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Carb Removal and Rebuild ( ST1100 ) *

Started by KoTAOW, August 03, 2014, 09:33:44 AM

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Submitted by Adam Frymoyer, STOC #949


Carb Removal and Rebuild ( ST1100 )


If your reading this, you’re probably in trouble with your carburetors. I hope you checked everything possible and performed a proper diagnosis before digging into this project. I’m a diesel engine builder by trade and have owned many bikes over the years. Primarily ST1100’s. I've also worked in a friends motorcycle / small engine dealership for a good number of years and have tuned, rebuilt and repaired everything under the sun.  I've been doing this for a long time and hope that this will give some guidance on ST1100 carburetors. Remember that this is only a guide (and my opinion) to rebuilding ST carbs. My pictures and procedures may seem vague, but remember, this is only for a little re-assurance. Please follow the factory service manual for the proper way to work on your bike.

This particular bike, I acquired from an estate sale. I don’t know the history of the bike, other than it sat for a long time. The previous owner never took care of the bike and it sat in a shed for 4 years while his estate was settled. It started right up and I drove it home. Soft on power, but it ran good.  With my busy schedule, it sat some more. Then the inevitable happened... The bike would fire right up, but only run on choke. The idle speed, slow speed and transitional circuits in the carbs were plugged. Time for a good carb cleaning and other maintenance while the carbs are out.

First things first, is to remove the bodywork needed to gain access to everything you will be doing. Seat, bags, top shelf, pocket panels and maintenance covers.

Next, will be removing air cleaner assembly.

Remove choke / enrichment cable and lay off to the left side of the bike.

At this point, some guys prefer to remove the fuel tank for easier access to the carbs. I never do. Just my opinion that it’s just more stuff to take apart and put back together.

Next will be loosening the upper carburetor isolator / boot clamps. I started with #3 only because I was on that side. (right rear carb)  Use a brand new or a very good condition #2 Phillips screwdriver. Best to use a JIS driver if you have one, or purchase some early before you start. The screw heads are very soft and strip easily.

Next will be moving up to #1 carb. This is a tricky one. The clamp is usually loose and spun. It’s difficult to see and access. You may have to take another longer screw driver and rotate the clamp back into position to get at the Phillips screw head. I took the best pictures that I could, showing the angles of the driver and how to access. You’ll also have to move the coolant hose and wiring harness out of the way to get the driver down in there. You’ll also need a good bright flashlight to shine down from the top to see the screw head. As difficult as it seems, you should be in and out of there in less than 5 minutes.

After #1 is loose, I moved over to #2. Left front carb.

Then to #4, left rear. This one can be a little tricky too, getting the screwdriver between the frame and cylinder head. Again, use another driver to spin the clamp to get a good bite on the screw head.

I’ve had this pry-bar for years, and it’s great for popping the carbs up and out of the boots. Don’t pry on the float bowls, linkages, etc. I pry “up” on the bottom of the plenum. Use the frame crossmember in front of the fuel tank to pry “down” on. Sometimes you’ll have to give it a good heave, but they’ll pop right out.

Next will be removing the throttle linkage. It’s best to wait until the carbs are popped up and loose, so you can get at the screws easier. The bottom screw is blocked with the carbs installed.

Then tilt the carb bank off to the right side and disconnect the float bowl drain hoses. You don’t have to slide the clamps down. The hoses will pull right off the nipples.

Disconnect the fuel supply hose from the vacuum / fuel shut down valve (if equipped). Just pull the hose going to the carbs. Leave everything else there to prevent fuel from spilling all over the place. Upon re-assy. I will be removing this valve and throwing it away.

After this is done also take the idle speed adjust cable and lay it over on top of the carbs now that the fuel lines are out of the way.

Move back around to the front of the carb bank, tilt the carbs upward and remove the fresh air intake hose going to the PAIR valves. You will more than likely have to cut it. The heat down in there welds / melts the rubber to the fitting. I had to cut mine.

The carbs are totally disconnected at this time. Pull them out. See all the mouse pooh and pee down in there? I removed the rubber heat shield and cleaned. While the carbs were out, I also removed the PAIR valves and replaced all the coolant o-rings, seals, hoses and clamps. Something to strongly consider while you have everything out.


Carb Removal and Rebuild ( ST1100 )


Now that the carbs are out, I always drain the float bowls into a clean container to see what, or if, anything comes out. I used the tailgate of the truck as a work bench to drain OUTSIDE in a well ventilated area. As you can see in my pic’s, the bowls are full of debris. Trouble....

The outsides of the carb bodies are filthy and need a good cleaning. Now is the time to do it. I used a whole can of carb cleaner and a blow gun / air compressor to clean them up. See the difference? Always start off with clean outsides so none of the junk gets into your cleaned carbs. Cleaning them will also help with the linkages, etc.

Now that the carbs are drained and cleaned, I can take them inside to the shop and start dis-assembly. First thing I’m going to do is mark EVERYTHING with it’s prospective cylinder #. Some thing’s I’m going to replace. Most of it will be returned to it’s original carb body, keeping all of the original factory mating surfaces the same. You’ll see I've also marked everything up top on the plenum and the intake snorkels. The top side is also coming off. Some guys don’t, but you’ll see why later-on that it’s best to remove it.

I started with taking off all of the float bowls. YUK! I’ve done countless carb rebuilds, but have never seen anything like this. Funny thing is, that the bike ran great, right up to the point to where it wouldn't.

After the bowls are off, I removed all of the floats and checked to see if there was any leakage. Shaking them to see if there was any gas on the insides. All checked OK. I cleaned them and set them aside. I also took a real good look at the float needles. Many times the rubber tip gets worn and grooved. LOOK THEM OVER GOOD. If grooved, throw them away and get new needles and seats! Surprisingly, mine were OK and cleaned up well. I take a shop cloth and spray carb cleaner on the towel, then gently twist the tip of the needle in the cloth. Cleans them up well.

Next, I remove ALL of the jets. Float seats, idle mixture, slow speed / pilots, main jet and emulsion tube’s. Check the screens on the Float needle seats. Clean as needed. I have both a homemade idle mixture screw tool and a Honda factory tool. Homemade one gets used on the bench. I counted the screw turns from lightly seated. All were between 2 and 2 ½ turns from the factory. It doesn't matter to me what they are, because I will be doing a jet change and the idle drop procedure when done.

Now that all of the bottom side is disassembled, I’ll move to the top and take all of the upper components out. I start removing the diaphragm’s / slide’s and inspect the slides for wear. Also looking at the rubber diaphragm’s to see if they are in good shape, not torn, or dry and brittle. Mine checked out OK, but had some weird white stains on them. Probably from sitting for a long time. Most of it cleaned up. Also have a look at the needles to see if they are worn / scuffed from possibly rubbing.

As you can see here, it is important to check and replace the secondary air breather filter regularly, just in front of the air cleaner assy. The breather filter on this bike disintegrated and filled the upper diaphragm chamber’s with foam filter material. Which also means particles went down into the carb vent, air-cut valve assy., choke vent and slow speed vent.

I now have a nice pile of carburetor parts. All numbered and placed separately with each corresponding cylinder number.

As mentioned earlier, some guys don’t take the upper plenum / tube assy. off when cleaning their carbs. It is relatively easy to remove it, but a pain to re-install. My opinion is that it must come off, or damage to the air-cut valve diaphragms, sooner or later, WILL happen. I've worked on quite a few bikes that were having fuel economy, idle speed, drivability and “running rich” conditions. All do to the “Shade Tree mechanic” destroying the air â€"cut valve diaphragms with carburetor cleaner, by NOT removing them. To get the air-cut valves out, the upper plenum must be removed.

Also take note that the plenum mounting screws are #3 Phillips. Using a #2 may work, but more than likely they will strip out. The screws are tight from the factory.

You’ll notice here that there are centering dowels on the bottom of the plenum. The screws go down through the center of them. Sometimes they fall out and get lost. Use caution when lifting up the plenum.

There are 8 dowels. I circled the locations in the carb bodies the best I could.

Now that the plenum is off, I can remove the air-cut valves. Sort of a challenging task, but can be done without breaking the carbs totally apart (major pain). I’ve taken the ST carbs totally apart once and vowed to never do it again! Linkages, springs, tubes, alignments, o-rings, etc.

There are two valves facing on the outer bodies of the carb’s. I take them off first. Then move onto the inner ones.

The inner valves upper screw can be removed with a #1 Phillips at an angle. Lots of inward pressure is needed so the screw doesn't strip, but breaks loose easily. For the lower screw, I use a 90 degree Phillips. Trust me, it can be done. I have to hold the throttle linkage out of the way for access. I used an old license plate wrench to hold the linkages open.

Check the diaphragms for dry rot, cracks, condition, etc... There are o-rings that will stick to the inner body of the carb. Don’t loose them...

I now have totally stripped carbs and ready for cleaning.


Carb Removal and Rebuild ( ST1100 )


Now that the carbs are completely stripped and safe to clean, I prefer to do them outside for ventilation. I’m going to start with the topsides of the carbs. Spraying into every hole I see and blowing out with compressed air. All 3 of these vent holes go to something and serve a purpose. Make sure they’re free and clear.

Next, I move onto the diaphragm chamber area. Same goes here. All of these holes serve a purpose and need to be cleaned out with carb cleaner and compressed air. Vents for floats, choke, pilot and air-cut circuits.

Next, I’ll move onto cleaning the air-cut valve passageways and plungers. Remember that this is also a part of the pilot / slow speed circuit and it controls the pilot mixture during deceleration.

This last picture is the air-cut plunger. I spray cleaner at it and move it in and out gently with a pick to make sure it’s not sticky. I also go easy with the compressed air.

All passageways and components are very important, and all of them work together. However, this is the most important part. The slow speed / pilot and transitional circuits are the first to get clogged and the hardest to clean, being so small in design by Keihin. These are the “first to go”. Keep in mind that the pilot circuit also supplies the idle mixture. If one goes, everything down line suffers.

Back flush / spray out the idle mixture hole in the mouth of the carb. Then spray into the mixture screw hole. VERIFY that carb cleaner is coming out the other side and at a good stream. Blow them out with compressed air and continue to verify that passages are clear and a full stream of cleaner is coming out! It may be necessary to do multiple times.

This next step is also critical. Get something to hold the throttle linkage wide open. Now I’m moving into the pilot circuit. I spray carb cleaner up into the pilot jet screw hole multiple times and look for it to come out of the idle mixture and the 3 little transitional slow speed / pilot holes at the throttle plate. VERIFY THAT CLEANER IS COMING OUT OF ALL 3 OF THOSE HOLES! Con pilot ports!

Next, I’ll flip the carb bodies over and spray out the choke / enriching supply(hold the choke open), pilot (again), main circuit, fuel bowl supply and upper half of bowl chamber. Verifying cleaner coming out the other ends with compressed air.  

The carb bodies are now done and I’ll be moving onto the bowls, jets, etc. I always try to be creative when cleaning out these little jets, and pieces. Some may think it’s un-orthodox, but it’s quick, easy and gets the job done without ruining anything. Very small vinyl pipe cleaner set, Q-tips, small strands of copper wire, etc. I won’t be cleaning out the pilot jets, because I’m replacing with new ones (bigger #40 sized). Later I’ll be tuning the bike to pre-emission Canadian spec’s.

I always make sure all the little holes in the emulsion tubes are free and clear using cleaner and compressed air. The rest is pretty much self explanatory.

At this point, EVERYTHING has been cleaned and verified. I can now start assembling the carbs.


Carb Removal and Rebuild ( ST1100 )


Assembling the carbs is pretty straight forward. Pretty much just “reverse order” of taking them apart. The first thing I do, is put the plenum / tube assembly back on the top. This gives me the rigidity needed for final assembly. I loosen the pivot screws just a little and the 4 bottom bracket screws that join’s everything together. This will give me a little “wiggle room” for alignment.

Setting the plenum down on top and getting the dowels to align can be a challenge. I always get one side of the dowels “just” started on one side, then PATIENTLY wiggle the other side down in. Bending and flexing the carb bank back and forth until all the dowels are in. It will take multiple times to get it set! Do not use the big plenum screws to draw it down, or a hammer. This will only bend, ruin and destroy the threads and dowels. Once the dowels are started on all 4 carbs, carefully put in the big plenum screws and only start them several threads. Check and make sure all of the rubber intake tubes are aligned properly with their prospective vent holes and vent tubes! Do this before tightening down the plenum. Once everything is aligned, the plenum will push right down into place. Then lightly snug the big plenum screws. I then tighten the pivot screws and bottom bracket. Then go back and final torque the big plenum screws and bend the tabs back over the screw heads.

I then MAKE SURE that the choke and throttle linkages operate smoothly and everything is aligned and secured.

Because I removed the PAIR system, I glued on a plastic cap to the breather nipple.

Flipping the carbs back over, I install the float seats. Because I’m re-using the needles and seats, I’m going to make sure I have no problems after assembly. I use Q-tips in a cordless drill with a little dab of metal polish on the Q-tip. This polishes the seating surface for the needle, making it like new. Remember that I cleaned the needle tips earlier.

Next, I will be installing all of the jets, etc. It’s pretty straight forward. Just remember NOT to over torque anything. Brass jets going into aluminum threads.... I start off with the pilots. I’m showing some differences between the Honda OEM #38 pilots, verses the #40 Keihin jets. Big difference.

REASONS to change the PILOT and MAIN jets:
To meet strict emission laws, the factory really has to run the mixtures lean. This is done at idle and at low rpm. The bike is on its kickstand or centerstand. An exhaust analyzer goes up the tail pipe for testing at low rpm. To meet these regulations, the factory sets up the carbs to pass Federal EPA standards.

Some states such as California, have even stricter regulations. For Honda or any other manufacture, the idle mixture and pilot circuits at set very lean to meet these regulations. Example all ST1100 49 State bikes have #38 sized pilots and #128 main jets, while the California and ABS bikes have a #38 pilot and #125 main jets. All US bikes got PAIR valves. If you look in the service manual, you'll see that non-emission countries (at that time) receive #40 pilot jets and #128 mains with a different idle mixture screw count AND no PAIR valves. After some experimenting and goofing around with different jets and settings, I've found that the pre-emission Canadian specs are the OVERALL best tune for the ST1100. Overall meaning, the best fuel economy, with the best power output and smoothest running. It's always a compromise with carburetion, that's why technology has brought us fuel injection(fuel injection not absolutely perfect either). This is the reasoning behind me changing the pilots to a bigger size and tuning the idle mixture screws to match their fuel rate.

The ST1100 responds VERY well to the upgrade, pulling much harder down low (off idle) and fantastic mid-range power. The main jets do not get changed because it carburets perfectly with the #128.  California and ABS bikes respond very well to all of the changes, including changing the mains from #125 to #128.

Emulsion tubes, main jet’s, etc, get put back in. The Idle mixture screw settings have been somewhat of a controversial topic in the ST community over the years. If in doubt, refer to your manual. Some guys count the screw turn’s from lightly seated during dis-assembly, then put them back in the original factory settings when putting them back in. Some guys take the average of the screw turns and set them ALL at that average. The most common being around 2 - 2 1/2 turns out. Another thing is that hardly anyone has the special tool to do the idle drop procedure. None of them are wrong, BUT all of them are not right either. Many have just “set-it and forget-it” and have successfully run many thousands of miles and all end up with a fairly good running bike(not a great or perfect running bike). To each his own, and I will just leave this topic as another man’s opinion. I’m starting from scratch with the new pilots, so I set mine all at a default of 1 7/8 turns (Canadian spec.). I’ll be doing the idle drop procedure later to finalize the tune.

Next, I’ll install the float needles and floats. In the picture you’ll see the two hoops on the retainer. They get positioned facing the center “V” of the carb bank /and or / float pivot pin. This allows the retainer to sit deeper in the float tab for a more secure fitment. There is a special way these are positioned. If you get it wrong the retainer just barely contacts the end of the float tab. In my opinion, there is a chance for it to fall out.

Next, I’m going to set the float level. As with anything else, the float level is very important. All of mine measured different from the factory. Factory spec is 7mm. 3 of mine were at 8mm and one was at 9mm. Very lean emission settings. I set them all to 7mm. Equivalent to .275” if using a dial caliper.

As seen in my pictures, the measurement gets taken from the side flat of the carb body to the outer top  seam of the float. This is how the factory does it, and so do I. If adjustment is needed, I use a pick, and either bend the float tab up, or down, to get the correct measurement. I also have the carb bank tilted up, so the floats won’t depress the springs in the float needles. The needles are lightly seated to get an accurate measurement.

Next, I’ll install the float bowls. The bottom side is completely done!

Onto the top side, I’m going to check and see if my carbs have needle shims installed. Some ST’s do, some don’t. Depends upon the emission origin of the bike. After taking the needle’s out, mine have them. However, they are thinner and don’t measure consistently. I’m replacing with .020” Keihin aftermarket shim’s. This will allow the carb’s to get on the main circuit quicker in the mid-range rpm.

A #1 Phillips fit’s down in the needle retainer. This does not get un-screwed. It is a cam design. Turn the screw driver one click counterclockwise (about 45 degrees). The needle retainer will pop up and can be removed. If equipped with a shim, it will stick down in there. A longer pick, or a piece of wire will get the shim out. Reverse order for assembly.

Now that the needles are all set, I can finish assembling the top side. All of the slides / diaphragms, springs and covers go back in. I always have a good look around and see if I missed anything. If it all looks good, DONE !


Carb Removal and Rebuild ( ST1100 )


Now that everything is assembled and checked over on the carb bank, I can now re-install the carbs back into the bike. It’s pretty much the same as before in doing the “reverse order” for the install. I’ve replaced the carb isolators / boots and have re-installed the black rubber heat shield.

When replacing the boots, don’t forget to install them with the raised tangs “facing out” as seen in the picture. This will aid in setting the carbs down into the boots. There are also notches at the bottom of the boots that have to be placed into the tab on the intake manifold. At this point, also rub a LIGHT film of grease, Vaseline, die electric paste, etc, around the insides of the boots, and on the outsides of the carb flange’s to also aid in installation. This must be done!

I then hold up the carbs down in the valley and hook up the float drain hoses. They push on easily. Make sure that the cut-outs in the rubber heat shield stay in their correct position.

I then let the carb bank sit inside the valley (back side tilted up) and hook up the throttle cables.

Getting the carbs into the boots is super easy. There is no need for crow-bars, broom handles, greased plexi-glass, bungee cords, ratchet straps, rubber mallets, etc. Here is the trick: Earlier, I’ve already lightly coated the insides of the boots and the outsides of the carb flanges with grease. I set the RIGHT side of the carb flanges fully down into the boots as you can see in the picture.

The LEFT side as you can see, is just sitting inside of the raised boot flanges.

I then take my right hand and put it centered on the carb intake plenum. With a GOOD FIRM push down, they will “snap” right in place, into the boots. Works every time.

I will then tighten up all of the carb clamps, install choke cable and other top end components. I am also  eliminating the auto vacuum fuel shut down valve. These are well known to fail at any given time. Leaking vacuum, or leaking fuel. I went 85,000 on my other ST without the valve, so this one gets the same treatment.

See that BRAND NEW HONDA OEM FUEL FILTER? I strongly recommend that one be used. I’ve seen lot’s of carb float bowls and jets with silt, sand, debris, rust, etc, by using universal aftermarket “screen” type filters. Not to mention debris stuck between the float needles and seats. I've just spent several day’s cleaning and setting up the carbs properly. No need to ruin the job by using a universal fly-by-night, junk fuel filter / screen. Just sayin’.....

The OEM Honda filter is $12 at my dealer and gets replaced every other year. Take pride in your work and don’t let ANYTHING get into those fresh carbs! Also consider a new air filter. Preferably the Honda OEM. The carbs are jetted to work with this air filter. Anything else may change the mixture and / or allow more grit into the intake.

Next, I always double check that everything is tight and installed. Check the linkage and cable operation for throttle and choke.

Now that everything is hooked up, I can start it up and check for leaks, etc. It runs like crap when first started. It always will. I just disturbed all of the carb linkages and synchro screws, springs, etc., when everything was apart. I shut it down and look things over. Good to go. I then hook up my carb sticks and synchronize the carb’s first. They will always be WAY out. But after adjustment, it’s now Smoooooth.... I also performed the idle drop and then re-synchronize the carb’s. So.... Sync, idle drop, re-sync is the procedure. Also remember that you’ll need to get a BIG fan or blower in front of the radiator to prevent the engine cooling fan from coming on. When the cooling fan comes on, it puts an electrical load on the alternator, which in turn lowers the idle speed. When the idle speed drops, all of the readings on the carb balancer will change quite a bit. You’ll be there for a long time trying to get the carbs balanced fighting engine cooling fan load. It’s that sensitive. Same goes for the idle drop. I set idle speed to 1000 rpm with the bigger pilots.

After that, it’s time to finish installing the rest of the top side. Air filter housing, filter, lid, etc.

I then put on the seat and take it out for a good run, up to operating temp. Everything is working fantastic, and the V-4 is running the way it was intended with those new pilot’s and the fresh tune. I can now finish assembly and put all that plastic back on. DONE !!!


Carb Removal and Rebuild ( ST1100 )

ST1100 Carb Part Numbers

I always use my local dealer for parts. I’ve been with them for 30 years and they give me wholesale pricing. However, most don’t have that luxury and use online sources such as Partzilla, Revzilla, Ron Ayres, etc.

Quantity 4 - 16011-MAJ-R00 Float Valve Set $41 each

Quantity 4 - 16010-MAJ-D00 Float / Carb Gasket Set $24 each

Quantity 4 â€" 16211-MT3-000 Insulator / Boot $9 each

Quantity 1 â€" 17211-MT3-000 OEM Honda Air Filter $38

Quantity 1 â€" 17253-KT8-000 Sub- Air Filter $3

Quantity 1 â€" 16900-MG8-003 OEM Honda Fuel Filter $12

You may also choose to upgrade the slow speed / pilot jets to #40. If California or ABS the main jets also.

QTY 4 #424-21 size 40 $7 each

QTY 4 #9901-393 size 128 $7 each

QTY 1 #009-396 needle shim $8 for a package

These can be purchased at in Oregon. 503-873-8992. These are genuine factory Keihin parts.

Adds up quickly! Potentially a $350 rebuild in parts alone. But, better than paying a dealer or bike shop! I hope this helps and drop me a line if you need any more help.


Thank You again for your contribution Adam Frymoyer, STOC #949