carburetor or coil problem?

[1980cm400t]

I am currently in the process of restoring a 1980 xs1100 and I have run into a carburetor problem that I am having difficulty figuring out and am looking for direction. I have tuned carbs on other bikes before but this is my first time doing a 4 cylinder. My current setup on the bike is 4 into 2 exhausts without mufflers and pod filters on the ends of the stock velocity stacks. The issue i'm having is the bike will start fine without the choke cold but doesn't want to start when the engine warms up. The engine also bogs and shuts off when revved which gets worse when the engine warms up. I have also noticed that it smokes (white smoke smells like fuel) out of the left hand pipe (which is connected to the 2 left hand side cylinders). Overall behavior suggests over rich air/fuel ratio.

I have equipped a common fuel supply system and non vacuum petcocks (which allows the petcock on either side of the fuel tank to fuel all four carbs. I also have installed a fuel filter and done a complete and thorough cleaning of the carbs.

I have also made the following adjustments/measurements
-carbs were pre-synced with a feeler gauge
-started with a main jet that was 2 sizes bigger than stock
-after testing main jet was brought back to stock (this stopped the right hand side from smoking)
-needle was also started at richer than stock setting and was incrementally brought back to stock settings
-ditto for pilot screws
-slow jets are stock size
-Pod filters were removed and engine run with open velocity stacks (to see if pods are too small and restricting airflow) no appreciable change in behavior suggests this is not the issue
-float level and float valve performance was checked with a manifold of clear hoses. The fuel level in all four bowls was consistent and at the correct level (indicates that performance issue is not being caused by a stuck float valve dumping excess fuel into a left hand side cylinder)

After writing this it has occurred to me that this might be a bad coil on the left hand side and not a carb problem. I also noticed that the right hand side (the non smoking side) has a little bit of yellow coloring on the exhaust pipes whereas the left hand side still looks brand new. Is there a proper way to test whether the coils are any good? In the event that the left hand side coil is bad is there a place to get stock replacements or are aftermarkets a better choice? The last time I tested a coil my method was to pull the spark plug in question sit it on the engine block and watch it while cranking the starter to verify that it is sparking.

[crazy steve]

It's a carb problem....

The coils are paired to cylinders 1/4 and 2/3, so if it were a bad coil that's how you'd see the problem. The carbs are paired 1/2, 3/4. You stated you did a bench sync, that's only good enough in most cases to get the bike started. You MUST sync the carbs via vacuum on the bike while running.

Open exhaust isn't helping either; these motors really want to see some backpressure, carb tuning problems are common with open pipes (and not always resolvable). Doublecheck to make sure which type carbs you have; float adjustment is different between the two types, as is jetting. Look here:
http://www.xs11.com/forum/showthread.php?t=35964
Check float type; brass or plastic?

But until you get a proper sync done, any other tuning will be a waste of time...

[TopCatGr58]

More on the carbs! You state a thorough cleaning so we'll take your word for it...that you truly totally disassembled the carbs.

Secondly, your year bike and carbs could be the bastardized sets...due to the model style change from 78-79 to the 80-81 series. You're stating Stock jets, but you're not telling us what actual sizes. The rare bastard 80 carbs use the smaller jets, but have the sharing tunnel between the main and pilot jet towers and so the pilot jet towers use rubber end plugs to direct their flow THRU the main jet tower and shared tunnel. Even with the sharing tunnel, they did use the later smaller main jet sizes of 110-120 vs. the earlier carbs that used 137.5 mains. And some folks will swap different years of carbs on bikes, the jetting is done according to the year of the carb, not the bike.

Also, there are a few different designs of pilot jets, and also different brands, and we have found that the K&L brand although labelled in the same range, the metering hole is much larger and so they run terribly much richer than true Mikuni pilot jets.

T.C.

[1980cm400t]

The carbs were soaked in berryman's b12 chemtool and run through the dishwasher, they are clean enough to eat out of. Using the carb identification thread on this site the carb bodies are the 80-82 type. they do have the older style bowls though. I have not checked for a connecting tube but there were never any rubber plugs in the carbs since I had the bike. The jets installed are 110 mains and 42.5 pilots just like the originals. Since the original post I have gotten a vacuum synch tool. The third cylinder from the left hand side (sitting on the bike) pulls significantly (roughly 1/3) less vacuum than the other three cylinders (the other three were nearly identical). No amount of adjusting the synchronizer screws made the slightest difference to cylinder #3. Thinking that something was wrong with the engine I then did a compression check and found all 4 cylinders were reading ~50 psi and were within 5 psi of each other. I then swapped the main jets and pilot jets from cylinder #3 to cylinder #1 (if i got a defective jet the problem should migrate from cylinder #3 to #1). When the problem didn't move I swapped the diaphragm assembly and needle. That did not make a difference either (I also swapped the pilot screw for good measure with no effect). The next thing I will try will be swapping the choke jet and see if that is causing the problem. Short of the choke jet what else can cause reduced vacuum pressure on the inlet of one cylinder while loading that cylinder's inlet air up with fuel? I am sure cylinder #3 is rich because when I turn off the petcocks with the bike running and let it pull the fuel level down a bit the bike reves perfectly without bogging at all. I should mention that I already checked the float height and all 4 carbs have identical level and are where they are supposed to be.

[DGXSER]

Ok, first things first. If you actually got 50 PSI and not 150 PSI on all four, did you have the throttle wide open when you did the compression check?? That is some seriously low numbers.

Another thing to look for is a vacuum leak. Did you soak the entire carb body in Berryman's?? That could eat the butterfly shaft seals and cause a significant vacuum leak. Try sproying the carb boots and then the carb bodies with carb cleaner, starting fluid, or an unlit propane torch to see if the idle jumps. When it jumps you hit the leak point.

[motoman]

The carbs were soaked in berryman's b12 chemtool and run through the dishwasher, they are clean enough to eat out of. Using the carb identification thread on this site the carb bodies are the 80-82 type. they do have the older style bowls though. I have not checked for a connecting tube but there were never any rubber plugs in the carbs since I had the bike. The jets installed are 110 mains and 42.5 pilots just like the originals. Since the original post I have gotten a vacuum synch tool. The third cylinder from the left hand side (sitting on the bike) pulls significantly (roughly 1/3) less vacuum than the other three cylinders (the other three were nearly identical). No amount of adjusting the synchronizer screws made the slightest difference to cylinder #3. Thinking that something was wrong with the engine I then did a compression check and found all 4 cylinders were reading ~50 psi and were within 5 psi of each other. I then swapped the main jets and pilot jets from cylinder #3 to cylinder #1 (if i got a defective jet the problem should migrate from cylinder #3 to #1). When the problem didn't move I swapped the diaphragm assembly and needle. That did not make a difference either (I also swapped the pilot screw for good measure with no effect). The next thing I will try will be swapping the choke jet and see if that is causing the problem. Short of the choke jet what else can cause reduced vacuum pressure on the inlet of one cylinder while loading that cylinder's inlet air up with fuel? I am sure cylinder #3 is rich because when I turn off the petcocks with the bike running and let it pull the fuel level down a bit the bike reves perfectly without bogging at all. I should mention that I already checked the float height and all 4 carbs have identical level and are where they are supposed to be.

Idle mix screw location(lean/rich) can drop vacuum significantly if notset correctly. From your input, sounds like your actually not even close to syncing and adjusting carbs. Early bowels on later carbs?.... unlikely as they don't fit. Sound like you have fuel levels the same, but what 'same'? If later carbs, that be 23mm. Also, could not tell by your input, but carb three is 'fixed' and others sync to it. Balance one/two carb, then balance #4 to three, then with center sync screw balance one/two pairs to #3. Re-adjust idle mix screws to JUST the highest vacuum on each one. Now go back and re-sync again.

[natemoen]

Idle mix screw location(lean/rich) can drop vacuum significantly if notset correctly. From your input, sounds like your actually not even close to syncing and adjusting carbs. Early bowels on later carbs?.... unlikely as they don't fit. Sound like you have fuel levels the same, but what 'same'? If later carbs, that be 23mm. Also, could not tell by your input, but carb three is 'fixed' and others sync to it. Balance one/two carb, then balance #4 to three, then with center sync screw balance one/two pairs to #3. Re-adjust idle mix screws to JUST the highest vacuum on each one. Now go back and re-sync again.

I think moto has it. You were probably using the idle mix screws and not the sync screws. There are only 3 sync screws because you can only adjust 3 of the carbs, #3 carb is your fixed baseline carb. All others adjust to it.

[1980cm400t]

50 psi is low on compression but the power source I am using to turn the starter is very weak and the engine barely turns fast enough to start. The important part of the compression test is that they were all the same. If I had a hung valve on the problem cylinder that cylinder would read a relatively low compression compared to the other cylinders, and would explain the vacuum issue. The rubber carb boots are brand new so they should not leak. The butterfly shaft seal would cause the loss of vacuum but a leak there would make that carb lean not rich as it would be pulling air around the diaphragm rather than through it (ditto for a leaking carb boot) right? To clarify when I said carb #3 I meant 3rd from the left hand side of the engine sitting on the bike (not facing the front of the bike) which is not the "master" carb it is the middle cylinder "slave" carb right next to it. I am aware of the difference between the pilot screws and the sync screws (there are 4 pilot screws and they are brass whereas there are only three sych screws and they are steel). I did not try varying the pilot screws from cylinder to cylinder though. I just set all of them a 1 and 1/4 turns up from slightly seated. The 1980 carb bodies are compatible with the "old style" i.e. brass hex bolt plug bowls. The original floats in the carbs were also plastic. The fuel level in all four bowls was the same and very close to the gasket level which if memory serves the spec is 3 millimeters from the gasket level. I did not take the time to measure the exact distance as I only looking for a variance from the problem cylinder to the others which would cause that cylinder to run rich. I will see if I can get cylinder #3 to match the others by adjusting the pilot screw, that would explain both the low vacuum and the rich ratio in that cylinder.

[DGXSER]

Ok, honestly, I would still expect higher numbers, but your point is taken that it is equally poor across all four.

As to synching, the main idle screw adjust the butterfly on number three, third from left as you sit on the bike. The synch screw between three and four adjust four butterfly as it relates to threes. The screw between one and two adjust one as it relates to twos butterfly. And the screw between two and three adjust two as it relates to three, bringing one along for the ride.

[petejw]

jat,
have u checked the valve clearances?
just because u have good compression doesn't mean
the valves are adjusted correctly.
what do the spark plugs look like after a good run?
and fwiw if ur running open pipes ull never get
the mid range running right.

[old_chopper]

I've ran mine with open pipes, it was off but ran ok. Yours will probably like some type of restriction better too.
I agree with most it sounds like you have a carb issue. If you start it up and let run for a couple minutes are all 4 pipes equally hot (sizzle test). That would be a good test.
If the trouble cylinder valves are tight you could have good cold compression and lose it as things heat up. Check your valves so you can mark that off the culprit list.
After running until the bad cylinder falls off are the plugs dry, wet, oily? Are the plugs good? They are cheap to replace so you can also mark that off.
I'd also suggest pulling the carbs and bench synch again using a small piece of wire.
Maybe someone near you has a bank of proven carbs you could borrow to test?

[natemoen]

The master static carb is the third carb from the left when sitting on the bike, we are talking about the same one.

[1980cm400t]

The valves and valve springs are all brand new I also had the valve seats machined when I had the engine apart. I managed to get the carbs synced. I had some trouble with the synchronizer tool I got (it is the style with 4 pressure gauges on it) because the needles were jumping around too much to actually compare them very well. I pull the carbs off, manually synced them with a micrometer and put them back on the bike. After restarting the bike all the cylinders read approximately the same vacuum pressure but the bike still will not idle (it is actually worse than before) and it dies and becomes impossible to restart once it gets warm. Also both exhaust pipes are now smoking like crazy. I pulled the carbs off again and verified that they do not have the hole drilled from the main jet to the slow jet. Now that they are all reading the same I am suspicious of the jets and will get some genuine mikuni jets to replace the main and slow jets.

[1980cm400t]

After doing a little research I found out that all genuine mikuni jets have a small box in the corner of a larger box logo stamped on them. All the 110 main jets and 42.5 slow jets in the bike had this logo. So the question now is how can a bike with a stock carb setup and genuine parts be running rich enough to smoke when there is no back pressure on the exhaust and no restriction on the intake. I was under the impression that reducing exhaust back pressure and inlet air restriction would cause a bike with a stock setup to run lean, not rich. I even took the pod filters off and ran it with open velocity stacks (in case the pod filters were too restrictive) and it still exhibited the same issues.

[natemoen]

You need to do an actual vacuum sync. It doeant matter how perfect you set the butterflies when they are off the bike you MUST do a vacuum sync. If the gauges are fluctuating to much then you need to pit resteictors in the vacuum lines to steady it.