retired my Motion Pro mercury gage. See pics here http://community.webshots.com/album/550655197ijvFrk

 

[NegoC8]

No, only checking one carb vacuum at a time. Gave it a shot last night, it was a bear!! Just when it seemed all was working well on a final tune of the #1, the RPM raced. I "twisted" it back down to the 2100 level I had been working at, then attempted to re-tune 1 and 3 to the level of #4 I had been working to all along at 2100 rpm. That made everything go out of whack again! Didn't help that it was midnight. I gave up and will try again this afternoon after work.

Enjoy your Memorial Day weekend. I'm determined to be riding before Sunday!!

 

[BigTom]

It sounds like you might be on to it...You know that you can buy a vaccuum guage at Checker/Shucks/Kragen for about $8.00? 2 makes the job 3x easier, and 4 is easy.

I have a 4 guage set up with dampner valves in line for each one. Takes me a lot longer to get to the carbs than to do it....

 

[NegoC8]

OK, this is embarassing, but more than that, FRUSTRATING! I've been in and around the carbs, synched, re-synched, checked timing, checked vacuum hoses /connections, pulled plug wires and plugs, checked coil connections, wiggled harnesses and it still isn't running right. I've used both suction guage and flow-meters for the synch procedure.

The symptoms at this point are a fairly decent idle, but when it gets in the ~3000rpm range of acceleration, it "bogs down." Really seems like a mixture thing but the pilot screws were never messed with before or after the original head coolant outlet replacement. Before that outlet cracked, the bike ran great! I miss riding it SO MUCH!!

Any other ideas would be gratefully welcomed.

 

[CarSalesman]

I don't think sync is your problem, but you should be checking it at as low an rpm as possible, definitely not at 2100.

 

[NegoC8]

At this point, I agree - I don't think the sync is the problem either. I'm now focused on the fuel system. I've also got a line on someone who works on these bikes, but so far his workload hasn't cleared enough to get me in, so I'm still on my own.

Recent discovery: Rather sizeable air bubble in my new fuel filter. Doesn't seem to go away. Really makes me wonder about the pump /pressure etc. There are no fuel leaks I can detect, so with a full tank, why would an air bubble persist??? Am I on to something?

 

[ST1300 Alicia]

I don't think the air bubble in the fuel filter is significant as we always have them in car fuel lines. Unless you bled the fuel filter and got all of the air out of it when you installed it and no air EVER came out of the fuel pump this would be normal. The filter is just a storage canister of sorts under pressure for a carbureted engine. I would look for a common denominator ignition connection, fuel connection or vacuum connection for affected cyl's.

 

[saaz]

I have mine apart to check a coolant leak, so can sympathise! There is the vacuum connection between the carbs for the CV part to work properly. Any chance a hose is connected in the wrong spot? 3000rpm is around the carb needles making a difference, so the operation of the slides may be the problem

 

[Reg_Siemens]

Have you tried bypassing your vacuum shutoff valve?

 

[NegoC8]

OK - it's fixed, and boy am I angry. To my recollection, there isn't a single reference in any of the material I've read on this subject that makes the following point:

When using the vacuum method:
MAKE SURE YOU HAVE BUTTONED UP THE ENTIRE AIR CLEANER ASSEMBLY EITHER DURING OR AFTER THE PROCEDURE, BEFORE EVALUATING THE MOTORCYCLE'S PERFORMANCE.

If using the flow meter method (I've used both by the way, and by the end of this you might guess where on the garage floor the air cleaner assembly was while all of the above thread was going on...):
MAKE SURE YOU HAVE BUTTONED UP THE ENTIRE AIR CLEANER ASSEMBLY AFTER DETERMINING THE AIR FLOW IN ALL 4 CHAMBERS IS THE SAME, AND BEFORE EVALUATING THE MOTORCYCLE'S PERFORMANCE.

Sorry to be so dramatic, but the only reason I put the air cleaner assembly back on was to take the bike to someone else to repair it. Imagine my disbelief when it ran the way I remembered...

If nothing else, I hope this thread is helpful to someone in the future - and particularly in the case of the post from "saaz" yesterday.

Finally - GRATITUDE. Everyone who contributed to this thread was genuinely concerned for the well being of a certain 1100cc friend of mine, so THANK YOU for extending your hand to a fellow ST'er in need!

 

[saaz]

The St11 does not run at all well on the road without the air cleaner assembly on. For synching etc, no problem. Reading back I should have realised that you were road testing. As I synck with the air filter (carbtune) on it has not been an issue. Did not realise you used the flow meter. We learn again not to assume the real basics are ok!

 

[edlee]

Bent Rod

Aha...it always helps to know the details. I was going to post the check to see if it worked reasonably well at higher throttle openings. You indicate it does, therefore I agree that it is most likely carb sync. At idle, if the sinc is off a good bit, it can completely shut off a carb, or two (or three for that matter depending on where the adjustment is off). But sync has only a small effect at open throttle conditions. A broken link or unconnected link would not get better at higher throttle openings. It is almost certainly just a sync problem. Have you got one of those fine SM flow meters? If so you can fix this in 2 minuites.
Ed Lee, Lancaster, SC :biker: