Odd vacuum gauge behavior while syncing carbs (positive pressure pulses)

[bjones2]

While attempting to sync my carbs on my 83 GS550L, this is the one with the two 2-barrel carbs, I have noticed that when a vacuum gauge is attached to the right-hand carb and idle set to about 1800, I will see vacuum pulses in the neighborhood of 17in with the gauge bouncing between zero and 17in with each pulse. Seems fine.
However, when I attach the gauge to the left-hand carb, I will see much different results. The vacuum pulses are much lower (around 10-12) AND instead of bouncing between the vacuum pulse and zero, it is bouncing between the vacuum pulse and a positive pulse of about 5psi.

I have also noted that the idle air screws on the left-hand carb hardly have any affect on the idle quality while the screws on the right carb, the one with the seemingly normal vacuum readings, have significant effect.

At first I wanted to blame a vacuum leak in the left-hand carb but how would that account for the positive pressure pulses I am seeing on the vacuum gauge?
At no time have I ever observed any backfire of any sort thru the carbs.

I am concerned that these positive pressure pulses are an indication of some unknown fault in the bike. Can anyone make any suggestions? Telling me I'm crazy would be a valid suggestion at this point.

Thanks,
Bruce

[Grimly]

I've never dealt with the twin barrel carbs on the 550L, but somebody who has will be along shortly.
My first thought is the restriction screws on each gauge line - do you have any, and are they adjusted right? Find mention of use of guages on that engine and see what any set-up procedures have to say about gauge restrictors and dampers. What you're seeing reminds me a lot of the unruly behaviour of undamped/ undrestricted gauges on any twin or four when the gauge valves are open - it shows up as an unreadable mess until damped out properly.

[bjones2]

I've never dealt with the twin barrel carbs on the 550L, but somebody who has will be along shortly.
My first thought is the restriction screws on each gauge line - do you have any, and are they adjusted right? Find mention of use of guages on that engine and see what any set-up procedures have to say about gauge restrictors and dampers. What you're seeing reminds me a lot of the unruly behaviour of undamped/ undrestricted gauges on any twin or four when the gauge valves are open - it shows up as an unreadable mess until damped out properly.

The gauge that I am using is a pretty basic automotive unit and is un-damped. You are quite correct that the resulting readings are a bit of an unreadable mess as each vacuum pulse registers individually with the needle swinging rapidly between zero and the vacuum reading.

I fully admit that I may be reading WAY too much into this when considering that I am using the wrong tool for the job.

It just struck me as odd that the "good" right carb showed readings oscillating between good vacuum and zero while the "bad" left carb showed readings oscillating between low vacuum and +5psi. I even when so far as to induce a vacuum leak in the "good" right carb via the petcock line to see what the results were. There were sort of what I expected to see with a vacuum leak, readings oscillating between low vacuum and zero, no +psi pulses as on the left carb.

If the real answer is "stop using that wrong tool for the job, ignore those "odd" readings and find the vacuum leak" I can accept that, I just thought it was, well, odd.

Thanks.

[Gorminrider]

undamped is annoying but the gauge is telling you something.... That the idle jet has no effect points to an issue-maybe carb rebuild, or check the valves (leaky?)
These automotive vacuum gauges usually come with instructions as to meanings of readings so look that up...

[Guest]

I will have to ask a couple of questions to make sure we are looking at things the same way.

1. I have not worked on a 550 with the siamesed carbs. Is there just one vacuum port per carb for checking carb sync or do you still have one in each intake runner?
I don't know why there would be individual ports, unless there is some mechanism in the carbs to adjust each throat, but I don't think there is. I have worked on another bike with the siamesed carbs, it had a solid throttle shaft with no adjustment, but also had individual vacuum ports for each intake runner.

2. This gauge you are using, ... is it a single gauge or one that has multiple dials or tubes to show vacuum levels for all cylinders? If it is a single gauge, definitely get a proper tool for the job. There is no way that you can move a single gauge from one carb to another and get meaningful readings while trying to sync carb. You have to be able to see all of them all at once to see how they interact with each other.

3. The positive pulses are puzzling. Could just be from an undamped needle, but if you are moving one gauge around, the difference would be the cylinders/carb, rather than the gauge. If you do have at least two gauges in use, swap them from side to side to see if the positive pulses follow the gauge. Might be a faulty gauge.
Then, there is another possiblity. The only way you are going to go positive there is if there is a leak in the intake valve. If an intake valve is not closing completely, it could pressurise the intake tract and show as a positive pulse on your gauge. Have you checked your valve clearances? That should ALWAYS be done before trying to sync the carbs.

4. No response to "MIXTURE" or "PILOT" (not 'air screw') adjustments tells me that you have blocked passages in the pilot circuit of the carb. Might be some crud in there, might be a broken tip of the adjustment screw, only a careful inspection will answer that for you. It is important to know that those screws control a MIXTURE, not just air. Turning them OUT will richen the overall mix, turning them IN will lean it out.

[bjones2]

I am beginning to believe that the "odd" positive readings are in fact a byproduct of using the wrong (undamped) gauge for the job as I was able to induce those same readings in the "good" carb.
I found that my two carbs (one vacuum port per carb) were wildly out of sync and that changes in the sync settings could induce these "positive pulses" in one carb or the other depending on adjustment.

I decided to ditch the crappy gauge and got a proper gauge, and was able to get everything in sync.
Lesson learned, get the right tools for the job.

Unfortunately, I still have issues with one of the carbs with one pilot screw having limited response and zero on the other screw in that same carb. The other carb responds as expected to pilot screw changes.
I have had the carbs out several times and "thought" I had cleaned the pilot circuit on the problematic carb several times though perhaps results indicate that I was not as successful as I had hoped.

I would imagine my cleaning methodology was flawed.
- Removed the pilot jets (2) in the offending carb.
- Cleaned the jets with a small wire and ensured the jet orifice was clear.
- Removed the pilot screws (2) from the offending carb.
- Injected carb cleaner into the pilot screw openings, observed it flow out of the pilot jet hole and the hole in the carb body (the one the needle tip extends into).
- Blew compressed air into the pilot screw opening, observed carb cleaner blow out the jet mount hole.
- Repeated the carb cleaner compressed air blowing into the pilot jet mount hole.
- Cleaned and reinstalled the pilot screws.
- Reinstalled the pilot jets.
- Observed the same non response to the screws.

I probably only did a hack job of "cleaning" the pilot circuit with the above. Live and learn I.

Thanks