This issue has likely been already answered somewhere in here, but I couldn't find anything using several search terms...nonetheless, I apologize in advance...

My '78 XS 400E starts easily on the kick-start (starter not working...winter project), idles fine at around 1300rpm, carburates smoothly during normal acceleration in all gears, except at around 5500 and above, and going up hills in top or 5th gear. Then it hesitates a bit, and won't give any more than about 6000rpm.

Battery is fully charged, new gas, carbs have not been rebuilt, but were dismantled and blown out with carb cleaner and compressed air. No crap noted in float bowl when taken apart. No rip/pin holes in diaphragms. Carbs visually bench synced. Petcock screen clean, petcock rebuilt, new vacuum and fuel lines, as well as carb boots.

Stumped.....

just a chance ? possibly ? maybe? it could be the same problem i had on one of my off-road bikes?

the rubber from the airbox to the carb is thin and old, so when you rev hard, the rubber collapses in on itself a bit.

that stop the engine from working properly, so you ease off the throttle and all is well again because the enginge is not sucking so much air.

as soon as you open the throttle again, it does it again!

when you are in your garage, checking everything, you dont see it as the rubber is it`s normal shape?

i had this problem before on an old Bultaco Persang if i remember rightly?

I'd check the the ignition timming

starting with the points gap using dwell if you have one.

then strobe the bike

drewps

Thanks, Drewps, yeah, I was thinking along that line, but needed a kick in the butt to get me moving in that direction. I have other bikes that require more immediate attention, but one afternoon I will sit down beside the mighty 400 and become one with the ignition....I'll let you know the results, eventually...add to the community knowledge....

Thanks, Drewps, yeah, I was thinking along that line, but needed a kick in the butt to get me moving in that direction. I have other bikes that require more immediate attention, but one afternoon I will sit down beside the mighty 400 and become one with the ignition....I'll let you know the results, eventually...add to the community knowledge....

what happened! how did you fix it. found myself having the same problem today screaming up a steep hill in 6th gear. it would not go past 5,000 rpm.

and dammit I found

from this thread that I might have a leak in one of my venturi slides. since my top speed is ~70... which is plenty fast for me but I don't stuff leaking.

I'm with drewpy- sounds like timing. Mine did almost exactly the same thing and after i re-gapped the points and static timed it, it started running great at just as high an RPM as I dared go. I static timed mine, which, if there is no mechanical problem with the ignition and advance, should be fine. I followed these instructions.

I just found it with a search.

I'm with drewpy- sounds like timing. Mine did almost exactly the same thing and after i re-gapped the points and static timed it, it started running great at just as high an RPM as I dared go. I static timed mine, which, if there is no mechanical problem with the ignition and advance, should be fine. I followed these instructions.

I just found it with a search.

damn looks good and not too hard. I'll give it a whirl tomorrow. thanks for the link. add that to the sticky drewpy!

ok I'm an idiot I can't even get the stupid cover plate off. it looks like there is a plug instead of a screw?!

do I have to drill it out

didn't realise you had electronic ignition, I think you need to look at the TCI box first. drop speedshop a PM as he can give better advice on these things!

BTW those are plugs and would need to be drilled aout and behind there should be screws.

didn't realise you had electronic ignition, I think you need to look at the TCI box first. drop speedshop a PM as he can give better advice on these things!

BTW those are plugs and would need to be drilled out and behind there should be screws.

thanks drewpy. sounds like I definitely shouldn't bother drilling out the plug then if I have electronic ignition. The haynes manual I have says that it should be trouble free and I should never have to mess with the TCI system and that "regular timing adjustments aren't needed". so I probably should look elsewhere for my rpm issues.. as I said before the bike runs great just doesn't seem to have any top end speed (this is probably a good thing.. evil knevil jr jr). I'm too the point where its full on riding season here so I don't plan on taking stuff apart full-on and jeopardizing riding time. but If I figure something out or some1 else runs into the same problem or ideas be sure to post as I will