Topic: Cb650 wouldnt rev past 6K (somewhat fixed,but have questions)

[Rubberman2003]

My cb650 would rev up fine in neutral, but under load would hit 6-7K and not go any farther.  Ive adjusted all the timing,advance,valve,pulse gap,carb ( carbs ARE clean) ect.  Here is what i find weird, i take the air filter out, run the airbox cover and side cover, but no filter, and i can rev to 10K, and loads of power.  Which tells me the bike is rich.  However the bike has a Mac 4-1 exhaust and stock 120 jets, wouldnt the pipe lean the bike out?  It just seems weird to me that the bike is running so rich with the stock jets!  BTW this is an 81, so not a lot of adjustments on the carbs.  Any ideas?

[curemode2002]

Actually one of the most common mods to the 650's is to remove the airbox cover and hold the filter in with just washers on the screws to get more air. I would think you would be a little on the lean side as well but these bikes really suffer for airflow. I actually run a K&N filter in mine with the airbox cover removed and I get pretty even running just a tad on the rich side. Some guys run with the K&N and drill a ton of holes in the airbox cover. But mixture wise the best thing to do is get it running and do a plug chop there is a post that covers that in detail. That will at least get you a good baseline that you can use to see what needs to be tweaked.

[Rubberman2003]

I will get some new plugs and see, i tired removing the airbox cover ect ect, using the washers to hold the filter in, only thing i can do that works is take it out.  Say i get plugs and it is showing it just right without an air filter, then what do i do?  Just get smaller main jets?  not a lot of things to adjust on the 81 carbs


Seems weird to lean a bike out from factory jets that has a pipe on it

[curemode2002]

Yes if they look good without the filter you would need to downsize the mains a little. Unfortunately mine is an 80 so I am not sure without looking at my manuals at home what the stock size should be. Z1 Enterprises is a great place to get jets they are a good price so you can get a set of a few different sizes.