YZ400 Accelerator pump problem

Hey guys hows it going? I have a 1998 YZ400 that has been sitting for a while. I decided to freshen everythign up, rebuild the carb, top end, new valves, valve job. Got the carb back on there and put fuel in it and have been pumping the throttle for a while and still am not getting any spray from the accelerator pump. Any ideas? Aside from a bad diaphram. How far is the plunger supposed to move when you open the throttle, it only seems to move a millimeter or two before it bottoms out.

Thanks for the help, I really appreciate it.

Check for a plugged discharge port, stuck check valves, or a ruptured diaphragm.

There's more info about the 400 pump under the "Taffy Mod" in the Common Threads sticky.

I am wondering the same thing

Check for a plugged discharge port, stuck check valves, or a ruptured diaphragm.

There's more info about the 400 pump under the "Taffy Mod" in the Common Threads sticky.

Thanks, Ill check out the Taffy Mod. The diaphram is fine, it was just replaced. My concern is the pushrod doesnt look like it is moving enough to pump the diaphram. Like I said above, it only moves about a millimeter or two. The linkage seems to be correct though.

But Ill head out there right now and check out the discharge port and check valves.

Did you get this figured out? I had the same problem and fixed it today for free. The spring is weak behind the black AP arm. I was able to in essence wind the spring up and have it tucked under the silver arm which comes down and moves the black arm. ill try to get you a pic. I guarantee this is your problem. I was getting a bog 1/4 throttle when I checked the AP squirt, it was non existent. This fixed it perfect!! better then wiring it up. Now I just have to do the taffy mod and im set, the bike feels like a new animal! GL

On the 98 and 99 the pump rod is exposed and gets dirty and rusted. Remove it clean it and reassemble. The 98 and 99 carbs are different then the later bikes. Good luck

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