Carb Prep

Bob Evans

The second most important piece on a Briggs stock class motor (cam is #1) is a good carburetor. We have seen almost .5 HP between an ‘average’ carb and the very best on a dyno. We will attempt to go through the basics of building any stock carb and then get into a few items that are a little on the trick side. Only two part numbers are legal stock carbs these days. 555129 which is the Raptor carb and #298397 which is the older service carb. The service carb will have breather holes in the airhorn where the Raptor carb does not have these drilled out. Legal carbs have a removable jet and DO NOT have a integral pull choke in the back of the carb.

After pulling the carburetor and tank off a new motor remove the three Torx screws that hold the carb onto the tank. It's my opinion that you should reuse these screws as they have lock washers on them and work quite well. Many builders will trash these and use allen bolts for replacements. All well and good, if you include lock washers. If not keep the stock screws as these will not back out under vibration as quickly! Screws that back out will lead to a broken carb!

If present, next remove the swirl from the throat of the carb. The secret here is to grasp the swirl with a good pair of pliers and twist the swirl slightly clockwise as you pull the swirl out. The only time you may want to keep the swirl is in the smaller purple plate restricted class. Opinions vary on purple plate motors but for me, my experience says to remove the swirl and open up the carb. The larger restricted plates motors such as blue or gold plates should definitely have the swirl removed.

With a 1/2" wrench you can remove the high speed needle and nut as a unit. Next take a good flat screwdriver and unscrew the brass jet. Use a screwdriver that fits the slot well, as the brass jet’s slot is very easy to damage. Damaged jets will not flow fuel as well. Try and find a screwdriver head that fits the slot exactly! Keep the jet as we'll modify it later for use in a methanol motor.

Now remove the four screws retaining the diaphragm cover and carefully pull the plate, diaphragm, cap and spring from the carb. Take note of their position as the rules state that they must be replaced in their stock configuration and position. Take the diaphragm cover and surface the side that butts up to the diaphragm. The easiest method is to lay a piece of 400 wet/dry sand paper on a flat surface such as glass and using carb cleaner or water for lubrication, rotate the plate in a figure 8 pattern until the mating surface is flat. This will insure good fuel pressure and no leaks! You should change the diaphragm about every 4 - 6 races. Remember it’s your fuel pump and methanol is rather harsh on parts. The new Teflon diaphragms seem to hold up to the methanol a lot better than the stock Briggs part, but are a tad pricey.

Now, back the idle screw out until it no longer is engaged against the body of the carb. This will allow the butterfly to be a bit more square with the bore making it easier to remove. Using a long thin screwdriver, reach down the throat of the carb and remove the screw holding the butterfly. Be careful not to mare the screw as it is directly in the air flow line. Again using the right screwdriver will be of long term benefit! If I remember one thing my dad taught me when working on motors, it was always to use the right tool for the job. Lord help me if he caught me with a pair of pliers on a nut or bolt.

After removing the butterfly screw, turn the carb throat down and shake out the butterfly. Finally remove the throttle shaft. Don't loose the felt or foam washer on the shaft. Using the long screwdriver knock the welch plug out of the back of the carburetor. Be careful not to damage the front of the carb.

Now we are ready to really work on the carb. First measure the backside thickness of the throttle shaft. WKA rules permit it to be a minimum of .086. Most stock shaft are around .095 to .110 so you can CAREFULLY take a small file and bring the backside down to around .088 to .090. Again this piece is directly in the air flow so every little bit helps. The proper way to measure this thickness is to set your micrometer to .086 and see if it goes over the backside of the shaft from any position. If they do, the shaft is illegal. Current WKA rules now state that the front edge can be no smaller than .040 so be careful there also. You can also purchase throttle shafts from most builders or kart shops already prepared to minimum size for less than $15. these are a much better choice.

While we are in this area, take a look around and find the shortest butterfly screw possible. Most are around .335 to .330. Just less to stick out into the air flow. You cannot legally cut or file the screw down for stock classes. For 1998 WKA now has a minimum screw length of .322. You can also look for a screw that when tightened on the throttle shaft, has the slot in line with air flow! There's something to find, a .322 length screw that aligns correctly. It can be Done! A word of caution here to not try and tighten the screw too hard just to get it in the right position. The throttle shaft is made of pretty soft brass and you can easily ruin a nicely cut shaft by stripping the threads out!. Find the right screw. Speaking of the right screw, if you have one of the Raptor II carbs with the Torx headed butterfly screw throw it in the nearest garbage can and use a slotted screw. Briggs has listened to the motor builders and now is using the older slotted screw on all new Raptor motors, as it’s smaller.

Next, lets move to the area left from removal of the jet and high speed screw. You will see two holes. For methanol racing these should be enlarged as follows. Use a #71 drill bit or reamer to enlarge the smaller hole. This bit is .026 and will keep the hole under the WKA minimum of .028. For the larger of the two holes use a # 53 bit(.059). This will keep the main metering hole under the .062 required. NOTE**** Do NOT use an electric drill for this modification. Get a jewelers hand bit holder for this operation and do it by hand. It is simply too fine an operation for an electric drill. Generally after you do this step if you look down the throttle bore you will see a small rise where the drill bit came through the bore. These are considered protrusions in the airway and are illegal, but we will get these out with the next step. If you are doing quite a few carburetors acquire small precision reamers in the # 71 and 53 sizes as these do a much finer job sizing the two holes. Smoother holes equal better fuel flow. Check the MSC catalog for these.

You can now turn you attention to the bore. WKA specs say the maximum bore size is .695. To get the bore to as close to this specification as possible, many builders us an adjustable reamer. You can purchase a fixed .690 reamer($30) from a great tool company, MSC Industrial Supply, located in Plainview NY. They can be reached at 800-645-7270 and will be glad to send you their 3" thick catalog for free! This is also where I purchased my small reamers and drill bits. If you love tools, this catalog will keep you busy for several evenings. Anyway, back to the bore. Take the reamer and starting from the rear of the carburetor ream the bore out by rotating the reamer all the way through in one pass. You can bolt the reamer in a vice and rotate the carburetor by hand. Use a light oil and keep blowing our the scraps of aluminum being shaved by the reamer. Never back the reamer up or reverse direction during this operation. Pull the reamer out the front after the single pass. If you want to get closer to the .695 maximum you can also obtain an adjustable reamer from MSC, which will allow you to ease your way up to the maximum size. These adjustable reamers range from around $25 up depending upon manufacturer and quality. To be on the safe side set the reamer at .693. Be sure and measure the width of the reamer on all cutting blades before using it the first time. Also EzBore offers a 693 reamer set up specifically for our use here. Great tool for around $150. Lasts a life time

Now you are left with a fairly rough bore. Most often, this finish will not flow as well as the original bore and finish, so the next step is an absolutely necessity. Honing the bore smooth. The best tool I have seen is a Flex-hone. These are the absolute best finish hones on the market for the money. They have small balls of abrasive on the ends of flexible rods enabling the hone to conform to the bore size. These are also available from MSC and Ez-Bore as well as a company in Ca. named Cylinder Head Abrasives. Many of the kart mail order houses also carry these hones today. The size you are looking for is 18mm. (.709) . You will want something in the 240 - 320 grit range. Most organizations such as WKA, KART and IKF only allow you to polish the actual bore of the carb, so you do have to protect both the front and back sides of the carburetor from being scratched by the hone. Basically only the area touched by the reamer. Check the current tech manual of your organization before polishing anything other than the bore!

You can cover the front of the bore as well as the back portion under the air horn by carefully placing clear sealing tape over these areas. Just be sure to clean the carb first with a good degreaser so the tape will stick. A one inch piece of 1/2” brass pipe connector works very well for protecting the front recess. Just add a bit of tape around the connector and it will fit perfectly into this front recess. On the backside of the carb be sure and protect the are around the 90 degree turn from the air horn to the bore. All Tech Inspectors are tough on this area. It must be a sharp angle!! Now using a slow speed drill, such as a portable, hone the bore in a back and forth manner for about 20 cycles. Remove the hone and wash the carb with a good cleaner. The bore should be very smooth and polished at this point. To check the size of the bore you can purchase tech tools from your karting organization or use a fairly inexpensive adjustable telescoping inside gauge set. Taking the smallest of these, I set them by using my micrometer locked on .696(WKA NO-GO) and locking the correct gauge down at that width. You can now use this to tech the bore via your organization’s technical manual’s instructions. These sets are fairly inexpensive ($12-$20) and will cover a wide range of sizes. Again check your organization’s specific rules for teching the carb.

If you have some of the older(non RAPTOR) style carbs, you can now plug all of the holes in the air horn of the carb. Plain old clear or blue silicone gasket sealer works very well here. I also use clear tape over the holes on the inside of the air horn to insure the sealer does not protrude into the bore(illegal). If you cover the holes with tape and then ***** a small hole to allow air to escape, you can fill the breather hole from the outside making it flush to the bore. Allow this to dry overnight and then remove the tape. J-B weld works well also.

Now you can assemble the carb in reverse order. One of the most frustrating things can be getting the butterfly and screw aligned. A long thin magnetic screwdriver will go a long way to resolve this frustration. I use a small awl to place the butterfly into the bore then use the screwdriver to set the screw. Remember don’t tighten tooooo hard. A pencil will also work quite well to hold the butterfly why you place it into the bore.

The final step is to adjust the throttle shaft angle to give the maximum air flow. This is where you really need a flow bench. If you have a Kart shop near by, many times they will allow you to use their bench for free or for a very reasonable price. By bending the throttle stop you can adjust the angle of the butterfly until you get the best air flow and fuel vacuum level. When flowing a carb you will find an position that allows you to get the maximum fuel pressure without killing your air flow numbers. If you have a choice go for the air flow. If you can't gain access to a flow bench, try and get the butterfly straight in the bore by looking down the bore from the front. This will get you very close. Most carbs set up for stock classes seem to end up set slightly beyond the straight ahead position when you're done with them on a flow. A touch more open in position than closed. If you are setting up for any of the restricted classes you will need to offset the butterfly and not let it be straight in line with the bore. Maybe a 1/8" offset toward the closed position. If you have a flow bench and are flowing for restricted classes, I would suggest using a gold plate(.575) to flow with, as the other plates restrict air flow to the point that it is very hard to see any change in throttle angle on the bench.

When bolting the carb back on the tank be sure and use two tank gaskets(max legal number). This will insure a good seal, cut down on vibration, and keep the long pickup tube from possibly touching the bottom of the tank and ruining fuel flow to the small bowl. On the newer tanks I like to use the tank insert to keep the small pickup bowl full of fuel.(part # 555520).

As a final step obtain several new jets from your local Kart shop and drill them with the following drill bits or better yet small reamers. These bits or reamers are again available from MSC.

1.25mm bit = .050 size hole
# 55 bit = .052 size hole
# 54 bit = .054 size hole
# 53 bit = .059 size hole( for purple plates)

As a good starting place, the .048, .050 or .052 jets will do for 99% of all stock applications. During the summer as the air heats up and becomes worse (thinner) you can move to the .048 jet, as long as you can keep your motor temperature at a reasonable range (<410 degrees). I really like to keep mine around 380-390 degrees. The .050 will work well during the winter when the air is denser(better). For a purple plate motor, use the .059, as you need to pull whatever fuel you can into the motor given the limited air flow. I know this sounds strange but the .59 just flat works on the small plate. Gold plates tend to use the same size jet as a stocker. Start with a .056 for Blue


Carefully check the hole drilled for the short pick-up tube and make sure the high speed needle body does not interfere with flow! You can carefully unscrew the small pickup tube with a 3/8 socket. This will allow you to see if the high speed needle assembly is blocking the small pick tube opening when installed. The needle point will be in view but make sure the body is not blocking the hole. You may have to try several different gaskets to find one that will be thick enough to keep the assembly away from the hole. Multiple gaskets are not legal here. Unfortunately, it seems Briggs is now using a much thinner gasket on the newer carb so look for the thicker one on older carbs. If you ever have a carburetor that seems to always seems to run hot on a motor, this is the place to check! While you have the small pickup tube out also check the hole for any obstructions. I prefer to leave the wire screen on to block any trash you might get in your tank. A lot of builders will remove the screen, but I have seen too many motors over the years just plain quit when the small pickup get clogged! Maximum size for the small pickup tube is .066 for WKA. This hole can be opened up with your .059 drill bit to add flow volume. The passageway for the fuel from the short pickup tube has no tech on size today and is an area than can be improved a bit!. Again, when bolting the carb back on the tank, be sure and ALWAYS use two carb/tank gaskets. This will eliminate any possibility of the pickup tubes being too close to the bottom of the tank and give you a bit of cushioning against vibration(carb breaker).

If you are really into looking for a good carburetor, try and find an older number 2 or 4 casting numbers. This number is located on the top side of the carb opposite from the diaphragm. The number will be sideways. The vast majority of carbs are number 5s. While there are many good #5 carbs out there, there are lots of really average #5s to boot. Frankly some carbs just plan flow better than others out of the box. Briggs has had many molds for these over the years , and some molds are better than others! I know of builders that will go through 100 new carbs just to get 10-20 good ones! No wonder they charge so much for those KILLER carbs. All #4s I've found are good carbs. The number 2s usually are found on older tiller motors. The number 4s were made in small supply about 5 or 6 years ago. The current RAPTOR III carbs from the factory all seem to be very consistent with very good air flow. Most of the older RAPTOR I carbs were not as good on the whole for stockers but seemed to make excellent restricted carbs. Good carbs for restricted motors are usually completely different from carbs that will flow well on stockers. Good luck looking in the lawn mower repair shops in your area. You will soon be scavenging everywhere you see a used lawnmower or tiller.

Butterflies for the Briggs also come from different casting. The numbers are cast right on the face of the butterfly. Number 8s are the most common today and are the thickest in cross section! The best castings are numbers 10, 11 and 6, as these are thinner than the #8s or 7s. Currently, WKA does have a minimum thickness specification for butterflies of .059 measured where the butterfly mates with the throttle shaft. Again, be sure and check the current tech manual for your organization. No polishing or changes are allowed to the butterfly. For stock classes use the thinnest butterfly you can find and still be within the rules. Restricted carbs are a different story. It seems the #6 works very well here verses the #11. Remember thin butterfly, throttle shaft and screws all mean there is less restriction in the air way. There are also now some offset throttle shafts available that will help with air flow and fuel pressure on most carbs. These are available from most shops or builders. Won’t say they help on all carbs but the majority will benefit from these. The Briggs part # is 299692 for the throttle shaft. Part numbers for the #11 butterfly is 211237, but they are out of production so you will have a hard time finding them in stock. The #6 butterfly part # is 211203. Check the box for the actual butterfly it contains!. Air flow wise the 6 and 11 seem to be about the same with the 11 having a bit of fuel signal advantage on most carbs.

If you are lucky enough to find that killer carb everyone talks about, measure every thing on that carb and find out why it’s better. Compare it to a dog or average carb and you will learn even more secrets for selecting a good carb. Lets just say, I carry my dial calipers to my Briggs Motorsports guy when I go looking for a carb.

While I’m thinking of it be sure and always use a air filter! Yes, there is a very very slight air flow restriction even with a clean oiled filter, but unless you get all of your internal motor parts for free, you will be well advised to run the filter if you're racing on dirt. Always clean and re-oil your filters after a weekend race. I actually use 2 or 3 during one days race, changing as they get dirty. An un-oiled filter is useless. On very dusty tracks the foam pre-filters and Outerwears are also a very good option. The best filters??? K&N. You will not need any filter over 4 1/2 X 4” in size. A dirty 4” filter can still flow more air than the stock carb needs so forget those 8” tall filters!

As a final note, be sure and protect the carb you have just built by using a tank brace. You can weld a brace under the tank for support or use the one that bolts to the bottom tank bolt and uses a large hose clamp to secure the top portion around the tank. The latest style carb brace bolts to the rear of the carb and the back two flywheel cover bolts on the head. These seem to be the best thing going today. Pay close attention to the three carb/tank screws as these will sometimes work loose even with lock washers. Just one of those thing you put on your pre-race maintenance list. Good insurance for that killer carb you just built!