Mods For Briggs Carbs?

If you guys want to read a good book, get Jimmy Glenn's flathead book. Show how to blueprint a stock 5 horse in detail. here is an except from the carb section on blueprinting and methanol conversion.

The second most important piece on a Briggs stock class motor (cam is #1) is a good carburetor. I 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 secret stuff side.

After pulling the carburetor and tank off a new motor remove the three screws that hold the carb onto the tank. It's my option 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!

If present next remove the swirl from the throat. 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 purple plate restricted class. Due to the small amount of air flow with this plate I believe the swirl will help with the mix of air and methanol

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. Us a good screwdriver as brass is very easy to damage. 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 the 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 a light oil, 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 6 races. Remember it's your fuel pump.

Back the idle screw out until it no longer is engaged. Using a long 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. After removing the screw turn the carb throat down and shake out the butterfly. Final remove the throttle shaft. Don't loose the felt/foam washer.

Using the long screwdriver knock the plug out of the back of the carburetor.

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 so you can CAREFULLY take a small jewelers file and bring the backside down to around .088. Again this piece is directly in the air flow so every little bit helps. 96 rules now state that the front edge can be no smaller than .040 so not much here.

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 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. 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's simply to fine an operation for an electric drill. Generally after you do this step if you look down the throttle bore you will see small rises where the drill bit came through the bore. These are illegal but we will get these out with the next step.

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 as possible many builders us an adjustable reamer. Personally I purchased an .690 reamer 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! 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 front of the carburetor ream the bore out by reaming all the way through in one pass. I bolt the reamer in a vice and rotate the carburetor. Use a light oil and keep blowing our the scraps of aluminum being shaved by the reamer. Pull the reamer out the back after the single pass.

Now you are left with a fairly rough bore. Not good for air flow, so let's now hone the bore smooth. The best tool I have seen is a Flex-hone. These are the absolute best finish hones on the market. They have small balls of abrasive on the ends of flexible rods enabling the hone to conform to any bore size. These are also available from MSC as well as a company in Ca. named Cylinder Head Abrasives(800-456-5474). The size you are looking for is 18mm. (.709) After obtaining the hone you must cover certain parts of the carburetor. Only the actual bore can be honed or scratched! You can cover the front of the bore as well as the back portion under the venturi 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. Now using a slow speed drill such as a portable, hone the bore in a back and forth manner for about 25 cycles. Remove the hone and wash the carb in a good cleaner.

You can now plug all of the holes in the venturi end of the carb. I use plain old blue silicone gasket sealer. Be sure the sealer does not protrude into the bore.

Now you can assemble the carb in reverse order. 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 a very reasonable price. By bending the throttle stop you can adjust the angle of the butterfly until you get the best air flow. If you can't gain access to a flow bench try and get the butterfly straight in the bore. This will get you very close. 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 1/4" offset. If you have a flow bench and are flowing for restricted classes use a gold plate to flow with as the other plate restrict air flow to the point that it is very hard to see any change in throttle angle on the bench. In a future article, I will tell you how to build your own flow bench for under $150 using high quality gauges!

When bolting the carb back on the tank be sure and use two tank gaskets. This will insure a good seal and keep the long pickup tube from possibly touching the bottom of the tank and ruin fuel flow.

As a final step obtain several new jets from your local Kart shop and drill them with the following drill bits

1.25mm bit = .050 size hole

# 55 bit = .052 size hole

# 54 bit = .054 size hole

# 53 bit = .059 size hole

As a good starting place, the .052 or .054 jets will do for 99% of all applications. During the summer as the air heats up and becomes worse (thinner) you can move to the .050 jet, as long as you can keep your motor temperature at a reasonable range (<420 degrees). I really like to keep mine around 380-390. The .054 will work well during the winter when the air is denser. 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.

nbsp;If you are really into looking for a good carburetor, try and find an older number 2 or 4 casting number. 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 5. While there are many good #5 carbs out their there are lots of really average #5s to boot. Frankly some carbs just plan flow better than others. Briggs has several molds for these, and some molds are better than others! I know of one Kart shop that will go through 100 new carbs just to get 10-20 good ones! Most #2s, and 4s I've found are better than average carbs. The number 2s usually are found on older tiller motors. The number 4s were made in small supply about 5 years ago. GOOD LUCK.

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 K&N filter, but unless you get all of your internal parts for free, you will be well advised to run the filter. Always clean and re-oil your filters after a weekends race. I actually use 2 or 3 during one days race, changing as they get dirty.

As a final note be sure and protect the carb you have just built by using a tank brace. I have used the one that bolts to the bottom tank bolt and uses a large hose clamp to secure the top portion around the tank. I have never broken a carb and I run on some pretty rough dirt tracks. Good insurance
Removing the swirl is probably the 1st thing that most people do, that know what they're doing. Give Jerry Dover a call. He is the carb guru. 704-782-7061.
Removing the swirl is probably the 1st thing that most people do, that know what they're doing. Give Jerry Dover a call. He is the carb guru. 704-782-7061.
I hear his stock carbs are awesome. I think the A carbs. He is a nice guy and has good prices on lots of stuff.
Removing the swirl is probably the 1st thing that most people do, that know what they're doing. Give Jerry Dover a call. He is the carb guru. 704-782-7061.
I have talked to my buddy and he agrees that removing it is better for performance applications, he also said that for purple plate racing it is reccomended. I must have misunderstood when he said it the first time. He does agree that stock engines need the plenum twist to atomize the fuel, not having the vacuum a performance engine does. Like I said my stock flatheads dont idle as well without it. I learn here as I go along!:thumbsup:
so if i was to remove it from my stock motor,would i lose hp? or would i gain some? i am confused!!! but i would love to get the most out of my stock 3hp and 5hps.
i didn't call him. i don't know him and wasn't sure if i should call him since he is a complete stranger(don't want to offend anybody). anyhow i was just wondering if anybdy has done this to a stock motor any gained anymore from it
I’m reviving this ancient thread to ask, did folks ever come to a consensus on wether or not to remove the spiral from a pulsa jet carb on a fairly stock motor?


Well-Known Member
I’m reviving this ancient thread to ask, did folks ever come to a consensus on wether or not to remove the spiral from a pulsa jet carb on a fairly stock motor?
I'm reviving it about a year later to say, uh, it could be a good idea to leave the spiral in on a fairly stock motor running on pump gas.
Seems fuel atomization will always be a problem and some also advocate leaving the carb and intake's rough casting imperfections intact.
There's always some casting flash at the base of the intake port around the valve stem... but the exhaust port can be smoothed out a bit as evacuating the bore is good.

I've read what korndog posted before but it's geared for a Briggs on methanol.
You need lots of methanol for that racing performance which is why the jets are drilled so big and the carb bore is reamed out.