Won't rev above 8k

[ltan667]

So I got my bike back from the shop a few days ago and my mechanic said the bike is having trouble above 8k. Rather than have him work on it yet, I decided to see what was wrong first. I took it out for a ride yesterday for about a good hour and for 1st and 2nd gear I could get to about 10k before the engine would stumble right on it's face. For the rest of the gears, I couldn't get close 8k even if I was super smooth with the throttle.

It seemed like it was very lean up top, and if I snapped the throttle back it would start to stumble but if I had rolled it VERY smoothly then it would slowly climb. I was riding at about 60 mph in 6th barely doing 6k and if I had rolled the throttle back at all, it would start dying and I'd have to downshift a few gears just to get it going again.

A couple things about the bike:
-It does have a Cobra F1s can on it
-Previous owner cut a HUGE hole in the airbox but my mech taped it up. (Before he taped it, I was able to rev to about 12-14k maybe this is the problem?)

So should I try removing the tape from the airbox first? I remember when there was no tape, the idle was not consistent at all. I've NEVER done carb work before nor am I a very mechanically incline person but I'm wiling to learn and try. Any suggestions?
And sorry for the long post lol.
Regards, Leon

[smooth operator]

  I'd take a plug reading to see what its doing. Were the jets chaged at all? It sounded like lean to me too but now your taking away intake air by taping over the hole in the airbox. So thats why I suggested checking plugs 1st and go from there.

[tomacGTi]

Did you mechanic change any jetting before messing with the airbox? If not, then there's your answer.

If the PO had put in a bigger main (would be bigger than 102.5 for a big airbox hole) and the mechanic then taped over it, it would be pig rich now. The Cobra F1S isn't a high flowing slip on but airbox work would definitely require some jetting changes.

Remove the tape and take it for a ride. If it surges at full throttle at higher loads, it's lean. If it accelerates as you roll back the throttle from WFO it's rich.

Unsteady idle is a whole 'nother can of worms. Probably need to mess with the mixture screws and synch it.

Barring looking into the carbs directly and reading the mains, that's the easiest option.

[ltan667]

Thanks for the reply fellas. As far as I've been told, the jets on the carbs are stock. There seems to be soot coming out of the tailpipe if I just rev it in neutral, is this overfueling? I'm going to try to take off the tape he put on today and see how it gets and I'll report back later.

Regards, Leon.

[ltan667]

Well I took the tape off and the beast was unleashed!! 

At first it stumbled at 6k but after a while of running it, it seemed to be perfect. Powerband kicks in from 6k to 12k but didn't go any higher than that. Still seems to be soot coming out when I rev it in neutral to about 12k. Now the problem is the idling,  it doesn't seem too stable. It'll idle REAL low, but I'll rev it so it won't die.

Thanks for all the help fellas!

Oh, does this mean that the carbs are not stock now?

Regards, Leon.

[andylad]

This is exactly what is happening with my bike now.  Currently it doesn't have a filter foam but thats on order.  In the mean time, I have a slug of foam cut out of a briggs filter foam piece and have it in the intake pipe with a screw in behind it so it wont suck through.  I have another issue too though, raw mist of fuel out the exhaust.