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BASIC RACE PREPPING OF A BRIGGS & STRATTON CARBURETOR by Mark Bergfelt
Many karting enthusiasts find themselves in
a dilemma. They have an intense desire to win, but more often
not, they don't have the financial resources to come up with the items
that they feel that they need to get to the front of the pack.
often the novice will spend many dollars on accessories that may not
them the desired result. The toughest part for the new karter,
the one with limited resources, is knowing just what parts and services
will make him faster on the track. He needs to know how to get
most bang for the least buck.
One way that karters can save money and get good results is by doing their own carburetor preparation. This article will illustrate the most basic preparation techniques to turn your stock gas carburetor into a WKA legal alcohol and air mixer. We will not dazzle you with flow bench information. If you are reading this in search of help for your racing effort, you probably don't have access to one anyway. That does not mean you can't make your carburetor do a satisfactory job. It's true that all Briggs carburetor castings are not created equal, and a flow bench is helpful for sorting them out, but these instructions will help you to do the best that you can with what you have. Getting the most out of what you do have is most important.
To accomplish our mission, we will be completely disassembling a carburetor. Before you start, get a container to put all of your small parts and screws into to insure that they don't get lost. I have found that an empty waterless hand cleaner container is perfect for this.
Removal of the carburetor from the engine is pretty straight forward. The first step is to drop the fuel tank from the bottom of the carburetor. Remove the support bracket screw down near the bottom of the engine, first. Usually a 3/8 socket wrench gets the job done. Next remove the three screws that fasten the tank to the carburetor. A Torx T-20 screwdriver will be needed to remove the screws currently being installed by the factory. Earlier designs needed flat scredrivers and even earlier ones, phillips type screwdrivers, for removal. They all got the job done, but my preference are the ones that the factory is currently using. You can replace them with 10-32 x 5/8" socket head (Allen) cap screws if you like. For those you will need a 5/32" hex key (allen wrench).
Removal of the carburetor from the cylinder comes next. Current production engines require the use of Torx T-30 screwdriver. Briggs & Stratton supplies a special tool for those screws. It is part number 19391 and allows the screws to be reached from the side. Earlier models of the five horse Briggs engine used a screw with a 3/8 hex along with two straight, crossed slots. You could use a flat screwdriver on them and a 3/8 open end wrench, sometimes. Briggs makes a special screwdriver for those screws. It carries part number 19305. It also allows access from the side. The screws used with that tool are my favorite method of securing a Briggs carb. Some racers prefer to use 1/4-20 x 3/4" socket head cap screw. They're OK and allowed by the rules. For those you will need a 3/16" hex key (allen wrench). Once you have the two mounting screws out, the carburetor will probably come off with them. If not, the only thing holding it on will be the adhesion of the gasket(s). Simply tug on the carb. and it will come off.
It's no fun at all to work on dirty parts, so it is strongly suggested that the carburetor get a good cleaning at this point. Some good parts solvent will get the job done. There are many available, so take your pick and be sure to follow the safety instructions. Most are flammable. If you have compressed air, blow dry the carburetor. Otherwise, wipe it off with a rag or shop towel. The carburetor that we are prepping in this article is brand new so we'll skip that step.
At this point there are a lot of directions that you could go and they would all be ok. I'll suggest that you take out the needle valve assembly first. A 1/2" box or open end wrench is what you will need. Simply crack it loose and unscrew it. Next, remove the brass jet. Briggs calls it a "seat-needle valve". Use a flat blade screwdriver to unscrew it. Put the jet and your needle valve in to your small parts container.
Removal of the bell crank that operates the throttle is a good next step. Use a 5/16" nut driver to remove the special shoulder screw that holds it on. With the bell crank loose, carefully unhook the throttle link and throttle link spring. You can disgard the bellcrank, since you will be replacing it, but don't loose the shoulder screw, throttle link and throttle link spring. Some people question the purpose of the throttle link spring. It is there to keep the throttle from "floating" or fluctuating, causing the engine to run at an unsteady speed, when a governor is used.
Removal of the diaphragm cover is really easy. All that you need is a thin blade flat tip screwdriver. Take out the four screws and put them into your container. The diaphragm cover might be stuck in place by the diaphragm. If it is, a sharp tap with the screwdriver handle will get it off. Be careful not to damage anything. Next, peel off the diaphragm. At this point the diaphragm spring and cap will probably fall out. Put them in your parts container.
A big screwdriver can be used to pry out the welch plug that seals off the end of the carburetor. Place the screwdriver into the airhorn of the carburetor with the end of the screwdriver against the plug. Pry hard and it will pop out. Watch that you don't poke yourself and pry away from your face and other people. If you are going to reuse that plug, place it over a 3/4" nut and tap it back into a dome shape with a ball been hammer. Put the plug aside.
Unless the fuel pipes are damaged, the only thing left to remove from the carburetor is the throttle shaft and shutter assembly. You will need a Torx T-10 screwdriver to remove the screw that holds the throttle shutter. Older carburetors used a slotted screw and a thin flat tip screwdriver with a long blade got that screw out. I prefer a magnetic type, but you can only appreciate the reason when you go to put the screw back in. Once the screw is out, the throttle shutter will fall out. At that point you can pull out the throttle shaft from the top of the carburetor. When you remove the throttle shaft, be sure not to loose the two small seals that are there to keep dirt from getting into the airstream through the throttle shaft bushing. It is illegal to race with out the seals and not too smart either since any dirt that gets in between the throttle shaft and bushing will wear them out quickly resulting in a loose and wobbly throttle shaft.
Inspect the screens on the two fuel pipes that stick out of the bottom of the carburetor. If they are clean, leave them alone. If they look even slightly clogged replace the plastic fuel pipes, To get them off, twist each one with a box end wrench and pull hard at the same time until you muscle them off. For the long fuel pipe you will need a 9/16" box wrench and for the short one a 3/8" box wrench.
At this point you now have a pile of miscellaneous pieces that used to be a carburetor and we hope will be a real killer when you are done with this project.
The main difference between a gasoline carburetor and one for alcohol is the size of the passage ways through which the fuel travels . You need approximately twice as much alcohol as gasoline, to get an engine to run correctly. For that to happen, there are three holes that we need to enlarge. One of these holes is the one through the center of the needle valve seat which will we will refer to as the "jet" from now on. The other two holes are located on the throttle body and were uncovered when you unscrewed the brass jet. One of the holes is larger than the other. The large hole, known as the main metering hole, must be smaller than .062". The smaller hole, called the Idle hole, must remain under .028". To enlarge the main metering hole you will need a number 53 twist drill (drill bit). A number 53 drill measures .059. You might want to risk the possibility of getting disqualified and use a 1/16 drill bit. A 1/16" bit is .0625" and is knats hair over the limit. Let your conscience be your guide. What ever you decide, use a pinvise to twist the drill through the hole by hand. Under no circumstances should you use an electric drill motor!
A number 71 drill bit measures .026" and a Number 70 is .028". Again let your conscience be your guide on choosing which drill to use to enlarge the idle hole. Be sure to use the pinvise.
The size of the hole in the brass jet is a non-tech item so you can and should experiment with the size of that hole. A serious racer has an assortment of jets to tune his engine to changing weather conditions. The carburetor that we are preparing here will be equipped with a .052" jet. Please note, this carburetor will be used on an unrestricted engine. Generally, with the Briggs carburetor, the smaller the restrictor, the bigger the jet that is needed. At first, this defys logic, but that's a whole different article and we're not going there right now. To drill jets, I like to put the bit into a bench vise with the bit sticking straight up. Then set the jet onto the bit. To make a .052" hole we will be using a #55 drill. Using a large screwdriver, screw the jet onto the drill bit until you completely drill through the jet. At this point it is advisable to mark the jet with the size or at least some type of mark so that you know what it is. This is important because you will be changing jets often if you want to have a chance of running consistently. I prefer to use 1/16" number stamps. It would probably be most cost effective for the average karter to by a set of drilled and marked jets that come in some type of case.
If that were all we did we could put the carburetor back together and it would run an engine on alcohol. Who wants to do that when there are a whole bunch of improvements that you can make at home and still be legal?
According to the rule book, "Maximum carburetor throat inside dimension is .695"." A stock carburetor has a bore that is relatively smaller than that. If the carburetor is going onto an unrestricted engine it can be reamed or honed to just under the maximum size. I use an adjustable reamer that is set to .693" to enlarge the carburetor throat. Place the square portion of the reamer shank into a vise, with the reamer sticking straight up. Place the carburetor over the reamer so that the reamer is inserted into the carburetor bore. Use an adjustable open end (Crescent) wrench to turn the carburetor on the reamer. A slight push on the opposite end of the carburetor with your free hand helps move things along. Also, a good shot of WD-40 on the reamer and carburetor helps produce a smoother job. After you are finished reaming the carburetor, clean it up throughly with solvent and blow it dry with compressed air. I recommend skipping the reaming step if the carburetor is to be used on a restrictor equipped engine.
The throttle shaft is the next item that the rules allow you to monkey with. The area on the throttle shaft where the throttle attaches is flat but it is not parallel to the opposite side of the shaft. The back edge can be as small as .086" and the leading edge .040". You can use a small file to file one or both of the edges until you reach those dimensions. Be sure to maintain the original shape. Rounding off the edges or changing the contour will get you disqualified in a GOOD carburetor tech. inspection.
The rules allow you to surface the diaphragm cover. To do that, get a piece of 600 grit wet or dry sand paper and lay it down on a piece of window glass, (plate glass is best, a surface plate is even better, but if you are reading this looking for answers you probably don't have one) with the paper side down. Spray some WD-40 on the sandpaper and lay the diaphragm cover plate on the sandpaper with the gasket surface down. Rub the plate on the sand paper in a figure eight motion until the entire gasket surface shows evidence of being sanded. At that point you are done. Clean up the diaphragm cover plate with your solvent and dry it off.
The rules allow you to crimp a coin or washer onto the mixture needle and you probably should. This makes the the small amount of adjustment that you can do with the needle valve a little easier. While holding a penny in the screwdriver slot, smash the screw head slightly in a vise until the penny is crimped in place.
At this point you can begin reassembling your carburetor. Begin by installing the throttle shaft and shutter. Make sure that your throttle shaft seals are in place when installing the shaft. Next place the throttle shutter into the carburetor bore and position it onto the throttle shaft. A pair of surgeon's hemostats are helpful here. A thin pair of long nose pliers may also get the job done. Use a long thin magnetic screwdriver to install the throttle shutter screw. Before tightening it all the way, wiggle the throttle assembly open and closed a few times to position the shutter. then tighten the screw.
Install a new bellcrank that is supplied with the stock carburetor linkage kits that are available at any kart shop. Reinstall the throttle link and spring and special shoulder screw in their original positions.
One of the most effective things that can be done to improve the performance of an "out of the box" carburetor is to insure that the throttle is opening all of the way at full throttle. This is best done using a flow bench but significant improvements to performance can be made without one. To do this, while looking down the carburetor bore, bend the stop tab on the throttle shaft with needle nose pliers so that the throttle shutter is straight or parallel to the sides of the bore. You can also use a die grinder to grind the stop to achieve the same thing. Both methods work and are legal.
Place a new welch plug into the end of the carburetor bore. Before doing this, though, apply a very small amount of grey epoxy cement in the recess where the plug snaps in. A sharp rap with the ball end of a ball peen hammer will lock it in place. Wipe off any epoxy that oozes out.
Screw in the enlarged brass jet. Tighten it with a screwdriver. Install the needle valve assembly next. Make sure the gasket washer is in place, screw it in with your fingers. Finish the job with a 1/2" wrench. Screw the needle valve screw in all of the way then back it out 1 1/2 turns.
If you removed your plastic fuel pipes, put your new ones on now. Twist and shove them on using the same box wrenches that you used to help remove them.
Place the diaphragm spring into it's cavity on the side of the carburetor body. Lay the spring cap onto the spring. Position a new diaphram into position over the spring and cap. Lay the diaphragm cover over the diaphragm and install the four retaining screws. Get them all slightly snug before finish tightning any of them.
If you are going to use an air filter, and you should, even if you race on paved tracks, install your air filter adapter now. Apply some silicone gasket sealer around the intake flange of the carburetor before installing the adaptor. The sealer will insure that all possible entry points for dirt are eliminated. It also helps retain the adaptor in the event that the retaining screws should loosen up. Install the adaptor and tighten up the screws. A little locktite on the screws is advisable.
At this point you should have a completed, race ready, Briggs & Stratton alcohol carburetor. The only thing left to do is install the carburetor and fuel tank in the reverse order that you removed those parts. It is wise to use brand new gaskets and add a drop of locktite to the threads of each screw since carburetor flange, fuel tank flange and fuel tank bracket screws all have a nasty habit of coming loose.
It's always satisfying to do well with equipment that you prepared yourself. It's even better when you are on a shoe string budget and can beat the guys with the thicker wallet. Good luck with your new carburetor.
You will need the following items to accomplish this task;
1. small parts container
2. 3/8 socket wrench and accompanying ratchet.
3. Torx T-20 screwdriver, or a flat
or phillips screwdriver (see text)
4. Torx T-30 screwdriver, or B & S tool part number 19391, or a flat
blade screwdriver, or a B & S tool part number 19305 (see text)
5. parts cleaning solvent and materials
6. 1/2" box or open end wrench
7. screwdriver, long blade with thin flat tip.
8. Torx T-10 screwdriver
9. pin vise
10. number 71 twist drill (drill bit)
11. number 53 twist drill (drill bit)
12. number 55 twist drill (drill bit)
13. 1/16" number stamp set (optional)
15. open end adjustable (Crescent) wrench
16. adjustable reamer
17. small flat file
18. piece of plate glass
19. 600 wet or dry sandpaper
21. dial calipers
22. long thin needle nose pliers or (hemostat)
23. stock throttle linkage kit
24. air filter adapter
25. silicone gasket maker
26. locktite or other thread locking compound
27. small ball peen hammer
28. 5/16 nutdriver or socket wrench
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