1982 550 LTD #2 cylinder issue

Well, I'll start with removing the carbs and start with another bench synch to get all my slides at a good starting point. I will then make sure they don't go too high at WOT and ensure that the throttle stop screw is adjusted properly. (This might not be the full issue, but something is definitely different with this screw since I had them apart). Once I got that done I'm gonna check the advancer and verify spark plug gap and spark. Then I'm gonna synch the carbs. If there is still a problem...then I dunno.

BTW, having #2 cylinder with a lower compression shouldn't cause this, would it? As long as it's above it's serviceable value it should be ok, right?

apbling wrote:

...BTW, having #2 cylinder with a lower compression shouldn't cause this, would it? As long as it's above it's serviceable value it should be ok, right?

Right.

With these carbs, would it have been possible to install the throttle slides backwards?

Good Fortune!

hmmm. Good point, I am not sure. I would say no, but I can't say for sure.
BTW, I soaked the entire area around the carbs with WD40 and carb cleaner, no change in idle, so I am nice and tight there.

Noone has mentioned anything about the chattering/popping exhaust. Does that tip anything off as the culprit? I mean, it really does it excessively now after having all the carbs apart. I rev it to 9k, close the throttle, it drops immediately to 1.2k, but chatters alot. It would sound pretty sweet if it was a big v-twin, but with an inline 550, not so tough sounding And it's not backfiring big pops, just a "chatter"

apbling wrote:

...No one has mentioned anything about the chattering/popping exhaust. Does that tip anything off as the culprit? I mean, it really does it excessively now after having all the carbs apart. I rev it to 9k, close the throttle, it drops immediately to 1.2k, but chatters alot...And it's not backfiring big pops, just a "chatter"

Might be too lean pilot circuits.

Could try applying a little choke to see if any effect.

Likely has nothing to do with the mid-range to WFO performance issues.

Good Fortune!

Patton wrote:

loudhvx wrote:

If the throttle slides go way beyond fully open, the mixture gets rich. I verified this on a wideband o2 sensor. The reason is, the needle raises up and lets in more fuel without any increase in air...set the stop so the bottom of the slide is just flush with the roof of the bore on the engine side....
...the needle raises up and lets in more fuel without any increase in air....

Am leaning something new here, as understanding has been that at WOT the mixture allowed through the needle jet is governed solely by main jet size, with jet needle being then so high as to have negligible effect. In other words, that at WOT the needle jet passes all mixture metered through the main jet, with no restriction by any then remaining skinny part of the jet needle inside the needle jet.

Carried to its extreme, my understanding would mean that at WOT, complete absence of a jet needle wouldn't make any difference in performance, as all fuel allowed (metered) through the main jet would be passing through the needle jet, irrespective of whether some minimal part of the jet needle then remained inside the needle jet at WOT. At which time the carb bore is of course wide open (with no restriction by the throttle slide).

But where interface between jet needle and needle jet continues to regulate volume of fuel metered through the main jet at WOT, such result seems to conflict with the "general rule" that jet needle governs mixture during mid-range and that main jet does all the governing at WOT.

Dang it! More homework for me. :laugh:

Good Fortune!

I found many jetting myths were not true when looking at the wideband o2. Everything affects everything, it's just a matter of degree. Another revelation is that even though the straight part of the needle is in the mouth of the needle-jet (needle/needle-jet interface you mentioned), raising the needle still gives more fuel. This is because the needle jet is a long tube. If you reduce the amount of straight needle in the tube, the fuel sees less restriction. What that means is, needle position affects mixture even at very small throttle openings, and even when it appears as though the needle's straight part is in the opening. In other words, the i"interface" is actually the entire length of the needle-jet bore and needs to be considered as such.

This is actually a good thing, though, because if the needle shape is not exactly right, you can make up for it by adjusting main jet, or pilot jet etc.

It also means if you are in a pinch, and can't get a bigger main, you can adjust the stop to get more than 100% throttle, and you'll actually richen the mixture at WOT. It's just another option.

Commonly, people recommend selecting the main jet for WOT first then adjust the needle for mid throttle. I did that. It turns out to be a more time consuming method. After getting WOT just right, I adjusted the needle upward for mid throttle positions, but then had to go back and reduce the main (now too rich at WOT) to compensate for the raised needle.

OK, so when I get a chance to take the carbs off and do this...

Do I back the idle adjustment knob all the way out and then turn each slide down until it seems to be seated, then slowly open the throttle using the idle knob and make sure all 4 slides are equal, then make sure none of them are openeing too far (and set the WOT screw), or should I base all 4 slides off their WOT throttle positions (they should all be equal this way too).

Also, I was thinking if I set them so they don't go past flush with the carb venturi, wouldn't I just be undoing what I did when I vacuum synch them, or is that so small of adjustment its ok???

Vacuum sync is to make them all even relative to each other. (Well, that's the idea, but that's not what really happens. That's a topic for another discussion.) Once they are even with each other, they should be within a tenth of mm or so. At that point you can use any of the carbs to set the stop to make them flush with the throat at WOT (on engine side). Idle position doesn't affect the WOT stop.

If you don't have them synced, then just do a visual sync to get them close. That should suffice to set the WOT stop. Then a vac sync can be done later when it's running.

Don't get confused with the fast-idle screw which is near the WOT stop screw. The fast idle screw raises idle when the choke lever is activated.

Am presuming bench sync procedure for carbs at hand is basically same as bench sync procedure for Mikuni manual slide carbs.

Start bench sync by setting the idle thumb screw in mid-thread position, which allows up or down idle adjustment as later required while syncing carbs on running engine.

Follow normal bench sync procedure to set all slides when closed at equal distance from bore. Sometimes a small drill bit or paper clip wire is used, but eyeballing may also be okay. At this stage, the WOT slide positions are of no concern.

With regard to sync, slide position is most critical at closed and barely opened positions. That's where sync really matters, at idle and pulling away from idle, largely governed by the pilot circuit.

The equal slide positions achieved at bench sync are the starting positions for the actual engine running sync during which their positions are re-adjusted to balance performance among the cylinders, complimented by contemporaneous pilot screw adjustments.

After completing the bench sync, may then check position of the slides at WOT and adjust the WOT limiting screw to where the "lowest" slide just clears top of carb bore when any further raising of slides is stopped by the screw. This has no effect on the bench sync equal slide positions at closed throttle. It simply prevents any slide from being raised too high at WOT when all slides are simultaneously lifted to WOT, while assuring that all slides are clearing the carb throat at WOT. While doing this, don't mess with the previous bench sync adjustments where closed slide positions were equalized.

Correct setting of the WOT limiting screw is immaterial to sync while engine is running at idle and near idle rpm.

Good Fortune!

So, like Patton mentioned, is is possible to put these slides in wrong? Seems odd to me that I could be this far out of whack from disassembling them all and maybe not putting all the same parts back into the same carb.

As it currently sits I could set the WOT screw so that it won't stutter, but I don't think I would be getting true WOT on some of them, while the others maybe are too far open. As for setting the WOT, would it be the lowest or highest? I would think the highest would be the standard, that way none of them would be too high.

The more we kick around ideas here the more I think this isn't the issue. I mean, I did just bench synch them before I put everything back together, but it's worth a shot.

BTW, hope you're not a cubs fan... my brewers are doing pretty decent tonight

The carb sync can get way out of whack anytime you remove the slide-arms from the throttle shaft. Everytime I do a needle adjustment I check the sync to get it perfect. Otherwise it can vary by several CM of mercury easily.

I can't remember if this was looked into, but what if the main jet is partially clogged on one carb. That would do it. It's not enough to just see light through the main jet. You should be able to see the hole as round, and you should be able to see the walls of the hole are clean.

electrical can be tested by doing some swapping if you know which cylinder is dropping out.

Pretty sure the mains are clear. They looked really good. The one thing thats really bothering me is Patton's procedure for the bench synch. If I put my idle knob 1/2 out (or in if we are positive thinkers), I don't think I will have enough adjustment on my individual slides. If I remember correctly, there was one slide where the screw/bolt only had a few threads left still making contact with the lock nut (because it was tightened down so far.)

Man, I may have gotten these carbs nice and clean with this lemon juice, but I sure goofed some stuff up!

Those syncing adjustments are all relative. Just reset all of the adjustments to the middle of their range to start over.