74' CB125S sputtering at high speed / load

Hi guys,

I'm fairly new to bikes but had a 70' SL100 as a kid. I picked up a 74' CB125S a few weeks ago and am having an issue when running in top gear and over 7k. After 5 minutes or so at this speed it starts sputtering or lurching. The engine loses power and quickly picks back up and starts this kangaroo hop effect. I rebuilt the carb last night and it was not too bad inside. Put in new jets and everything except the float. Took it out for a spin and it still has the same issue. Was hoping to get some ideas as to where to start troubleshooting.

It does have a pod filter FYI... I have the original airbox though and am in the process of stripping and repainting it as it was in bad shape. The previous owner had turned it into somewhat of a cafe style and after a bunch of reading I hear that these motors hate POD filters.

Do you have the oem exhaust?

Yes its a stock muffler.

seems to me if it was just the pod filter it would happen straight away in all gears at those revs. as its only in top gear and after 5 minutes it could be a fuel supply problem. have you got an in-0line fuel filter, could be clogged and as you're using more fuel at high revs its taking more effort to get through and once the revs drop off it gets a chance to refill the carb ? have you checked the airbleed hole in the fuel cap, it might be tiny but it will be there somewhere, if its blocked it'll vacuum lock the fuel supply and cut off the fuel until the vacuum is dropped or broken (by the engine dropping its demands for fuel) and will then let more petrol through. try stopping as soon as it happens and open the cap, see if there is any sort of hiss or sucking noise, this would indicate a vacuum lock problem. if you try this, does the bike still have the problem when you set off again or does it only recur after another 5 minutes?

have you tried running at 7K in lower gears for a couple of minutes at a time to see if the same thing happens?

doesn't sound electrical (coil, ignition etc) as that could cause the same problem but again would happen in all gears and would usually pop up after the system warms up. I've had coils crap out and the problem seems to come lower in the revs and gets better when you thrash the hell out of it. that was on much larger capacity multi cylinder bikes but the effect should be the same

don't know if I've solved the prob for you but maybe some stuff to look at

good luck

ps this is my 800th post, how cool is that

spotty,

Thanks for the reply. This gives me some things to try out for sure. Your thoughts about it not getting enough fuel make a lot of sense to me. She does have an inline fuel filter that the previous owner installed and although it looks clean, maybe it is just restrictive enough to cause issues anyway. I am going to pull the petcock and clean it out good, remove the inline filter, and give her a test run. If the problem continues I'll check for vacuum lock on the tank. It has a newer cap but it's still worth checking.

I have not tried running at 7k+ in lower gears because I live in town and I'd feel like a dick trying that. Might get out to some country roads and give it a try though.

Well, I removed the inline fuel filter from the line and took her for a ride in the country. No issues! Thanks a ton for your help spotty!

excellent, glad to help (had to happen one day), I shall have a beer to celebrate

maybe put a bigger filter in that won't restrict the flow but still gives you the protection