Very Strange Carb Sync Problems

[cowboyup3371]

Steve and I are stumped by a problem with Charmayne's '81 550L that we'd like to give to the Hive Mind for consideration.

Since we got her 550L on the road a few years ago, she's experienced problems with the bike cutting out at lights/stops. The plugs are looking slightly lean on the pilot circuit but the mixture screws for 1, 2, and 4 were set for over 3 turns out while number 3 was at 2-1/8 (it actually looked richer than the others). The intake boots and o-rings were replaced within the last two years and still look/feel good (no obvious cracks, cuts, or tears - visually and physically checked by running a finger around them inside/out). I also confirmed the air filter is still in good shape and doesn't look dirty or overly oily in my opinion.

With that, I asked Steve to come over this morning to help me troubleshoot the problem further. After not getting the carbs to synchronize at all and continuing to see the bike cut out even on the center stand, he recommended that he take the carbs home and re-clean them himself. We found that one pilot jet was definitely clogged from what looks to be bad gas so he re-cleaned all of the jets (full carb dip was done for the second or third time last year). All floats were set to the correct setting and all jets visually re-verified to be stock (40 pilot and 92.5 main; 150 air). The carbs were then reassembled and bench sync'd then re-installed on the bike; mixture screws are now all set to over 3 turns out.

Unfortunately, our problems continued. We could get 1 and 2 to run at the same level together and pull a lower vacuum then what 3 and 4 are pulling. When they are offset like that, the bike sounds like it runs well. However, when we try to configure all four carbs to pull the same vacuum, the bike starts to run very rough, dies out, and experiences problems restarting.

One last note as I don't know if it's pertinent but the pipes are very hot close to the engine but are almost cold (can hold them) at the muffler.

Does any one have any suggestions as to what could be causing the issues here? I'm sure Steve will be by later to add/clarify anything I wrote here but I tried to explain it as best as what I saw the bike doing while we worked on it today/tonight.

[Steve]

Checking in, ... he explained the problem quite well.

I will add that the vacuum was low enough that we had to turn the Carbtune unit upside down, but all the strange stuff continued. On a hunch, I swapped the hoses on the gauge. Moved #1 hose to #4 and #4 to #1. Same with #2 and #3. Strange stuff continued.

Just to eliminate one more variable, I will take my mercury sticks (and probably my Carbtune, too) over tomorrow.

.

[GregT]

When did you last check the valve clearances ? It'd also be worth putting a timing light on both sides of the ignition to ensure both sides are advancing equally.

[Brendan W]

....... and valve timing ?

[cowboyup3371]

Valve adjustments were done 2000 miles ago (about 2 years) as she doesn't ride much without me. We had discussed checking the timing but ran out of time last night so will put that as a "to do" today.

[bwringer]

The following is an unorganized pile of random thoughts and musings; things your report made me think about, nothing more:


A rich #2 makes me think of petcock problems; a leak through the vacuum diaphragm can let excess fuel into #2, not to mention letting the petcock shut off unpredictably.

I'm also thinking in the direction of electrical/spark problems. As suggested above, a good timing light can help you make sure the sparks are happening consistently. Getting nice clean regular flashes? (Or you could just get totally ridiculous about it and hook up an oscilloscope...)

Electrical issues also tend to be affected by heat, which pushes my thinking thataway as well for anything that's intermittent. I once diagnosed a defective Dyna by removing the ignition cover; on a crisp fall day, the bike would run great for as long as you wanted with the bits out in the breeze, but with the cover on, two cylinders would cut out after 10 or 15 minutes of running. And we couldn't get this to happen idling or revving on the bench; the engine had to be working harder than that.

I'm sure you've already checked voltage at the coils. I've tracked down several intermittent electrical issues caused by wires or connectors that were making contact, but very poor contact. Voltage was fine, but the bad wire or connector couldn't pass enough actual current to be useful. Sometimes you can wiggle or massage the wiring or connectors to catch these sorts of things, or add a small load like a test light to see if voltage drops. And of course, feel the coils to see if one is heating up more than the other.

Also some obvious stuff like checking or replacing the spark plug connectors; at least take them apart to see if the resistor inside is borked.

One other thing I'll mention is that it's common for plugs to foul when a bike with an aggravating problem has spent a lot of time being started up over and over again, idling or running in the garage. Might be worth the six or eight bucks to toss a new set in there.

A compression check might be a good idea as well... or pull the valve cover and look for something exotic.

All the above said, issues that affect 1 and 2 vs. 3-4 (rather than the more common 1-4 and 2-3 split) could derive from things that happen when a bike sits on the kickstand; fuel stays in the "downhill" carbs longer, so these can have more issues.

[Steve]

The following is an unorganized pile of random thoughts and musings;

Sorry, Brian, but I think the emphasis here is on "unorganized" and "random".

Rather than my usual quoting of individual clips, I will do just one quote and insert comments/rebuttals.

A rich #2 makes me think of petcock problems; a leak through the vacuum diaphragm can let excess fuel into #2, not to mention letting the petcock shut off unpredictably.
#2 is not running rich. The petcock is not shutting off unpredictably. When the engine dies, the bowls are full of fuel.

I'm also thinking in the direction of electrical/spark problems. As suggested above, a good timing light can help you make sure the sparks are happening consistently. Getting nice clean regular flashes? (Or you could just get totally ridiculous about it and hook up an oscilloscope...)
No scope handy, so ridiculousness is averted. Have not yet verified spark or the lack thereof, but the engine does not die suddenly, like the switch is turned off, it stumbles once, then twice, which slows the engine a bit, then dies. A process that only takes a couple of seconds, but is not immediate.

Electrical issues also tend to be affected by heat, which pushes my thinking thataway as well for anything that's intermittent. I once diagnosed a defective Dyna by removing the ignition cover; on a crisp fall day, the bike would run great for as long as you wanted with the bits out in the breeze, but with the cover on, two cylinders would cut out after 10 or 15 minutes of running. And we couldn't get this to happen idling or revving on the bench; the engine had to be working harder than that.
I have also experienced heat-related issues, but this is a bike that has been started and warmed up just enough to not need the "choke" to keep running.

I'm sure you've already checked voltage at the coils. I've tracked down several intermittent electrical issues caused by wires or connectors that were making contact, but very poor contact. Voltage was fine, but the bad wire or connector couldn't pass enough actual current to be useful. Sometimes you can wiggle or massage the wiring or connectors to catch these sorts of things, or add a small load like a test light to see if voltage drops. And of course, feel the coils to see if one is heating up more than the other.
No, the coil voltage has not been verified. We are not suspecting voltage problems, but have not yet ruled them out.

Also some obvious stuff like checking or replacing the spark plug connectors; at least take them apart to see if the resistor inside is borked.
I am thinking that bad connections/components in the plug wires would show up as a problem that is visible at the plug. All the plugs look great. Good color, all dry, indicating proper burning.

One other thing I'll mention is that it's common for plugs to foul when a bike with an aggravating problem has spent a lot of time being started up over and over again, idling or running in the garage. Might be worth the six or eight bucks to toss a new set in there.
The plugs have been visually verified and are in good, although used, condition.

A compression check might be a good idea as well... or pull the valve cover and look for something exotic.
I would be happy to do a compression check, but do not have a gauge. Valve cover is next, mainly to verify cam timing.

All the above said, issues that affect 1 and 2 vs. 3-4 (rather than the more common 1-4 and 2-3 split) could derive from things that happen when a bike sits on the kickstand; fuel stays in the "downhill" carbs longer, so these can have more issues.
Agreed, but the bike is sitting on its centerstand on a concrete floor. The current issue (see details below) are appearing after a carb tear-down and cleaning, so no "downhill fuel" issues.


As a recap to the whole situation:
The bike in question is an '81 550L, all stock. The only modification is that the ends of the mufflers have extra holes, which only makes a bit more noise.

The issue is that the bike will run just fine, but stall when stopping at in intersection. It stalled as if the idle speed was set too low. Turning the idle speed screw up a bit would allow the engine to restart, but it would then idle about 3000 RPM.

Yesterday, we removed the carbs, took them to my place and did a forensic investigation. The reason the pilot screws were turned out so far was because the pilot fuel jets were at least partially obstructed on more than one carb. This would account for the bike running well, but dying at idle. Did not bother to dip the carbs, but did backflush all the passages with carb cleaner spray and compressed air to verify they were all clear (they were). After re-assembly, a bench sync was performed, using my light table to compare the sliver of light around the barely-opened throttle blades. The pilot screws were set to my usual pre-set of three full turns out from lightly-seated.

The problems we are experiencing now are rather directly related to carb sync. In spite of a very close visual sync on the bench, when installed on the bike, it appeared that the sync screw between carbs 2 and 3 was off by about two turns. Although it is not the proper sequence to sync the carbs, carbs 1 and 2 were perfectly matched, as well as carbs 3 and 4, but the two sides of the engine were off considerably.

What made it even more interesting is that turning the 2/3 sync screw to match the vacuum levels made the engine run worse. We turned the screw so far, the throttle would move 3 and 4 before taking up the slack in the 2/3 adjuster, then finally opening 1 and 2. This would imply that 1 and 2 were fully closed, but the engine did not sound like it was running on only two cylinders. Adjusting the 2/3 sync screw so the vacuum imblance was worse made the engine run better. That is why I am taking my mercury gauge over this afternoon.

Along the same line, although the gauge said they were fine, I adjusted the balance between 1 and 2. The farther I got away from balanced, the better it ran. VERY puzzling. Again, will be checking with a different gauge to rule that out.

The one bright point of all of this is that the bike is no longer dying, because the pilot jets are now clear. When we finally get the engine running properly, we will do the fine-tuning on the pilot screws, but they do not have any effect on vacuum levels in a carb sync.

I appreciate the suggestions, but I can't see how some of them relate to the issue at hand, given the circumstances. I am trying not to be like some others that have posted, asking for help, then rejecting the suggestions, that's why I am explaining why I don't think it would help. If I need a figurative slap up the back of the head, let me know.

.

[tom203]

A local guy to me with the exact same bike has been experiencing this same issue .bike cutting out at stops stuff. Thought it was coils, so upgraded to dyna 3 ohm ones, then dyna s ,but problem persisted . Discovered he had water in gas, so got fresh stuff and thought problem was solved - but issue resurfaced quickly. I suggested petcock filter inspection .
I am waiting to hear back from him.

[LAB3]

The only thing I haven't seen mentioned yet are the butterfly valves. Bench sync was mentioned but it may be worth checking that they're opening and closing properly and in the same position relative to each other at rest, stuff sometimes gets bumped, overlooked etc.

Without beating a dead horse, I still wouldn't rule out an ignition problem. We've often mentioned that carb problems are sometimes ignition problems and vice versa. A weak spark can affect the way a bike runs, right?

[Steve]

Well, in the process of re-assembly the butterflys are checked. Sorry, don't have an angle gauge to see that they are all opening the same amount, but they SEEM to operating together. The bench sync merely assures that they are STARTING at the same time.

I would like to revive that dead horse as much as anyone, but right now, the bike is at least running. I can not suspect the ignition at this point, because it is not dying. The difference in the vacuum levels while trying to sync the carbs is what is baffling now. The ORIGINAL problem was that the bike was dying. Dirty pilot jets were causing that. They have been cleaned, the bike is running.

.

[cowboyup3371]

So it looks like maybe the carb sync is settled though we didn't really do anything special as both my Carbtune and Steve's mercury sticks show the same levels. However, the engine is still dying after it warms up. After verifying the ignition timing (maybe a degree or two retarded), valve timing (where it is supposed to be), voltages at the coil (12v when cold but seemed to drop to 9-10v after it warmed up), and even a valve clearance check (done when slightly warm - one valve potentially out of spec and will be rechecked later when the engine cools off), we are left with not knowing for sure what's going on. So, after talking with Steve, I will do what I don't want to do and just throw parts at it. I'll first replace the coils with spares I received today and ride it for a day or three to see how it acts. If we still have problems then I'll try to put the ignitor from my bike onto hers.

More to come later

[Redman]

....
................................I will do what I don't want to do and just throw parts at it. I'll first replace the coils . . . . .

Plug caps are cheaper, and can present strange symptoms.

[cowboyup3371]

Thanks Dave; I'll talk with her about ordering some.

[alke46]

Thanks Dave; I'll talk with her about ordering some.

Huh? can't you just get them from your local spot like Iron Pony?

[Rob S.]

Pardon me if I'm stating the obvious, but like carb adjustments, just throw one part at a time at it so you'll (we'll) know which part caused the change.

Definitely got a bunch of us on the edge of our seats.