#2 and #4 not firing after rejet

[KEITH KRAUSE]

Just saw your update. Quick reply 'cause I gotta go out.
Trying to make sense of your latest test...opening up the throttle as you did would certainly bypass the pilot circuit (and any fuel flow problems the pilot circuit may have). You'd be running on the jet needle at that point. Your test suggest you got more fuel into the cylinder. So that's good.
The backfiring isn't good though. Most backfiring can be from raw fuel collecting in the cylinder and exhaust and at some point the whole collection combusts in one big bang or smaller bangs. Problem is, you say there was no fuel entering until you decided to open up the throttle. No fuel could have collected before you opened it up. If that was true, then your needle circuit also has a problem and/or, the spark is intermittent. I say this because your test was while running on the needle circuit.
A weak spark will obviously cause backfiring but so will too much fuel.
I don't have time now but I'll try to think of the problem and come back later if you still need help.
Something's causing poor combustion and it's not compression. So that leaves a weak spark or carb problem. 1 and 4 are fed by one coil, so it suggests the coil isn't the problem (1 is supposedly good), but it's still possible that other conditions/differences exist between cylinders/carbs 1 and 4 that help #1 keep firing, though it may have a weak spark too.
Do you have a manual and multi-meter? Test the coil resistance's and voltage first because it's simple. If it passes test, then you just have to check/clean any plug cap or plug lead problem. That would cover any #4 spark problems. HOWEVER, I'm not that familiar with the ignitor tests on your bike. Seems to me the ignitor can cause a weak spark at two cylinders also. Anyone? Perhaps your manual will help?
Anyway, check the coil, plug connections and possibly the ignitor. Basic maintanance that should be done anyway. If it passes, you KNOW it's carb related and we can focus on that. If you have a meter, the electrical tests are much quicker/easier than tackling the carb.
Hope I'm not making you do any unnecessary work, but backfiring can be either fuel or electrical and we have to eliminate one so we can focus on the other.

[J_C]

Keith, thank you for your replies!

I do have a multimeter, though I'm unsure why the problem wouldn't have followed the coil / cap to #1? That's why I haven't focussed on that earlier than now, though I can certainly look into it.

Before doing that, however, we just noticed something else when we went to start it cold...

We put it on choke for around 30 seconds, #4 did not heat up. I closed the choke and opened the throttle just partially (my idle adjust knob threads are warped so they dont work right). As soon as I closed the choke and went to normal idle, the backfiring (is it backfiring? Its a loud popping coming out of the pipe) started and the #4 heated right up. Is it possible that the choke for whatever reason isn't working on the #4 and it isn't until it's on normal idle that the combustion begins happening, resulting in the burning off of all the fuel already there?

The popping was happening at 1500 rpms, which I assume is still only the pilot circuit, since I only had the throttle cracked enough to keep the bike running. Does this pose any additional clues that a guru such as yourself might be able to use?

[KEITH KRAUSE]

Keith, thank you for your replies!

I do have a multimeter, though I'm unsure why the problem wouldn't have followed the coil / cap to #1? That's why I haven't focussed on that earlier than now, though I can certainly look into it.

Before doing that, however, we just noticed something else when we went to start it cold...

We put it on choke for around 30 seconds, #4 did not heat up. I closed the choke and opened the throttle just partially. As soon as I closed the choke and went to normal idle, the backfiring started and the #4 heated right up. Is it possible that the choke for whatever reason isn't working on the #4 and it isn't until it's on normal idle that the combustion begins happening, resulting in the burning off of all the fuel already there?

The popping was happening at 1500 rpms, which I assume is still only the pilot circuit, since I only had the throttle cracked enough to keep the bike running.

OK. As for the first part of your reply, yes, a weak coil will effect both cylinders it feeds, but, I've seen weak coils have a more noticable effect on one cylinder more than the other because of other conditions that effect spark. You CAN have one cylinder with apparently no spark and the other cylinder can have a weak spark but still firing. "Other" conditions can be lots of things I won't get into now. If, at the exact time the problem is happening, you should at least remove the suspect plugs (1 and 4) and look for a blue spark. That may tell you enough. But it's still best to also test with your meter and follow your models manual info. This is just basic checking/troubleshooting. With what history I know of this bike, I tend to think your problem is carb related, but it would be nice to know the electrical is good and we can focus on just that replacement carb.
If you read any of my posts, I'm a real believer in going through the basics when there's a problem. It may seem like more work, but I always look at it as it needs to be done anyway and actually saves time sometimes instead of looking for things one at a time. Basics are compression, good valve clearances, good spark/resistances/voltages/timing/connections, VERY CLEAN carbs with good inner o-rings/rubber parts, floats set, bench synch followed by vacuum synch, good manifolds/any manifold o-rings, clean air filter, and any other things that your bike's mod's may require.
Anyway, back to your latest cold start up results.
Yes, it's possible your choke isn't working, but I don't think that's your main problem. I tend to think your #4 choke is working. You can see the mechanical movement/operation? When they "fail", it's usually because they won't seal and you run rich. If mechanical, it's usually obvious just watching it and comparing movement to the others. Many things effect cold start up so it's not easy to make sense without being able to see the bike myself. Temperature, engine temp, mixture screw adjustment, amount of choke, clean carbs, etc, all can change things.
If the throttle is closed, you're COMPLETELY on the pilot circuit with ASSIST from the choke circuit. The moment you open the throttle, this effects vacuum/pressure and the choke circuit is bypassed. SOME choke circuit fuel still enters but you're still bypassing it to some extent and the more you open the throttle, the more the choke is bypassed 'til it really supplies an insignificant amount, if any.
AS for opening the throttle "a little", most jetting charts will say that you're mostly on the pilot circuit but the jet needle will start to overlap very soon. So it's not a positive thing to say "I'm at 1/8 throttle... so I must still be 100% on the pilot circuit". I've seen varying results as to how early the jet needle starts overlapping the pilot circuit.
In your test, backfiring is better than no firing. If it won't fire at all until the throttle is opened somewhat, this suggests that the increased vacuum is drawing fuel from the jet needle at least partially. It also suggests the pilot circuit isn't operating right and there's not enough fuel in the mix until you open the throttle, which will (depending on how much throttle) at least start the transition from pilot to needle circuit. The bike SHOULD get all the fuel it needs at idle if the pilot is operating right. The choke will be needed for starting, but it always depends on how cold it is outside, etc, as to when you can shut off the choke and hold an idle 'til it warms up well. My bike only needs about 30 seconds of choke this time of year.
Remember, I'm assuming the floats, o-rings, synch, etc, are all good. I assumed earlier that the carbs were clean, but it appears they may not have been. Also, you can't be sure any small passages are clean without compressed air. You can try to verify by watching for carb spray exit a hole whenever you shoot it into a passage but that still doesn't mean the passage is completely clear. If you don't have access to compressesd air, the best you can do is spray or soak, then carefuly poke a wire in the passage and possibly blow through a piece of small tubing. Do you see spray exiting?
Hope I answered your questions.
What exactly have you done to try to fix this problem?
Is the backfiring ONLY from the right side pipe?
Is the #4 replacement carb 34 or 36mm? There was some confusion here?
Are you positive the primary and pilot air jets are very clean? They're near the bottom of the carb throat, filter side.
Did you bench synch the carbs in the factory order? I forget the order, but I believe you have to follow a specific order on your model. This is not purely your problem, but it could be a contributor.

[J_C]

Hope I answered your questions.

Keith, I owe you a keg of beer for all the help you've given me. I say this from time to time, but I'll reiterate; one of the reasons I bought a GS (as oppossed to any other model) was the generous nature of this board.

Do you see spray exiting?
What exactly have you done to try to fix this problem?
Is the backfiring ONLY from the right side pipe?
Is the #4 replacement carb 34 or 36mm? There was some confusion here?
Are you positive the primary and pilot air jets are very clean? They're near the bottom of the carb throat, filter side.
Did you bench synch the carbs in the factory order? I forget the order, but I believe you have to follow a specific order on your model. This is not purely your problem, but it could be a contributor.

-The carbs had been dipped for several hours and cleaned using the method detailed in the gs garage, though I may have missed the pilot jet hole the first time through. Yes, The spray shoots out the tiny hole into the area next to the slide cylinder. It also sprays out through the air jet. It may have been clogged before but that hole is wide open now, as is the pilot jet.

-To fix it I have attempted the following. Rebuilding (taking all parts out and then all back in), remeasuring float height, checking carb plunger movement, checking for fuel in the boots, checking fuel line, cleaning pilot jet, pilot air jet, pilot hole, main to cross over channel, main jet, needle jet etc, adding new orings (during original rebuild), and checking the diapragm

- Popping is only right pipe, and immediately consistent with the sudden increase in temperature of #4 header

-This is a 34mm carb

-The pilot air jet it clean... primary air jet? Is that the hole without the screw in it?

- The bench synch was done 3,4,2,1. Those were the directions I got off of xs11 for the mikuni carbs. I just received my mercury sticks friday, would these help more experienced people thant I diagnose this issue?


At what point in time do I kind of throw in the towel on this one? The old man wants to take the carbs in, which I am vehemently against, but they might be able to see some thing right off bat that I am not accurately describing. I'm also going to try the old carb body today, crack and all

[KEITH KRAUSE]

Primary air jet is located near the bottom of carb throat, filter side, usually the opposite lower "corner" of throat. If pilot air jet is about "5:00", then primary is about "7:00". Primary jets can be "fixed" or removable. In any case, be sure it's clean, though the pilot air jet would be more suspect in your tests.
If you can see spray exit all the tiny pilot circuit bypass holes, near the top of each throttle plate, then you're probably OK, though high compressed air is always best.
A good vacuum tool synch is mandatory, but it has to run well enough to attempt the synch.
Sure sounds like the pilot circuit, but if you're turning the throttle enough to involve the needle circuit, and it's only improved from no fire to back fire, that suggests more carb problems than just the pilot circuit not delivering fuel. Tape your throttle housing and grip and mark for closed and about 1/3 throttle positions. Allow for any slack before the throttle actually opens. Where does the backfire begin? Does opening it up to 1/3 continue the backfire?
I'd still like to see the electrical tested, if possible. I assume that, at least, the plug cap connections and coil terminal connections have been checked for clean and tight?? Plugs new or at least very clean? Gapped?
Installing the old carb would possibly help figure things out. If the old carb works but it's cracked you'll have to try to find a known exact replacement. How have you tried to temp repair the old carb? J/B weld or ? What was the damage to it?
I'm against taking the carb in too, but only because most mechanics aren't good at these CV's. But that doesn't mean all mechanics are that way.
If you CAN get decent results from temping in the old damaged carb, that would certainly suggest you just have something wrong with that replacement carb and we can focus on all the things that it could be.
You're the one doing the work, but I'd probably try the old carb too...if you believe it's repaired enough to make a good test. Don't forget to check it for float level, mixture screw adjustment, etc, before trying it.

[KEITH KRAUSE]

One other thing. I checked the factory manual for an '80 GS1000E (BS 34SS carbs) and it says the #3 carb is a master carb. That means #3 shouldn't even have an adjuster on it. Does it?? If no adjuster, the factory order of bench synching is to set #2 first...then #1...then #4. The vacuum tool synch is the same order of course. If you do have an adjuster, then disregard this.
Only extra thing to know is, on ALL CV equipped Suzuki's, with stock intake/exhaust, you must vacuum synch the middle (2 and 3) cylinders a little lower than 1 and 4. I believe about 1/2" lower mercury will be OK. That's with stock intake/exhaust only.
Like I said, this info is for the same carbs apparently, but on a '80 1000. I would think they're physically/mechanically the same. There shouldn't be a difference with how the synch procedure is done. I'd trust it before referring to a Yamaha 1100 article.
Don't know how much a possibly poor bench synch is contributing to your problem, but it's just another thing you can do right and eliminate it as a possibility.
Today is yard work day for me, so I'm outside for several hours. I'll try to check back in around 3:00 if you have any more info or need help.

[J_C]

I'll go out and mark the throttle and test beyond 1/3. The plug cap connections have been tested. I'm not exactly sure what the coils look like but I'll see if I can give them a look and perform those electrical tests.

The spark plugs only have about 15 minutes of running on them. I checked the gap before putting them in, and switched #3 and 4 yesterday to ensure they weren;t the problem (The spark plugs worked before the new carb body but just checking to make sure)

The old carb body cracked (hairline crack, not broken) while attempting to remove a seized air screw. I haven't attemped sealing it yet, but I will today with jb weld (Assuming the threads still line up)

The gs1100 does use BS 34SS carbs. #3 does have the idle adjuster on it, though the threads are worn on it so it tends to slip with the spring installed. I took the spring off to bench synch it.

I'm beginning to think I should hold off on the vacuum synch until the pipes are here (they got delayed a week) since I will need to redo it again after it comes anyway.

In the meantime I will see if I can test the 1/3 throttle and try to repair the old body. Thanks Keith!!


Edit: Results:

At idle (Choke): No popping
At idle (no choke): 4th header now heating up (maybe somewhat slowly compared to 1,2,3). Popping intermittent out of right pipe.
1/6 throttle. Popping frequent, increasingly frequent as throttle increases
1/3 throttle. Popping has almost disappeared

This afternoon I will go out for an audio recording. This might help?

[J_C]

Here are the audio files. Unfortunately The popping is hard to hear... its more prevalent in the second file.

The files are sequential ( no time lapse) Starting from cold choke. The bogging is when I let the idle too low (Again, no adjust knob). At some point I rev to 1/3 and the pops go away.

http://socal-letsride.com/storage/GS1100L/CIMG0015.WAV

http://socal-letsride.com/storage/GS1100L/CIMG0016.WAV

My uneducated guess says that there is some kind of synch issue with #4 since it won't pilot circuit on choke, but it will when I use the throttle like an adjust knob to 1100 rpms. There must also be some kind of problem with the jet needle.

[KEITH KRAUSE]

OK. Heard that, and it wasn't pretty!
My opinion is that's not backfiring...that's the sparkplug or mixture intermittently firing.
Every time you hear a pop, that's the plug spark igniting the mixture. You obviously don't have the correct mixture or correct spark for continuous combustion.
This exact popping can be from a mixture problem, or, poor spark.
If the mixture is wrong, it will only ignite some of the time.
If the spark is poor, it can only ignite a very rich mixture. Once every several cycles, your mixture would collect to the point that a weak spark could ignite it. It should fire every power cycle of course.
I still would like proof that the electrical is good, but you seem convinced it is. While I think so too, I'm just naturally skeptical. I'll forgo the electrical because I want to simplify and just help you. We'll do whatever can be done to find a carb problem and hope it is a carb problem. It does make sense mainly because you're sure the connections are good and that #1 is receiving good spark. So...
It clears up pretty good at 1/3 throttle, anything less creates popping. I also think I heard some fuel starvation in the audio, though you say that could be just you allowing the rpm's to drop too much because of an idle adjuster problem. I have to say it sounds like fuel starvation, NOT you allowing it to drop. Most bikes pick up much quicker when the rpm's drop too low for a moment. Yours sounds like it's really struggling to pick up? Hard to say because I'm not turning the throttle. The fuel starvation thought certainly fits though.
What's interesting is it seems to be improving some? At one time you didn't see any fuel wetting the throttle plate at #4. I assume you did some more cleaning to the pilot circuit? Or is the improvement mostly because you're turning the throttle more? What color is the #4 plug after a minute or so of running? Only problem with a plug read here is because the choke may/will darken the plug and you may need more time for the plug to show a true color that's doesn't have any choke effect. It would be nice to compare the plug to the others though, in any case. They're all under the same start up conditions.
The more we talk about these carbs, the more it seems is wrong with them. First the idle adjuster, now the synch mechanism. I have to wonder what else may be wrong? A poorly synched throttle plate means incorrect vacuum. Too much or too little, as compared with the others, will cause major problems. In your case, it sounds like you don't have enough vacuum to draw sufficient fuel through the pilot jet and to a lesser extent, the jet needle. If you're POSITIVE the pilot circuit and jet needle/main circuit are clear/clean and the jets are identical to the other carbs and the floats are adjusted the same, then that would eliminate any fuel flow question. That would mean the only reason fuel can't be drawn up through the pilots passages/jets, is because of poor vacuum. Poor vacuum is most commonly a throttle plate issue. Poor vacuum can also be a bad or unseated diaphragm or spring problem but you say the diaphragm assembly is good. Besides, a torn diaphragm generally doesn't accept ANY throttle beyond idle. Poor vacuum could also be an intake leak, but without reading back, you say there are no leaks, correct?
So we're looking at a vacuum problem it appears. Opening the throttle increases vacuum and as you open the throttle the bike runs better and better.
So, knowing you have a problem with the synch adjuster and it appears the other things are checked, it does sound like the synch or throttle plate is the problem. How does the #4 plate compare (visually) to the others.
It can't be opening "behind" or "ahead" of the others. A very tiny amount is acceptable for an eyeball bench synch, but not any more than that. It also has to operate smoothly and the same as the others.
It's hard to watch the plates under running conditions because these CV equipped bikes run badly with the air box off. You may have to wait to see the true vacuum you have when you get the synch tool.
Problem is, your bike may not run well enough to synch it. But it might. You'll have to be careful of mercury rising too much if you have to attempt a synch in this condition. But maybe we'll find the problem before that.
Let me know what you try. Trying the original carb will be interesting, if you do.
Be patient. If you're giving us good/complete info, I think you're getting there.

[J_C]

Thank you Keith. The frusteration from yesterday has subsided; I'm just glad I have great help! I agree that the electrical should be tested, I just didn;t have time to learn how to do it today When I get back on Friday I will start giving it a once over. (Bike lives in Burbank, I travel to Houston every week for work)

There do seem to be minor improvements every day, though God knows what I'm doing to get them. I did clean the pilot circuit extenively so that may have been a step in the right direction. I don't think it was just turning the throttle. In fact, I have it idling on its own now after adding some thread tape to the knob threads.

I listened to check whether the popping was cyclical or random, and it definetely seemed more random. I think a key indicator here is the plug color. I ran it on choke for about 45 seconds, then let it idle between 1100 and 1400 for about 3 minutes. After that I checked all four plugs. 1,2 & 3 all were a light off-white gray, while #4 was almost completely black. What's causing that I don't know.

That would mean the only reason fuel can't be drawn up through the pilots passages/jets, is because of poor vacuum. Poor vacuum is most commonly a throttle plate issue.

I think this just might be the case. I will need to wait until next week to perform the carb synch with the mercury sticks to see if that might shed some light.

Does the plug color tell us anything?


Thanks again, I'll have to bring you a friendship cactus or something for your yard :-D

[KEITH KRAUSE]

Yes, the plug colors are a good indicator. In your case, the problem is the blackened plug. It's very close to fouling out. The causes of a black plug are either a rich mixture, excessive vacuum drawing more fuel (as compared to the other cylinders), a weak spark, or combination of these.
You can always try a new, gapped plug, but it's not likely to be a bad plug.
I still tend to think the coil is good but think it should be tested to be sure. Also, coil terminal connections should be checked/cleaned with a wire brush before testing. The battery must be fully charged before testing too.
If OK, then I'd focus on why the rich mixture or excessive vacuum.
Since you're gone all week, I'll let this go for now. Later this week I'll post everything I can think of that relates to your problem. You can look at it and we'll eliminate things if need be 'til we find the problem.
The vacuum synch/adjuster at #4 concerns me a lot. So does the fact the replacement carb body may still have a difference we're not aware of. It could be interesting trying to synch if it's not operating as it should. One way or another, the synch has to be done at #4, at least closely.

[KEITH KRAUSE]

The other interesting thing is #4 seems to have gone from no fuel (the throttle plates weren't even wet) to possibly too much fuel (the black plug).
But a previously blocked pilot circuit and a second problem, such as incorrect vacuum or weak spark, would explain this. We'll see.

[J_C]

OK I Removed the new carb body and replaced it with the JB Welded carb body. Took all parts from one and put them in the other, remeasured float height, installed.

Started up. 1,2, and 3 got hot while on choke, 4 did not. Once choke was released, #4 began to slowly heat up (Heat from block most likely). 1/4 throttle was applied and #4 instantly heated up, with lots of popping. Removed pods from 1 and 4 to have a visual comparison. Somewhere around 1/3 throttle the cylinders began to raise. They appeared to be raising on cue with each other.

My plan at this point is to do the following:
1. Wait for new exhaust; Install
2. Retest. If problem exists, next step.
2. Rejet all carbs to tested and proven settings achieved by chef
4. Retest. If problem exists, next step.
5. Replace all jets and choke assembly with spares.
6. Retest. If problem exists, next step.
7. Cry.

[KEITH KRAUSE]

Just a quick reply to let you know I'm still watching this.
Been busy all week/OT at work and today I'm going riding (not trying to rub it in) and then I have Fathers Day activities to go to, so I won't have time to reply today if you're working on it and need help.
I re-read this entire thread and it's a bit of a head scratcher. Possible vacuum, fuel, or air problems, or even a combination.
Obviously, the problem is just #4 now. #1 was once a problem too but it appears that extra cleaning or re-assembly or ? has solved its problem. I'm not positive, but it seems #4 is a LITTLE better than at the beginning, because it's at least popping and trying. This could be due to you opening the throttle more than during earlier tests, so I'm not sure. It also could be because of extra cleaning, etc.
The black plug at #4 only is bugging me though. That would suggest too much fuel (you say the spark is good). But your earlier tests showed a LACK of fuel entering the cylinder. Heck, the throttle plate wasn't even wet. But I'll stick with where we're at now.
By design of this carb, it draws fuel from the pilot circuit at closed/idle. At this time, you have no combustion. But as you open the throttle to 1/4 and then 1/3, you're getting well into the jet needle circuit. Now it's popping and combustion is improving. Hmmm.
So what's happening here, besides transitioning from the pilot circuit to the needle? Under "normal" conditions (all parts in good condition/correctly adjusted), we're introducing more mixture, both air and fuel into the cylinder. We're also increasing the vacuum. One, or a combination of these things isn't enough or happening at closed/idle position.
First and most common thing (considering what you day has been checked) would be a clogged carb passage somewhere. I say this because you haven't used high pressure air to help you clean the carbs. I have to say you MUST use high pressure, a good 150 or close to clean carbs after spraying/dipping. So that's a possibility.
Any chance you can take the carb someplace and blow it out with high pressure?
I have to go now but we'll check in later.
Quick thought... what exactly is wrong/damaged with the #4 synch adjuster? Does it "visually" hold its synch when you compare plate adjustment with the others after testing on the bike?
Following your EXACT starting procedures as before and using closed throttle/allowing it to idle, what happens if you place your palm over #4 intake and completely seal it off for several moments? Any change at all?? What if you let in just a little air by moving your palm a little? Any change?
Talk to you later.

[J_C]

Keith, I appreciate your time whenever I can get it You're not superman you know!

The thing about the pilot being clogged... I know it's possible... but is it likely that after a carb dip (without compressed air) a carb body could have a plugged pilot passage leading to exactly the same symptoms as the other carb body with plugged passages? I was thinking that it might be one of the individual parts since the same parts were used on both bodies and both bodies exhibited the same symptoms. I can still find somewhere with compressed air... At what PSI does a gas station air pump put out air?

When I put the JBWeld body in I did bench synch it using very wire gauge from a stripped trash bag tie. It was actually already perfect from a bench synch perspective, which kind of makes sense since it ran before and I didn't have to adjust 1,2, or 3 when I put on the unbroken carb body in.

we're introducing more mixture, both air and fuel into the cylinder. We're also increasing the vacuum. One, or a combination of these things isn't enough or happening at closed/idle position.

I suppose the only way to test this is with the mercury sticks to at least eliminate vacuum... I put together my mercury sticks but will have to wait until I'm back next weekend to try them.

There is nothing wrong with the synch adjuster. The cracked part is actually the neck of the air screw. When the adle adjust plate it pushed with a finger all appear to rise together.


I actually did the hand trick over the vacuum yestrday. I #1 as well to have a comparison. #1 the idle increased (makes sense, right now 1,2 and 3 are running lean as seen by the only slightly gray plugs) but I don't remember what happeened with #4, other than both 1 and 4 spitting gas back at my hand if I held it there too long. I will try it again before I leave for Father's day festivities.