I have a flat spot at 4000 rpm

Check two things. First the fuel pressure when it starts to cut back. You may be loosing pressure. And second, do an exhaust system pressure test to see if you have too much back pressure in the system restricting exhaust flow.

You didn't say whether your engine is fuel injected or carbureted. If it's equipped with a carburator, it could be that you need to replace the accelerator pump diaphragm. But, you'd notice this even at lower speeds as well with a hesitation when you go to take off. It could also be something to do with the exhaust. Have you modified the exhaust system in any manner? The installation of drag pipes is usually the cause of such a "dead spot" It causes the engine to fail to respond to a slight throttle change and feel "sluggish" in that RPM range. This is due to the absence of backpressure in the exhaust system. The engine performs best with a little backpressure but not so much as to be restrictive. I usually correct this by installing a quarter inch diameter bolt about one and a quarter inches long in the bolt hole that holds the baffle in near the exit end of the pipe. Put three nuts on the bolt on the inside of the pipe. This creates just a bit of backpressure without changing the sound. Usually just enough to solve almost all of the problem. I hope this helps but I may be off in left field as I don't have a lot of information ot work with.

Good Luck and Ride Safe
Steve

Get your bike set up on a rolling road (Dyno) this should cure this problem. Whenever you modify the induction or exhaust system on a production Engine system there will be a compromise. Manufacturers set up these systems to be optimised in all conditions, this will mean less power or torque in certain parts of the rev range.

Hope you have kept notes on all the carb setting changes.
Did the problem start when the exhaust was modified? If not it may not be a carb problem.
Does the problem change if the engine is cold or hot?
Does it make any difference with the choke on?
Is the air filter new and clean, new plug, valve clearances adjusted, new oil.
I know I havent answered the question, but hopfully gives you a few more things to check

Hi Tom your vehicle is equipped with a Rev limiter it will not allow you to rev the engine above 4000 with the trans in park; as for the first problem you may have one of two things, one of the spark plugs or spark plug wires is bad or you have a flat spot on the throttle positioning sensor in which case have be replaced. Good luck?

Your Honda is designed to give you many , happy, trouble free miles. Honda's are known for there stock quiet. Your Honda will NEVER sound like a Harley. When you modify it , it will only sound like a lawn mower with a blown out muffler. If you still want to proceed , you can drill out the baffles if you have a drill bit that is a foot long. If you decide to modify your stock system you have to drill through several chambers of the baffles to relieve the back pressure. With this method if you are disappointed with the out come you can not put the stock muffler back on. Drill only one hole at a time and drive the bike for a day to see if you have achieved the desired affect. If you relieve too much back pressure and develop a flat spot in your acceleration you will have to put in new main jets or a jet kit with needles to restore your performance. You will also have to install a jet kit if you change the exhaust system. I do not have a recommendation for a system . If you go this route be sure the system you pick has a baffle that is adjustable or changeable so you can adjust the sound. Warning: all aftermarket systems will be louder than you want them to be and ear plugs may not be enough to help.

Hi there, there are many exhaust, or mufflers out there to make your bike sound great. Depending in what Country you live in, there are several brand names available to choose from.
I have personaly used cheaper version ones, with great results, sound and performance was the same as expensive ones.
It really comes down to how much money you want to spend. A second solution, is to have your standard exhaust system modified, and de restricted.
You need to contact a Yamaha motorcycle shop. They can advise you, and order your exhaust system for you.

3500 is about normal for an xb at 65 mph. My previous ride was a pickup and rolled at about 2000-2500 on the highway. But the XB's engine is way more "tightly wound". Your rpm gage should run at about half of of your speed...ie: 70 mph will be about 35 (3500) rpm, 80 mph will be about 40 (4000) rpm. Freaked me out when I first got the thing, but after researching it Ive found out that with a stock engine it actually runs more efficient at the higher RPM's. The 06 boxes won't redline until 6000 rpm, so unless you are driving 120 on a frequent basis you're good. You can probably have it modified to run lower, but you will lose the awesome gas mileage and overall it is not going to improve the way it runs, and would have no benefit............

change tranny fluid with mercon v also can add friction modifier (ford part# m-19546-a) and be sure to drain torque converter as well. should fix ya