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.