Rebuilding Walbro carbs tutorial

[400brian]

start: http://jdsleds.com/jdsleds/forums/viewt ... 37#p138437

Go here : https://drive.google.com/file/d/1yFonnM ... 2MOIc/view


I am going to add a couple other points here. When you set the metering arm; You measure across the bare face of the carb body, no gasket installed. The stock setting was .005 to .020 above the body. Many of us have found that with the aftermarket rebuild kits you need to go lower. Flush with the body or just making contact with the straight edge. If it is set too high, the carb will overflow out the throat. If the engine runs great for a bit then falls flat on it's face, only to pick up and run again... over and over, most likely it is staving for fuel and the metering arm is set too low. The metering arm adjustment can be the biggest hassle on these carbs, get that right and life is good.

In the third link above, it shows how to remove the plug between the choke and throttle shaft. So far I have never removed one of these. On the last carb I made an attempt, I heated the area with a propane torch, but I couldn't crack it loose. I didn't want to ruin the throttle return spring, so I ended the attempt.

Also, in the tutorial, no mention is made of the packing washers that go on the mixture needles. The kit should include them. The high speed sits on top of the carb body with a cupped washer under the spring of the needle. On the idle mixture screw it is in the bore of the carb body. You have to dig it out and replace it. There should be a brass washer on the needle between the packing and the spring.

On adjusting and tuning your Walbro; The idle mixture and High speed mixture screws each can be set at one turn open to begin. The engine needs to be up to operating temp to get good adjusting results. You will also get the best results if you do the tuning at outside temps close to what you will be riding in.

The engine should idle at a steady RPM without fouling plugs. The mechanical condition of the engine and the tune of the ignition system can both have an effect on this, making good adjustments difficult. If the engine dies at idle, it is most likely too lean, open the idle mix screw a bit. If the RPM is alternating up and down, the mix is too rich, turn the screw in a bit. If you are fouling plugs on startup, same deal, it is too rich. Some engines will idle happily at 1000 - 1500 RPM, others need 2000 - 2500. The Walbro is a sequential flow carb. The idle circuit continues to flow fuel up through wide open throttle. Because of this, fine tuning the idle mixture and idle speed can often resolve or minimize a bog on acceleration.

The high speed mixture is to be set for best performance at wide open throttle, then open an additional 1/8 turn. Temps you are running in make a difference here. Check the plug colors after a high speed run to confirm you are neither too rich or too lean. I prefer to see a plug that is a medium brown.

Experience running the Walbro is a huge advantage. After running the machine for several years in varying conditions, you will learn to adjust the carb by performance, engine sound, and plug colors. The advantage of the Walbro is you can quickly tune it to run well in most any temp or conditions.
A backfiring engine is too lean, and must be addressed ASAP. An engine that is spitting back through the carb ( sneezing ) especially on a warm day, is too rich and closing the high speed mix a bit will fix that. The caveat is that you need to remember to return the high speed adjustment to its original adjustment when the temps turn cold, or you may burn the engine down from a now too lean condition.

If the engine accelerates nicely then suddenly falls flat, most likely the metering lever on the main body is set too low and you are outrunning the fuel supply. Adjusting the metering lever is fussy work, but getting it correct is very important for good performance.

Keep in mind that Walbro equipped engines require 1/2 to full throttle applied while starting the engine.

[400brian]

One of the updates for the '73 JDX8 is swapping out the original WDA-34 Walbro for the WRA-31 that was used '74 up.

The WDA-34 was too rich in the mid-range, and they actually held classes showing how to peen the orifices closed to reduce the flow.

I had bought 3 WRA-31s off ebay in preparation of this project. Some time ago I pulled one apart to find the metering plate screws seized, and when you twist those off in the main body, you now have a junk carb.

Recently I pulled number 2 down, and was relieved to find it all came apart OK. It then occurred to me that I might as well rebuild two, have one on the shelf as a spare. Also, when we get to February, and JDJR still needs a carb, I can say neener neener.

However, number 3 turned out to be just as junk as number 1. Bummer.

OK, so the rebuild candidate was pretty clean, and everything moved. After removing it from the solvent and getting all the old gasket material off, I noticed there was a fair amount of corrosion under the gaskets on the carb plates.

The orange umbrella check valve that goes in the above pictured plate ( in the circle in the upper right ) used to come in the full Walbro kit. Being as that kit has not been available for years now, I generally don't remove that check valve. In this case, it appeared that there was corrosion under the valve, so I popped it out to clean the area.

The orange umbrella valves are avaiable from New Breed.

This carb saw a couple seasons time on a 340/5, then has ran on the 440/21 on the 73 X8 since 2020. It has worked great to date.

[400brian]

I rebuilt the engine in my 74 X8 back in 2021. I rebuilt the Walbro WRA-31 at the same time. I had put 2000 + miles on the sled after having gotten it running in 2007. Excessive end play at the crankshaft was the reason it was rebuilt.
Since the rebuild, I have not had a good time with the sled. Despite having good success with Walbros up to that point, I just couldn't seem to get the rebuilt carb dialed in. The problem ranged from over fueling to running short of fuel at the top end.
The most recent apparent issue seemed to be over fueling. If you refer to the first photo, you can see the black goo has been spraying out of the exhaust. In my experience this indicates gross over fueling, or an engine running on one cylinder. The spark looked good, so more tinkering with the carb seemed in order.

Last night I pulled the plate stack off and adjusted the metering lever. Normally I just eyeball it, and they work. As noted though, recently that has not been working. I laid a piece of .030 thick steel across the gasket surface and wrote down where I estimated I had it set. I then pulled a spare WRA-31 I had taken off because of the same issue, adjusted the metering lever, noted and recorded that position, and put the carbs in a box for testing today.
For whatever reason, I felt the most confident about the sled's OEM carb. I bolted it on, fired up the sled, and it ran terrible. Smoking profusely, no throttle response, and would not idle. I didn't waste much time with it before deciding to try the second carb.

When I originally acquired this carb, it was a mess. Corroded, rusty, and oxidized. Guys told me not to bother with it. However, the three metering plate screws were successfully removed with some heat, so I went through it . I think I tossed one plate as too badly corroded, and the main body got a dab of JB weld to fix a sealing surface. I had tried running it a couple times over the past three years, but it didn't work any better.
So today I bolt it on and crank her over. She barked right off, and idled nicely! The throttle response was excellent, and the smoke was at normal levels. I decided a test rip was in order. We got a dusting of snow this afternoon, so the lawn looked like the test track. I would like to test it on a longer run, but there simply wasn't any place to do that.

I pulled it out of the garage and fired it up. After getting it lined up on a short straight, I punched it. Holy cow! That sucker hooked up, pulled the skis off the ground, and hauled ass! I couldn't believe it, the sled hasn't ran like this for ten years. So after ripping around the lawn for a bit to verify this wasn't a fluke, I put it back in the garage to get the goo cleaned off and everything put back together. Dang it, it is going to have to go in the trailer with the rest. Heading for the Ride with the Champs next week.

[400brian]

Another thing I have just learned is a tool to clean the Walbro inlet seat. It is called a tortilon, it is a tool made of wound paper, and is used to blend charcoal and pencil art work. The seat is not replaceable in the Walbros, so the best you can do is polish them. Some have tried Q-tips, but it has been pointed out that a 3/16" dia Tortilon can be used with a little polishing compound to shine them up. I found these on Amazon.

Posted on FB:

We've been having some good discussions lately regarding the WD, WR, WDA series of a Walbro Snowmobile Carburetors. Here is another important tip to consider when you are rebuilding one.
Unlike the Tillotson models, the inlet seat is secured into the throttle body and it is not replaceable. These are made of brass and like most non-ferous materials, it is subject to corrosion. A corroded or dirty inlet seat can cause inlet needle sealing issues that result in hard starting due to loss of prime and/or excessive flooding.
The fix in this case is to use something called a tortilon. These are available at any artist supply store like Michael's or Hobby Lobby. Office Max or Office Depot may also carry them. Of couse, there are plenty of online sources for these too. They are made from tightly rolled paper and come in various sizes.
To resolve a corroded or dirty inlet seat, use a 3/16" size tortilon with a small amount of polishing compound or cleaner wax to the tip and the sides. Toothpaste also works since it's a mild abrasive. It'll slide in and since it's made from paper, it'll conform to the taper at the orifice of the seat. Twist it around by hand for a bit and the tortilon witll blacken as it cleans the dirt and corrosion from the brass. Use a Q-tip to clean out the seat occasionaly and to check your progress. Once the Q-tip comes out clean, you've gotten it as clean and corrosion free as you will likely manage. Use some solvent to thoroughly clean any residual compound from the seat, install a new inlet needle (Walbro PN 82-30-7) and you have a fully refurbished inlet needle and seat.

[400brian]

Another good post from FB:

The documentation on inlet lever height is somewhat fuzzy. Both of those types of metrering diaphragms are original Walbro equipment. I have more commonly seen the "buttonless" style on the right used with carburetors meant for a multiple carburetor arrangements like the WD2-1, WRD -1 and the WD1-1. The remainder of unmolested carburetors that I have run acrosss had the button type on the left.
Service instruction documents illustrate the inlet lever adjustment both ways: 0.005" - 0.020" ABOVE the throttle body gasket surface and 0.005" - 0.020" BELOW the throttle body gasket surface ( see photo below).

It's important that the metering lever allows the inlet needle to close firmly under the full spring pressure, a flooding condition will surely occur if it doesn't. It also has to lift enough when fuel is drawn out of the meterming chamber to open the needle. If it doesn't, a lean condition occurs because the metering chamber cannot refill fast enough. To make things more complicated, OEM Walbro metering diaphragms incorporated the gasket whereas aftermarket types use a separate gasket. This allows an extra source of air leakage, variation in gasket thicknesses and variation in button heights. This has been one of the primary reasons rebuilt Walbros malfunction. This has been going on for years.

There are solutions to these issues. First, use a straight edge scale (on edge) and lay it across the metering diaphragm and gasket that you plan to use. I usually do this after I have laid them on the top of the fuel pump stack after rebuilding. This will place the metering diaphragm in it's relaxed state. Note the distance between the button and the scale (or the disk and the scale if it's buttonless type). You can use a feeler gauge if you need too but I have noticed that most aftermarket kits it ends up being nearly zero. Next, adjust the inlet lever above or below the throttle body gasket surface as needed to take up the distance you measured between the scale and the button (or surface of the disk). So in the case of the aftermarket kits that measured zero, you would place the arch of the inlet lever even with the gasketing surface of the throttle body. If the metering diaphragm that you plan to use is the buttonless type, you would have to raise the metering lever above the throttle body whatever the height of the button is (it's usually about 0.020"). The whole point here is to determine where the inlet lever height needs to be in order for the inlet lever to fully close under spring pressure and also for it to activate quickly when the metering diaphragm lifts. This was a little hard to explain, hopefully this makes sense?

The air leakage issue between the metering diaphragm and the metering gasket on the aftermarket kits usually is a non-issue. Even if they initially refuse to seal up well, after the gasket absorbs a little fuel, it softens and swells creating a good seal. Sealing ridges don't exist on the throttle body but they do exist on the metering plate in the fuel pump stack. The ridges effectively seal the metering diaphragm to the metering plate but they don't do a great job sealing the metering diaphragm to to the metering gasket. Of course, with the OEM Walbro metering diaphragms that were one peice, this wasn't an issue. If you encounter a particularly stubborn situation here, and you cannot attain a decent seal between the throttle body and fuel pump stack, a little Aviation Form-A-Gasket between the metering diaphragm and the metering gasket usually resolves it. You essentially recreate the OEM type by adhering the two. I have also had to occasionally flat-plate a throttle body that has had damage preventing a good seal.

[400brian]

The OEM Walbro inlet needles are much harder to find than they once were. This is the current part number.