Igniton? Float levels ?Misfire above 8500 RPM only 5th gear

I had a good 5 hour 650 Seca ride today and had a chance to run it hard on some deserted back roads.
It pulls to the 9500 RPM redline in a real hurry in the four lower gears and up to 8500 RPM in fifth and then starts what feels like a light misfiring from 8500 rpm on up in fifth gear. I think it will pull to redline in fifth but didn't want tokeep pushing it and we do have speed limits.
Any ideas where to look? My plugs and coils and high voltage leads all appear good and I have no problems riding in heavy west coast rain.
Any one else experience misfire only in fifth gear at high rpms?
I have over 50,000 miles on it but it isn't tired yet!

 

Your vacuum-operated petcock might not be able to keep up.

The next time you decide to try some over-the-ton running, try it on PRI and see what happens.

Something to keep in mind: Once you get up over 8500rpm, the motor is fully on the mains and in top gear it's seriously using fuel.

So yes, besides the petcock, float levels can make a difference, as can any carb crud that would affect full flow. ANYTHING that hinders full fuel flow will be a suspect.

Also might want to check and be sure all 4 plug caps are screwed securely on their wires.

Another remote but very real possibility: throttle shaft seals that are just starting to fail...

Even more remote but also possible (it happened to me) be sure all of your little rubber vacuum plugs are actually still rubber and doing their job. Mine had become petrified and two of them were "dancing" around under vibration and certainly not sealing anything.

Check all 4 vacuum nipples to be sure none are loose in their intake manifold. (Experience again.)

 

Thanks, bigfitz
On my bike the throttle shaft seals are not a remote possibility, they are on the to do list for the next tear down as I see some fuel staining on the body around the shaft.
I was wondering if it was fuel supply related as well because it red lines real quick in the lower gears where it only takes a second to get there.
the fifth gear pull takes a little longer and maybe I am starting to need more fuel than can be supplied.
It is just a real light missing like two cylinders might not be supplying their full output.
I have gone over everything in the ignition again and all looks good and the battery is good.
I suppose it is the different cam ? but my 650 sure revs faster than the 750 but the 750 has more torque and it easier to ride on long runs.
One of the guys I rode with a while yesterday could not believe my bike was from 1982 and thought only the old two stroke Yamahas were that fast!

 

If you think that it's Fuel Related:
Check the Fuel Lines for "Kinks"
Check the Filter for Debris and Flow
Check the Fuel Lines for Correct Size (I.D.)

Try running on PRIME at High rpm's.

After that you get into:
Beenie Screens flow.
Diaphragm Piston Movement (Stiction)
Partially blocked Emulsion Tubes.
Petcock Tower Clogged (Especially when low on Fuel)

Ignition Breakdown related to Charging System
Alternator Brushes
Battery condition.

Old Coils unable to handle the load at extreme rpm's?

 

Thanks, Rick

I will work my way through the list.

If it is the coils, wouldn't I have trouble at full rpm in 4th? It will run up to redline in 4th with no noticeable problem, but I am only at full rpm for a split second until I shift to fifth where the trouble starts after 8500 rpm.
Do the coils act up after time at full rpm?
I will pull the petcock out and see what the screen looks like, the last time I changed the fuel filter after one year there was a bit of water in it.
I could try again with the fuel filter out and see how it runs but I am worried about getting dirt or rust in the carbs.

 

5th gear pulls a heavier load then 4th gear. meaning more fuel as suggested. or quite possibly be a compression problem, such as valve adjustment. or maybe. you weigh too much. ( not to be rude). a 150 lb. person can go faster then a 300 lb person.
motorcycles are light weight. it don't take a lot of weight to weigh it down.

electrical should be working since you have no problems in 1-4 gears.

quite frankly, your obviously getting up to speed. i wouldn't worry about trying to break the speed of sound.

 

We have Speed Limits ... depending on where you're from.

I was in the 3rd Lane of the 4 Lane I-95 Speedway between Portland, Maine and the Massachusetts State Line.

I had the Cruise set at 85 and kept pushing on the Gas to keep-up with Traffic high-balling it under the Green, during the mid-afternoon pre-rush hour giddy-yap.

I'd be pissed if I needed an additional 500 -to- 750 turns to pass some big-rig and keep-up with the rabbit whose taillights I have my eyes glued on started to get-away and leave me handing-out all alone in the warp-speed lane not being able to react to the clues that Johnny Law was just over the next rise with a Citation Book, Bic Pen, Instantaneous bad-day maker and knocking-down only those lead foots who are zinging-along at more than 20 over Par.

 

Hi, snowwy66
It is not the top speed that is bothering me, not running perfectly in 5th means there is something wrong with my bike and I want it to be right, not OK or pretty good.
That is the whole fun of working on my bike, I know that it is set up right and working right, it hits 160 kmh with no problems but it seems there is something wrong in the fuel supply or carb setup and it will be very satisfying to find the problem and put it right.
I expect the operation will be better in the lower gears, too.
The full out test is the only dyno I have, this bike is one of the few I have had that will pull to redline in top gear, but it is just not 100% now and I will not be happy until I get it right
I get a satisfaction knowing my bike is running better than most and I know why!
There is also the safety factor knowing my bike will not die or start bogging down when I pull out to pass, we have a lot of 2 lane and mountain roads and I like to limit my time on the wrong side of the road.

 

Eliminate one Factor that would rob you of getting Full Bore at a time.

Scrub the Bores.
Use a Strip of ScotchBrite slightly wider than your finger.
Give them Hell.
ScotchBright -- Gray -- Medium
Followed by:
ScotchBrite White -- Fine Alternatively Polish

Scrub (L) -- Polish (C) -- Finish (L)

 

Some bikes will not even hit their top speed in top gear, that's how much additional load is placed on the motor when running WOT in top gear...

It's not unusual for very subtle problems to show up in this condition. But most common would be lean condition on one or all cyls, as discussed, or possibly a coil that is degraded in such a way that its ability to saturate between firings has been compromised. You'd only notice this at high rpm, extreme high load condition.

Problem is, nobody has or knows how to use an ignition scope anymore...which is the quickest way to see this problem.

 

Your Mixtures might be just Rich enough that the Bike isn't going to make that extra power.