Katana  GSX 1100F Carburetor Service

Both the Suzuki Service Manual and the Haynes Service Manual are excellent references for performing a Major service on the Mikuni BST34SS carburetors on the GSX1100F motorcycle. This on line reference is to serve as a supplement to address certain concerns with 'how to's" that are not covered in the service manuals.

The service manual recommends a total disassembly of the carburetors for service and in fact this is not necessary for most work, they can remain together, coupled with the set plates top and bottom. Matter of fact in the attached photo you will see that the carb's as a group can be managed conveniently by using vice grip pliers on the set plate to hold the rack stable for performing work on them.


If you do disassemble them, then make sure you align the bodies as shown in the picture so they function well together. There can be shaft-binding etc. if they are not aligned together as illustrated.

Next, get a spray on carb cleaner product like Carb Medic. Many of the brake clean type sprays will work equally well. Spray the entire exterior of the carbs with the cleaner, brush any hard to remove dirt with a paint brush and then blow off the residual debris and cleaner fluid with a hi-pressure air spray to remove any exterior dirt. Now disassembly can occur.

As the attached photo Fig. 2 shows it is helpful to have a clean work surface and a means to separate the parts for re-assembly. The carb parts should always be kept separate and a muffin tray works well to do this. Have a pencil and paper to note the values of the jets, needles and needle clip settings by carb location.

First remove the top diaphragm cover and the slide with diaphragm. Inspect the diaphragm for any perforations. Fig 3. Refers. If none exist you should clean the slide with the spray, wipe off and inspect the faces and side edges for wear. Normal minor scuffs will show on the surface but it will still be smooth. If there are any rough abrasions or the edges are becoming sharp the slides will need to be replaced. These can be expensive so go through a complete carb assessment before buying any parts. In this way you can decide the merits of buying another set if yours are badly worn. Mikuni carb's are quite durable, so with proper care they should last you as long as the motorcycle. If the slides and diaphragms are good take time now to apply a light spray of Armorall or similar silicone product to the surface of the diaphragms, rub it in slightly and allow it to soak in. These will restore the flexibility to the fabric and allow them to last longer. Check and clean the inside slide bore as well. Fig 4.

Next disassemble the float bowls from the carb bodies. Note, make sure these bowls are separated so they go back on in the same order. It is best to do this task with the carb's inverted and supported in a vise with vise grips. See fig. Removing the bowls exposes the floats. Unless you have had a float needle leaking which you need to replace I wouldn't remove the floats and needles and seats because I have found on re-assembly they begin to leak. You need to make a judgement call regarding their replacement. When the carb's get 50,000 miles on them, it is for sure time to replace the needles and seats. Before, only when there is a known problem. Now is a good time to check the float levels.

Checking Float levels

Float levels are critical to the fuel supply of the bike. If the floats are set too high, the bike can run lean or starve for fuel, if too low, it will run rich, use more fuel, and the top end performance will be impacted. The correct height is 14.6 mm. An easy way to check these is to take a venier caliper and establish the 14.6mm measurement. Next, what I do is cut a 14.6mm. or  5/8" notch out of a piece of sheet metal that forms an inverted L. Allow a shoulder to remain on the sheet metal piece that is just wide enough to span from 1 float bowl surface to the body of the adjacent float bowl. Make sure these cuts are square. In this way you can make sure the jig is square when you measure the float height. Adjust the little float tab accordingly to get the right measurement. Fig 5.

Next, remove the pilot air adjusting screw, noting on a piece of paper what the screw in to tight number of turns is. For US bikes, the pilot airscrew has been covered over with a tamper-proof cover. To access the screw you will need to drill through this cover and pop it off. Be careful not too drill thru or you will ruin the screw below. Use the carb reference guide to establish what the correct adjustment is but for most bikes it is 2 1/2 turns out. I found 2 in spec and 2 open too far at 2 3/4 turns.

Remove these screws, spray carb cleaner down them and then blow the channel out with air. Leave these out until re-assembly.

Next remove the main jet from the needle jet. Note the sizes of the main jets on your paper because they differ from the two external cylinders to the 2 internal ones. Check these jet sizes against what the reference table says should be there. Someone else could have made a mistake before you and you would just be re-creating the mistake on re-assembly if you donít verify it. The main jet removal now allows you to 'tap' up on the needle jets to remove them from the bore of the carb. IT IS CRITICAL THESE BE REMOVED AND INSPECTED.

The needle jets have minute pinholes in them to meter fuel through at different throttle settings. If these pinholes are blocked they will create flat spots in acceleration dependent on what the needle exposes. Clean these thoroughly with carb cleaner and blow out with air. Inspect the bore of the needle jet with a magnifying glass and bright light. These are made of brass and can wear and elongate creating an egg shape, which will alter the correct mixture of fuel flowing through. These should be inspected in conjunction with the needles themselves. Obvious extensive wear should have you replace them with a full new set. Pay particular attention to any visible striations on the needle itself and the setting of the clip. None US bikes use a 5DL7 -3 needle, which means the clip, is in the 3rd slot on the needle. Raising or lowering the clip will affect the mixture and is a quick way of doing minor mixture adjustments based on altitude changes.

 The Basics on How to Troubleshoot Carb Performance

Remember the Mikuni carb has three controls over fuel metering and air supply, in addition to the Choke setting.

At idle, there is an idle circuit and pilot jet that control fuel metering. Additionally there is the pilot air jet that controls air supply at idle and the pilot air adjusting screw. Check the Suzuki manual for the jet sizes for your particular model. Unless you have a US model the general setting for the pilot airscrew is 2 1/2 turns out from seated.

At approx. 1750 RPM the bike begins to be influenced by the needle jet and needle. As the throttle is opened and more vacuum is created the slide moves up in the bore and begins to expose the small pin holes in the needle jet, resulting in more fuel flow. At the same type the aluminum slide is opening to allow more free flow of air through the venture bore. As I said before, flat spots in mid-range can be generally determined to be influenced by these settings.

By approx. 4000 rpm the engine is now under the influence of the main jet almost completely as the needle has retracted up the needle jet, and the aluminum slide is wide open. From here to redline the bike is influence by the main jet and float height adjustment. If the floats are set to high (when measuring their height inverted), the bike will not have enough fuel in the fuel bowls to supply the engine needs. If the floats are too low, fuel will be in too close proximity to the main jet as can create an overly rich flow essentially pushing fuel up through the main jet instead of allowing the venturi action to draw it in. Of course this high fuel level condition from too low a float bowl setting will affect fuel economy through all ranges since this fuel "push" will always be occurring.

Just cleaning the carbs, checking pilot air adjustments, float levels and needle/needle jet conditions will remedy most basic problems with these carbs.

Remember that the needles and needle jets are paired and they should be replaced together. Pay particular note to your needle jet codes. US bikes take 2 different needles jets/needles the (5DL11 needle - P2 needle jet), and the (5DL16 - P0 combo). If the 5DL16 combo can not be found then the non-US (5DL7 -3 P0) combo can be used. You should change your pilot air jet from a #155 to a #150 at the same time. Please see the chart for all the correct jet matching.

Most Common Pairings

Needle Needle Jet
5DL7- 3                Suzuki Part # 13383-48B10 P0                                             Suzuki Part # 09494-00586
5DL11                  Suzuki Part # 13383-48B20 P2                                             Suzuki Part # 09494-00535
5DL16                  Suzuki Part # 13383-48B30 P0                                             Suzuki Part # 09494-00586

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