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Old 08-22-2002, 04:50 PM
Mike Hart's Avatar
Mike Hart Mike Hart is offline
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Join Date: Jul 2002
Location: Seattle, Washington
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Blaine's XS11 Carb FAQ

Blaine's XS11 Carb FAQ
Work in progress,suggestions and additions welcome

by Blaine Hoopes, last modified 10-16-1999

NOTE: This information comes from my direct experimentation using '79SF, '80SG and '81SH bikes with carbs from '78E, '79F, '80G, '80SG and '81SH (plus similar carbs from XS650s of various years). I have read anything and everything I've been able to get my hands on but I am reporting my experience NOT simply repeating what I have read. Most of the information available on carb tuning applies to carbs that are very different from the emissions type carbs on our bikes so some of my statements will be different from the standard carb tuning info you may have heard previously.

Q: Are all of the carbs on all of the XS11's the same?

A: NO! In fact they are slightly different on nearly every model even if they look the same on the outside. Differences I have encountered are:
'78 - '79 carbs are completely different than '80-81 carbs and few parts are interchangable.
Some models had different needle jets on carb 2-3 from those on 1-4 to richen the mixture for the 2 inner cylinders and keep them cooler. '80G & '81H had larger main jets on 2-3 than 1-4 for the same reason, SG & SH did not.
SG & SH had uncovered pilot jets and do not have the hole linking the pilot jet to the main jet. Putting the rubber caps over the pilot jet on one of these carbs would cut off all fuel from the pilot circuit (neither the service manual nor the Yamaha parts fische indicate this and Yamaha will gladly sell you rubber caps and tell you they are required for your 80SG even though they will completely kill your pilot circuit).

It seems to be a common practice to replace a damaged carb with one from a parts bike of a different model and year. I have found carb banks that have been intermixed like this are very hard to sync since one carb may have larger or smaller drilled holes in the carb body and may even have missing holes that cause a mismatch.

Q: There are 4 hoses going to the carbs on my 79SF but only 2 going to my friends' 80SG. What's the deal?

A: The '78-'79 carb banks have hoses conntected to two "T fittings" (between carbs 1-2 & between 3-4) that allow air to vent into the float bowls. In '80-81 these vents were relocated and don't have hoses or T fittings, they vent through small openings in the carb intake bell.

Q: I have individual filters on my '78-'79 carbs and nowhere to run those float bowl vent hoses. Can I just plug them off?

A: NO! If they are plugged the engine will not run. Atmospheric pressure must be able to reach the float bowls since it's the difference between the atmospheric pressure and the low pressure in the venturi that causes fuel to be drawn up into the airflow.

Q: What is the basic operation of the carbs?

A: If you place a straw in a glass of water and blow across the top of the straw this will create a low pressure (a vacuum) that draws the water up the straw and creates a spray of air and water. A carburetor is simply a more sophistocated version of this. There is a throttle device to control the amount of air flow (and consequently how much fuel can drawn), a circuit for idle and low speed, another circuit for high speed and something to make a smooth transition between the two (slide, needle and nozzle), an enrichener for cold starting, float valves to maintain a steady level of fuel etc.

Q: What does constant velocity mean?

A: When you open the throttle the engine draws more air through the carb. A large quantity of air must move faster being drawn through the same diameter venturi than a smaller quantity of air does. In a non-CV carb the air would be speeding up and slowing down in the venturi depending on the demand of the engine. In a constant velocity carb the venturi automatically gets smaller when the engine isn't drawing much air and gets larger when the engine draws more air, this keeps the velocity (speed) of the air close to the same at all throttle openings and provides a more consistent metering of fuel in the venturi. It is "supposed" to give better fuel economy, cleaner emissions and adjust itself to altitude changes. Most tuners dislike constant velocity carbs and use round or flat slide carbs in which the slide is raised directly by the throttle cable instead of indirectly by airflow as in a CV carb.

Q: What is a jet?

A: A jet is device that meters a precise amount of fluid (technically, air is a fluid too). The main jet is a very simple jet consisting of a single hole of a specific size in a brass plug. The needle jet (aka nozzle, aka emulsion tube) is much more complex being a tube with a hole of specific size in one end for a tapered jet needle to slide in and several holes down the length of the tube to allow air to bleed through and mix with the fuel as it's drawn up the tube. This jet adjusts over a wide range by the movement of the tapered jet needle.

here are jet pictures for reference

a air jet - b main jet - c pilot jet (some have different # of holes)
d various needle jets (emulsion tubes) - e two styles of float needle valves and float needles

Q: What does the slide needle do?

A: It's called a "jet needle" and when the needle is fully down it plugs up the needle jet (the long brass tube the needle slides in) and doesn't allow any fuel/air mixture to be drawn into the engine from the needle jet. The needle is tapered and as it pulls out of the needle jet the tapered section allows more and more fuel/air mixture to be drawn out of the needle jet. Raising or lowering the needle clip changes the point where the taper starts to open and close the needle jet. The shape of the taper affects the mixture through the midrange as the throttle is opened. A short, steep taper will change from fully closed to fully open in a short amount of needle travel. A long gradual taper will gradually open the needle jet over a longer amount of needle travel. The needles in our carbs have two tapers, one that handles about the first 3/4 throttle and a steeper taper for the final 1/4 throttle opening.

Q: I've heard people say to raise or lower the needle but mine doesn't seem to have an adjustable clip that they mention. How do I do it?

A: '78 -'79 have 5 notches on the needle for adjustment, '80-81 do not. To adjust an '80-81 needle up (richer) you place one or more thin washers under the clip (between the clip and the plastic doughnut). To adjust these needles down (leaner) you need to machine the doughnut thinner or remove the doughnut and use a stack of washers instead. The doughnut has a pin on the bottom and a ridge so the sanding/filing should be done on the top of the doughnut and the bottom left alone. Use calipers or micrometer (and great skill) to make sure all four doughnuts are consistent. Once the doughnut is thinned you will need to use washers if you decide to raise the needle. Here is a picture of a needle from an '80SG with the doughnut thinned to allow two washers below the clip, I also left two washers above the clip so I can adjust either way without searching for washers.

Needle (jet needle)

Q: What is the screw on the top of each carb for?

A: This is the idle mixture screw. Some carburetors have an "air bleed" screw to adjust the idle and pilot circuit mixture. Most tuning manuals discuss adjusting the air bleed screw and don't mention the type of idle mixture screw we have on our carbs. On stock XS11 carbs this screw only affects idle and very little else. It meters a preset (set by pilot jet & pilot air jet) mixture of fuel and air through a small hole in front of the butterfly valve. This air/fuel mixture is further mixed with air leaking past the slightly open butterfly valve to produce the idle mixture. Turning this screw clockwise leans the mixture, turning it counterclockwise richens the mixture. Most tuning manuals will tell you that the mixture screw affects the first 1/4 throttle range, on our carburetors this screw affects only one of four holes in the pilot circuit and has almost NO affect once the throttle is opened enough for the other three pilot feed holes to come into play. I prefer to tune the pilot circuit by changing the pilot jet or pilot air jet and then adjusting the idle mix screw for proper idle mixture.

Q: My bike backfires when I coast to a stop in gear with the throttle closed (called over run, engine braking etc.) is this a carb issue?

A: It could be. This usually means too lean of a mixture or sucking air through a leak in the intake manifold, vacuum hose or exhaust mainifold. If it only happens with the throttle closed and you don't seem to be leaking air through the vacuum ports on the manifold boots or the vacuum port on carb #2 you can probably cure it by turning your idle mixture screws out a half turn or more.

Q: Why do I need to "triple clean?"

A: I never heard the term "triple clean" before joining the XS11 list and personally I think it's nonsense. If you get it clean the first time you don't need to triple clean but these carbs are 20 years old and have many places for gum and varnish to accumulate. Many people have found that they get a little more crud out each time they clean and I guess 3 is the magic number?

Q: Why is it important to balance the floats each time I work on my carbs?

A: The floats control the fuel level in the float bowl. Most of the things we can adjust in a carb affect only a narrow range of throttle but the float level affects all ranges of throttle so it is very important to have all four carbs set to the same float level.

Q: Do I need to adjust how far the floats drop to let fuel in?

A: Yes! If the floats drop too far into the bowl they are likely to bind on the side of the bowl. The floats should be adjusted to drop just enough to open the valve sufficiently to keep the bowls full. You can test that the floats don't stick by holding the assembled carb bank in your hands (no fuel in the bowls) and blow into the fuel line (you should hear the air going through the float valve into the bowls). Now slap or tap the float bowls a few times and very slowly roll the carbs upside down without shaking or jolting them. You should NOT be able to blow any air through the fuel hose now, if you can then one of the floats has probably gotten wedged against the side of the bowl and is stuck open.

Q: Fuel leaks into the airbox. What can cause this and how do I fix it?

A: Many people "fix" this by shutting off their fuel petcocks or rigging up an inline fuel shutoff valve as a patch for leaky petcocks. I believe the floats must be able to stop the flow of fuel properly even if the petcocks are left on prime. This is the only way you can be sure the floats are maintaining a consistent fuel level. The floats are supposed to do exactly that, float, as the fuel rises in the float bowl. As they rise they press against a needle and push it into the needle seat to stop the flow of fuel into the bowl. If the float doesn't float or is adjusted incorrectly it won't be able to push the needle tightly enough to stop the flow of fuel. If the needle doesn't fit correctly in the seat (corrosion, chips, worn, dirty) it won't be able to stop the flow of fuel no matter how hard the float pushes it. If fuel is able to leak around the seat it will continue to flow even if the float is working correctly and the needle is sealing in the seat. '78-'79 carbs have a fiber gasket under a threaded seat, '80-'81 carbs have an o-ring around a "slip in" seat. This o-ring can get loose and allow fuel to leak around the seat.

Brass floats can develop leaks. Test them by plunging them into boiling (or near boiling) water, if there are leaks you will see a stream of bubbles coming from the leak. I've had good luck soldering up the leaks but the recommended fix is to replace them. (NOTE: If you hold a float in boiling water for several seconds it will heat up all the air inside and no more bubbles will appear, just cool the float by running under cold water or let it sit in the air and then plung it back into the boiling water for a fresh stream of bubbles from the leak)

Brass floats can be caved in by squeezing them too hard or by using compressed air on an assembled carb, if they are caved in they won't float as high and the fuel level will not be correct. Verify that all the floats have the same boyancy by floating them in a tray of water.

Another cause of overflowing carbs can be floats sticking, usually against the float bowl itself. When reassembling carbs after cleaning/adjustment it's a good idea to test the floats before and after putting the float bowls on. I like to hold the carb bank upside down and blow into the fuel line (careful not to blow fuel into your eyes) and lift the floats and hear the air start and stop as the float is lifted and released. I roll the carbs from upside down to rightside up while blowing and verify that the floats operate correctly. Then after putting the float bowls on I test this again, frequently one or more floats will stick with the bowl on even though it worked perfectly with the bowl off. This indicates the float is rubbing against the bowl and you may need to bend it a little (but keep the float level correct) so it can move freely with the bowl in place.

Someone posted recently that they had their air and fuel lines misplaced. On the '78-79 carbs there were T-fittings to vent the float bowls with hoses going to the airbox. These fittings look just like the fuel fittings and will fill the carbs but have no valves to stop the fuel flow.

Q: What does the pilot jet do and what effect does changing to a larger or smaller size have?

A: The pilot jet meters fuel from the float bowl into 4 tiny fuel feed holes in the carb throat that feed the engine at small throttle openings. Our pilot jets are an air bleed type and allow air to mix with the gas before entering the carb throat (where it mixes with more air). The reason to mix it with air first is to help atomize it but more importantly it dilutes the fuel with air so it is easier to meter precisely where it enters the airflow in the carb throat. Larger pilot jet means richer mixture at idle and first 1/4 thottle, smaller pilot jet gives leaner idle and first 1/4 throttle. (On '80-'81 specials the pilot circuit stays active over more of the throttle range than other models)

Q: What does the main jet do and what effect does changing to a larger or smaller size have?

A: Main jet size affects the mixture mainly from 3/4 open to wide open throttle but does affect the mixture a little at all throttle settings above idle. The main jet should be selected for proper mixture at wide open throttle and other adjustments should be made for mixture at low and mid throttle settings. Larger main jet gives richer mix, smaller main jet gives leaner mix.

Q: What does the pilot air jet do and what effect does changing to a larger or smaller size have?

A: Pilot air jet bleeds air from the intake bell to the pilot circuit and dilutes/atomizes the fuel before it reaches the 4 holes in the carb throat. Larger pilot air jet gives leaner mixture to pilot circuit and will deliver more mixture so idle speed will increase, smaller pilot air jet gives richer mixture to pilot circuit and will deliver less mixture so idle speed will decrease. picture

There is a subtle difference between changing air jet or fuel jet to adjust the pilot mixture. If you richen the pilot circuit by decreasing the size of the pilot air jet you will need to open the butterfly slightly to achieve the same idle speed and this will begin to expose the three pilot holes right at the butterfly. If, instead, you increase the size of the pilot jet to acomplish the same richness you will need to close the butterfly a little to achieve the same idle speed and cover the three pilot holes a little more. In other words richening the pilot circuit by decreasing the size of the pilot air jet speeds up the transition from idle to off-idle. Riching the pilot circuit by increasing the size of pilot fuel jet slows down the transition from idle to off-idle.

Q: What does the main air jet do and what effect does changing to a larger or smaller size have?

A: This jet is pressed in and cannot be easily replaced. It does tend to get gummed up and must be cleaned very well. This jet bleeds air into the needle jet to emusify the fuel and dilute it for more precise metering by the jet needle. Larger air jet gives a leaner mixture and smaller (or plugged up) jet gives richer mixture.

Q: Other Jets?

A: The starter jet controls extra fuel that is released to richen the mixture for cold starting. The needle jet is the long tube the slide needle goes inside, this is often called an emulsion tube and the service manuals call it a nozzle, Mikuni refers to it as a needle jet most often. The small hole drilled into the carb body that the idle mixture screw goes into is a jet though it isn't called one usually, and the three small holes just behind the butterfly valve are jets (in my opinion anyway) in that the size and number of these holes controls the mixture when they are active (at just off idle). I suppose you could even call the small hole in the slide (that allows vacuum to raise the slide) could be called a jet of sorts since the size of the hole affects how quickly air can flow and how quickly the slide moves. This is a stretch and they are never refered to as "jets" but they do control things based on the size of the opening.

Q: What are those long brass tubes above the main jets, what are the holes down the side for and why are they mismatched in my carbs?

A: They are called Nozzles, Emulsion Tubes or Needle Jets (not to be confused with needle and seats in float bowl) depending on which manual you read. The holes down the side of the tube allows air that bleeds in from the main air jet to mix with fuel being drawn up the tube and whip the fuel into an emulsion (hence the term emulsion tube). The manual only specs X-2 for this jet in all years/models but there are at least three different tubes with different number of holes found in various models. More or larger holes lets more air in makes the mix leaner, less or smaller holes lets less air in and makes the mix richer. Certain years have less/smaller holes (richer) and also have smaller main jets (leaner) to balance it out. Some models mixed two different needle jets having less holes on 2-3 and more holes on 1-4, probably to richen the mixture on the inner cylinders for better cooling. The numbers I've seen are 266 with 16 holes in '78 and some '79, a combination of 266 and 301 (12 holes) on some '79, 300 (4 large holes & 2 small holes) on '80 and '81.

Q: Do jets wear out?

A: In general they don't but the needle jet and jet needle can. The needle should be held by the plastic parts in the slide so that is can't spin around in the slide, if it is loose enough to spin around it can wear out and become sloppy in the slide. If the needle is loose enough in the slide to rub on the needle jet it can wear out the jet. Using anything even mildly abrasive to clean a jet can change it's dimensions and it doesn't take much to radically alter the mixture. I had a set of carbs that would always soot up the sparkplug in one cylinder only, no matter how many times I cleaned, checked and adjusted everything. Finally I discovered that the needle jet in that carb had a very slightly enlarged hole (where the needle slides in). The needle was not worn or loose and the jet looked like it had been really gummed up and cleaned agressively by a previous owner - I suspect the PO had used fine sandpaper or steel wool or something to clean it with - at any rate the amount that the hole was enlarged was very slight (.2mm maybe) but it was enough to make the low and mid range very rich. Replacing the jet solved the problem.

Q: Why does my '80-'81 Standard have rubber caps over the pilot jets but my '80-'81 Special doesn't. Why does my '80-'81 Standard have larger main jets than my '80-'81 Special? Are these two things related?

A: I can only guess "why" Yamaha did this. With covered pilots there is a fuel feed between the pilot jet and the main jet so fuel must go through the main jet to get to the pilot jet. Once the throttle is opened enough to lift the needle and allow fuel through the needle jet the pilot circuit gets very little fuel and no longer affects the mix. With uncovered pilot jets there is no hole linking main jet and pilot jet and the pilot jets suck fuel directly from the float bowl and continue to deliver fuel to the engine at all throttle openings. I believe this is why the main jets are smaller on the Special, to compensate for the extra fuel being delivered by the pilot circuit at all throttle openings.

NOTE: recently someone posted to the list that he had two '81 specials and both had covered pilots. I have not seen this myself but it's possible Yamaha released different versions of the carbs. All the '80-81 specials that I have first hand knowledge of had uncovered pilots.

Q: Why should I leave the throttle alone when starting a cold engine?

A: Our "choke" isn't really a choke (it doesn't block off the air intake), it is an enrichener circuit. It adds a rich fuel/air mixture through a port on the engine side of the butterfly valve. If the butterfly valve is open (throttle twisted) there is less vacuum to pull the enriched mixture from the port and there is also more air coming in past the open butterfly which mixes with the starting mixture and leans it.

Q: Where can I get spare parts for my carbs?

A: I've found that XS11 carbs are in short supply at bone yards and some of the parts are not available or very expensive at the dealer. I discovered that XS650 carbs share many of the same parts and there are zillions of XS650's lying in boneyards. 1980 and newer XS650 carbs use the same slides and diaphragms as '80-81 XS11, I believe (but haven't tested yet) that '78-79 XS650 carbs use same slides and diaphragms as the '78-'79 XS11. The needle jets and jet needles are different as are the springs. The choke plunger is the same, pilot jets are the same, idle mixture screws are the same. Needle, seat and floats are the same but the XS650 didn't get the black plastic floats until '81, XS11 had them in '80. XS650 is also a good source of chrome tops for the inner two carbs on '80-'81 XS11.

Q: My bike doesn't run very well, what size jets should I change to?

A: I've been on several mail lists and nearly everytime someone complains about their bike not running well someone else will suggest they put in larger main jets. The general thinking seems to be that since most of us have aftermarket exhausts and air filters we need to put in larger jets. My personal opinion is that most people just screw up their mileage and performance by rejetting and unless you have changed compression ratios with new pistons or combustion chamber mods, or put in different grind cams you really don't need to rejet. You can make a significant difference with 1-2 mm float level changes. Raise the level of fuel in the float bowls and you will richen up the whole mix, lower the level in the bowls and you lean up the whole mix. Also, if you start rejetting to solve a problem that is actually caused by something gummed up or leaking (or even an ignition or cam timing problem) you have thrown in a whole new set of variables that make it really hard to find the real problem. I say leave the jetting stock, get the bike running correctly and adjust the floats to get the spark plug color right.

Q: What about the springs that push the slides down? Do they wear out?

A: The short answer is that stiffer springs seem to cause the mixture to be leaner and softer springs seem to be richer. I believe they do soften up over time and I think it's important that all four carbs have matching springs. Here is my experiment:
I have several XS650 carbs as well as XS11 carbs. Many of the parts are interchangable. I noticed that the XS650 slide springs are about 10mm shorter than the XS11 springs (both from '80 models). I have been fighting an "off idle" stumble on the XS11 since I've owned the bike. I got to thinking that shorter springs would let the slides open quicker and probably improve the performance (WRONG!) -- it's a good thing I had the shorter 650 springs to try instead of cutting my XS11 springs shorter. I discovered, to my surprise, that the engine didn't run well at all with the shorter springs - it wouldn't take any throttle, just sort of gagged on it unless opened very gradually - it seemed to exaggerate the problems I had been having before. So, I decided to try lengthening the springs instead and started wondering how much tension they might have lost being compressed for 20 years. I carefully stretched the stock springs out about 8mm longer and reassembled the carbs. It accepts the throttle much better than it ever has. NOTE: my bike has low restriction air filters and exhaust and perhaps this is unique to my setup and not something required on a stock system. My impression is that the springs had softened over time and were allowing the slides to lift too quickly, causing an overly rich mixture at low rpms - but it may just mean my exhaust setup requires slightly stiffer springs than stock.
Mike * Seattle * 82 F'n'XJ1100 *

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