A common problem with 4-stroke carburetors is the dreaded bottom end bog. This usually takes place when the throttle is snapped quick from idle and the fuel is not delivered to the cylinder quick enough and the bike cuts out for a split second. The bog can definitely be eliminated or adjusted by re-jetting, but I wanted to try a simple technique using an o-ring.
The idea in this modification is to tie the throttle pulley linkage and accelerator pump linkage together, in hopes the accelerator pump (AP) will then keep up with the throttle pulley. The video shows the step by step process of modifying the carburetor with an o-ring and also included are a few close up pictures for clarity.
Watch the video
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Tools and parts needed:
Step by Step
Right side of the carburetor. The black cover needs to be removed, if carburetor is removed from the bike. This step has already been done.
Right side of the carburetor with black cover removed.
The linkage will begin to separate at about 1/2 to full throttle until the connecting rod bottoms out the accelerator pump. The picture below shows the gap. A similar gap occurs at initial snap of the throttle due to flex in the spring before it “catches up”. This is one reason for the bog. If the throttle is rolled on slowly, the AP linkage will keep up and bogging doesn’t occur.
Putting the o-ring over the adjustment screw and AP linkage helps keep the two together as the throttle is initially opened. The o-ring used here is 1/4″ inside diameter. If the o-ring is too large, it won’t do any good. If it is too small, it may just snap. Using a small tool, stretch the o-ring over the linkage as shown in the pictures.
Now when the throttle is snapped, the o-ring holds the linkage to the screw until about 1/2 or full throttle. This should remove, or minimize the bog experienced when hitting the throttle from an idle. This is assuming all other jetting is correct or close to what is recommended for the altitude where most riding takes place.
This o-ring modification is simple and easy to do. When a bike runs good, there may not be a need to mess with the jetting, so a simple adjustment may be all that is needed. Let me know if this works for you and I will update how this bike performs in a later post.