After I recently installed a new cylinder, piston, & rings in my 400ex and have noticed a severe bogg (like a vacuum leak) when the rpm's are low. After the RPM's climb a little higher, everything clears up and runs fine. It has been about a month since I last had the machine running and had no problems before. I have checked the plug, timing, and for vacuum leaks already. Thanks!

 

Sometimes when you bore a cyclinder or put in a larger one, you'll lose a little bottom end, but it shouldn't bog. The carb has an accelerator pump on it for lower RPMs you may want to check it for proper operation. Maybe the diaphragm has a hole it.

 

Try opening up the pilot jet underneath the front of the carb. See if it makes a difference. Mine had a hesitation off idle. It was more of a hiccup/backfire than a bog, but opening the jet 1/2 turn helped me a lot.

 

Thanks for the reply's. The bog is not a small hickup or a little bit of low end lost. This is a huge bog that is on the pilot jet only. It is probably stopped up with sludge or something (since it did set a while). I was just wondering if someone else had this happen to them and it would give me a head start on fixing my problem.