midrange loss of power? suggestions!

[norcalatver]

I'm hoping one of you has a suggestion which will help me solve my mifiring only while riding during midrange on my 400ex. I've tried changing the carburetor, cdi, spark plug, coil, stator, valve adjustment, and ignition. It misfires terribly only while revving in the midrange. Idle is perfect and so is w.o.t. This problem occurred for no apparent reason while riding last wekend. Any suggestions would be greatly appreciated.

[Aallron]

Check the packing on the silencer if any.

[norcalatver]

what would no packing in the silencer do? I know its not plugged.

[cals400ex]

you talking about the middle gears or the middle throttle position? does it do it in each gear if your talking about mid throttle positions?

[Aallron]

With no packing in the silencer there is reduced back pressure in the exhaust system. On a normally aspirated 4 stroke motor there needs to be some back pressure in the exhaust to have the motor run at peak efficiency... same thing is accomplished by adding - subtracting disk's from a disk type exhaust.

[norcalatver]

Thanks for the help Mickey, my problem did lie in the exhaust valves. They weren't seating properly. I don't know why the seats flattened out within a year of having the motor, maybe using Ultmate 2 race fuel isn't as good as VP would suggest? I've always used C-12 in the past and now wonder if the change in fuel caused the premature failure.

[norcalatver]

Has anyone heard of any problems with the oxygenated fuels?

[MIKE400EX]

Is the oxy fuel unleaded?, Is it actually the valve faces or the seats in the head? Valve float will also pound out the seats quickly. Did you add a rev box or higher rpm cam? If you recently had the head worked on, did they really cut the seats too? Many automotive oriented shops don't have small enough seat cutters for the head, however the small valve size still fits in most valve grinders. Ie...did someone only do "half" the job? Just some thoughts. :confused:

FWIW A buddy uses only Ultimate 4 in his Ducati racer and I've never noticed any unusual wear.

[norcalatver]

Thanks Mike, FDO did the work, I'm pretty sure they did a complete job. It's the valve seats that are pitted. I added a very high rev cam but have the stock rev box in there because it doesn't need to rev high being a 493 stroker.

[MIKE400EX]

Norcal,
I'd bet that FDO did the valve job correctly. Usually excessive heat is the cause of valve seat pitting. Is it only on the exhausts? Is it predominantly on the contact/sealing areas or all-over the seats? Maybe the oxy fuel is causing your bike to run too lean. Do you do a lot of riding wide open for extended periods? A wider contact area on both the seat and valve may help you, maybe along with slightly fatter jetting.

[norcalatver]

Thanks again Mike. on the jetting, I had the quad dynoed with a sniffer on the dyno. i was using oxygenated fuel at the time and the operator actually re-jetted it slightly fat at sea level and I mostly ride at about 4,000 ft. elevation. I've never had a problem with VP C-12 so I think that's what I'll use from now on. And yes, the pitting is exactly where you describe and on the exhaust valves only. Thanks for all your help.

[MIKE400EX]

If it's not a really worthwhile difference to run the Ultimate 4 then stick with the C12. Otherwise maybe even a little fatter settings or possibly a FST style aux. oil cooler might keep the head cool enough. I assume you are already running a 9Z plug. When they did the "sniff" test on the dyno was it only at WOT, if so then maybe the midrange or needle setting is too lean? If it's possible, do a sniff test with a load against the bike at a few different rpm points that are less than WOT.