Bogging down at 5000-6000 RPM

Hey guys, all the rest of the issues are straightened out on the XS400...
Took it to work yesterday, maiden commute voyage... on the way home I noticed a bogging down between 5000 and 6000 RPM, after that all the throttle in the world doesn't seem to make a difference. A bit of chugging like it's starving for fuel, below that it runs great....

Thought, bad vacuum hose on the petcock, replaced that and no difference (needed to be done anyways). Went for a long run this afternoon, everything else is working great..
Other thoughts are.....

1) hole or tear in the petcock membrane
2) clogged gas cap vents (I did get a small rush of air opening the gas cap afterwords)
3) Main jet issue

fixes..
1) found a place to order a new membrane
2)how do I clean a clogged vent? looked at it earlier, not sure how to proceed.
3) yet more carb cleaning

Think I'm on the right track here?

 

The Diaphragm Pistons could be sticking.

Those Bores are not exempt to Aluma-oxidation. They get a film on them that needs to go away for a smooth application of power.

Do a "Clunk Test" (Search Clunk Test)

If you have full and unimpeded movement ... we look elsewhere. If not, you're on the right track to getting the Tach moving-over to the right a little bit quicker!

(I've been Polishing the Bores on the Carbs sent to me for Cleaning and Restoring. A Dremel Polishing Pad fits in there with JUST enough room to do the job GREAT!)

 

Thanks Rick, wouldn't have thought of that one.....
I'll add that to the to do list as well....glad I picked up that Dremel when it was on sale over the summer...more than paid for itself already

Guess I'm going into the carbs...again

Wanted to check on the main jets as well....I was talking to a guy that had a similar problem with his 400 Seca...when he cleaned his carbs, he swapped his main jets by accident.... I know I didn't do that as I did min one at a time, but who's to say what a PO may have done.

I know what I'm dong tomorrow, that and cleaning up an old Shoei FM2 fairing I picked up....

 

OK, tore the carbs apart again.... interesting stuff.....

1) found main jet at the bottom of car bowl on left side, guess that explains the sooty plug....
2) compared numbers on the jets.... 127.5 on right, 117.5 on left
Should be 127.5 on the Left and 120.5 on the right, 117.5 was for the Seca 400 (same engine, different trans)
3) Lots of fine rust sediment at the bottom of the bowls, getting past the filter, or was just left over before the filters were there????

Did a clink test, the passed, but I gave them a quick buff and polish anyways to be sure.

ready for a test run..... will report....

 

Report is in...

WOOHOO!!!!
MUCH better...., starts are smoother.....a lot smoother, a lot more subtle response on the throttle now...
Did a couple of shifts at 7000-800 RPM... no stutter.....
Better speeds in all gears.....]

Lessons learned...
Don't forget to tighten your jets after cleaning.
NEVER!!!! assume a PO put things back together correctly.
And thanks to Rick, I now have shiny bores

 

I have a similar problem with my 1982 Seca 650. In the lower gears, i do not get as pronounced a lag, but in 4th and 5th, btween 5000 and 6000 rpm's is a dead zone. I can twist the throttle and nothing is happening. My carbs passed the clunk test, my filter is clean, the carbs are clean, beeen seafoamed and now I al scrubbling the slides and resetting the needles 1 notch richer (Dynojet jet kit installed) to richen up the mixture at that rpm level. I know the Seca 650 has an inherent flat spot at this level due to epa lean mixture from the factory, but hopng i can get around that a bit. cruzerjd

 

If you are CLOSE to being "Dialed-in" ... you still have the Option to Tweak it "On-the-fly."

Find a place where you can leave a Parking Lot ... get on the Highway and Return and do a "Kill and Coast-in" to where you can safely do your tweaking.

Just do a Plug Read and make INDIVIDUAL adjustments to the Pilot Mixture Screws. The Plugs will tell you the story ... which are Clean-Lean and need some more Fuel ... and, those that are a little too dark ... needing a little bit of tweak IN to lighten-up the burn.

I had to add some to my bike ... even though I thought I had it right on-the-money.

After a couple of outings ... you'll have found where the Mixtures are at their best and you'll have a nice burn and good power as the result.

 

Thanks for the tip, I'll see how close i can get , probably next weekend....
I need to give her a good run now to see how she fares