kz305 bogs high rpm wot

rebuilt top end.
cleaned carbs and replaced diaphragms last year.
adjusted idle screw to 1.5 turns out this year.
adjusted static timing as close as i could get it. left side is perfect, right is a little off.
new gas.
float level is perfect.
new plugs. they look like a good colour when i pull them, havent done a plug chop though.
new carb boots.
pretty sure the baffles in the mufflers are shot
new oem air filter.
feels like less power than before i rebuilt the top end.
bogging starts at about 7k rpm in every gear.
runs great otherwise. just getting annoyed by the lack of power at high rpm.
any ideas? jet sizing?

...adjusted static timing as close as i could get it. left side is perfect, right is a little off.


I'd look at it running with a strobe, and ensure it is advancing and retarding. If your pipes are nearly shot the motor might sense a difference in backpressure and it may need upjetting.

A WOT plug chop and check timing would be a good start.

just to clarify, it feels more like a hesitation than a bog, almost like its not getting fuel.

On my way home from work I tried closing the choke half way and the hesitation went away and I got a slight power improvement.

Guess I need bigger main jets?

It's weird though. Last year before I rebuilt the head I was running the same mufflers and only a peice of foam glued to the air box that the PO put in and this never happened.

If the bike ever ran ok with the current jets I definitely would NOT change jets. What is the longest period the bike sat unused since you last cleaned the carbs? If the bike sat with fuel in the carbs for more than a couple weeks the carbs may have gas varnish built up inside them. That varnish will muck things up in a hurry, and can cause exactly the symptoms you describe. Also, if you are not running an inline fuel filter you may have crud getting to the carbs.

I would install a fuel filter and very carefully clean the carbs before fiddling with anything else at all. Ed

Ugh. Really didn't want to pull the carbs again but I'll do it and clean them out again. It sat all winter but the carbs were off the bike with no gas in them. Sat maybe a week when it was back together.

The bike had a filter on when I bought it but it was too restrictive and the engine would stall after 20 minutes because the gas couldn't keep up so I removed it and it ran about half the summer plus 500kms this year like that.

When I cleaned the carbs the first time I never removed the jets and cleaned them proper. I guess I can start by doing that.

Took carbs apart and cleaned the jets. Found lots of black crap at the bottom of the bowls so I'll be purchasing an inline filter. Took it for a ride and its still doing the same thing.
Have to put the choke on half to get any power past 7k rpm.

Noticed the one main jet said 82. Forgot to look at the other one. There was nothing blocking the holes at all.

Clear-tube test?

Test-run with fuel tank cap ajar (not snapped down)?

Good Fortune!

i have done the clear tube test several times and the level is about 2mm below the seam.

will try the gas cap unlatched. i have noticed a sucking sound once and a while from the cap after a ride. figured it was normal?

Janz3n wrote: just to clarify, it feels more like a hesitation than a bog, almost like its not getting fuel.

Janz3n wrote: . . . noticed a sucking sound once and a while from the cap after a ride. figured it was normal?

Inadequate fuel tank venting, as could happen with a clogged stock vent, may cause a vacuum inside the tank, which impedes fuel flow from the tank, resulting in "fuel starvation."

Perhaps such vacuum is being evidenced by the sucking sound as outside air is pulled into the tank.

Test-running with the fuel cap ajar aids diagnosis by providing more than adequate tank venting.

If the issue disappears with the cap ajar, it will be be time to correct whatever is obstructing the stock tank venting system.

If the issue persists with the cap ajar, there are other possible causes for fuel starvation that may then be addressed.

Good Fortune!

Tried leaving the cap open first thing in the morning with the same results. After about 10 minutes of cruising I decided to punch it with the choke off and it seems to have fixed itself.

There a small flat spot around 8k but its not bad. Stumped.