Power Above 6000RPM Lost

[Peter G]

Hi everyone,

My '83 xs400 maxim has been treating me very well lately. I've installed new handlebars and new slip on mufflers. Looks and sounds great in my opinion!

The issue arises ever since I put on the new mufflers. the bike accelerates approximately the same to about 6000rpm, and anything past that is mainly just engine rev noise but hardly any power. In an effort to "fix" this I replaced the main jets in both carbs to one size bigger from stock, and since doing this there hasn't been any real improvement in the upper portion of the power band. My air filter is clean and I've been checking the plugs frequently and its not running too lean, maybe the tiniest bit rich if anything.

Any ideas?

Thanks in advance guys.

 

[xschris]

From the looks of the mufflers you have the emgo shorties. Those are about the worst ones to use/tune. Your power loss is probably from little to no back pressure. I have never tuned a dohc bike or one with those type of mufflers. One of the dohc guys may chime in on this.

 

[Peter G]

From the looks of the mufflers you have the emgo shorties. Those are about the worst ones to use/tune. Your power loss is probably from little to no back pressure. I have never tuned a dohc bike or one with those type of mufflers. One of the dohc guys may chime in on this.

You're certainly right about the back pressure since there is definitely a lot less baffling. But as I understand, back pressure isn't as necessary when you're wide open on the throttle? I could be wrong, but if it were a back pressure issue, am I not loosing power in the wrong place?

 

[xschris]

If you still have stock air box/filters your letting more air out with the same air going in. The stock exhaust and intake was designed to give the bike the best power over all the rpm range. When you change one of those everything goes out of whack.

 

[16VGTIDave]

With those mufflers, try leaning out the mixture. Smaller than stock main jets (less fuel, less power). While they sound "cool" (i.e. obnoxiously loud), those mufflers are what you paid for them. I'd wager they are causing reversion and reducing performance, like open pipes are known to do.

6000 RPM is the top end of the needle's range, and the beginning of the main jets operation. If changing the main jets doesn't help, try adjusting the needles. I'm going to guess lower, but you will have to test and tune accordingly.