Topic: Stock CB650 runs rich...unless no air filter....

[BikeKayakSki]

Hey folk, I have an odd issue with my 81 cb650. The bike refuses to pull above 6k and has no power up there, it just starts misfiring. I rebuilt the carbs and used stock main jets, and cleaned the hell out of everything else. The bike also has new plugs, and a new stock style K&N air filter in the airbox. The bike idles fine, and pulls off from a start pretty well.

Here's the weirdness, if I run the bike with no air filter, it pulls and runs great all the way to redline, NO ISSUES. Not a hiccup anywhere. Which is weird, because the bike should be running super lean with the filter out, and is indicated as such through the plugs, but not as lean as I would expect. This is really confusing to me because of the fully stock setup I have going on here. But the stock setup seems to be making the bike run rich, which is leading to misfires at high rpms.

So what do I do here? I was seriously considering putting a screen in the airbox to keep serious junk out and run it as such, but I feel like that's the easy way out.

Anyone have an idea?

[Steve_K]

A dirty filter can cause that.  Maybe an old one could not flow due to age. 
I assume there is no sock over the intake opening?
Steve

[BikeKayakSki]

Hey Steve, as stated it has a brand spanking new stock style K&N air filter in it. My old filter was very clogged, and the bike wouldn't do much above 5k with that in. The new filter got me about another 1 to 1.5k more in the RPM range, but thats it.

As far as a sock over the intake...no there is not. Where would that be located if it was installed?

[jonda500]

Sounds like going one size smaller main jets could be worth a try? (or get an oem air filter or use less oil on the k&n filter.)
John

[scottly]

Did you over-oil the K&N filter? They usually come pre-oiled, and can be installed straight out of the box, after removing the cellophane wrapping, of course.

[BikeKayakSki]

Jonda: I'm really thinking about going the smaller main-jet route. A thought occurs too, I took the vacuum petcock off my carbs, so the fuel is being gravity fed now. With this kind of mod is it necessary to rejet?

Scottly: I didn't put any extra oil on the K&N, so its strait out of the packaging at the moment.

[jonda500]

Jonda: I'm really thinking about going the smaller main-jet route. A thought occurs too, I took the vacuum petcock off my carbs, so the fuel is being gravity fed now. With this kind of mod is it necessary to rejet?

Changing the petcock wont affect the mixture - you did close off the vacuum port that used to open the petcock right?
John

[BikeKayakSki]

John: Yeah I plugged the hole in the #2 carb.

I did a bit more research, and found that some early 80s 650s run way better with the airbox lid off...so I found some washers to hold the air filter in place and ditched it. Runs like a champ now.

[DH]

John: Yeah I plugged the hole in the #2 carb.

I did a bit more research, and found that some early 80s 650s run way better with the airbox lid off...so I found some washers to hold the air filter in place and ditched it. Runs like a champ now.

You might check this out.....http://manuals.sohc4.net/cb650/technical_reference/SB650_1.pdf