My 1992 Yamaha Seca II (XJ600s) will not rev past 7000 RPM

Alright, brace thyselves for a wall of text.

Last year I bought an xj600s to use for commuting and pleasure. Cosmetically it was beaten to hell but it was a strong runner.

Over the winter I resolved myself to get it running optimally. So I swapped out the valve shims and this spring I did a carb sync/mixture adjustment.

On the first two rides of the spring there were some issues with bogging during acceleration. The plugs looked rich but I decided to go the extra step and re-clean the float bowls/passages.

I put the floats back together and reinstalled the carbs...

This is where the nightmare begins.

After reinstalling the carbs the bike now refuses to rev past 6-7000 RPM. It idles very stabily and purrs like a kitten up to 5000 but after that it's like it hits a wall, begins backfiring and drops back down to the idle range of 1250 RPM.

I figured that maybe I put the floats together wrong so I pulled the carbs and redid everything but nothing has changed.

Things I've tried:

1. Fuel levels - all carbs are reading the correct fuel levels when tested.

2. Float levels - correct, as specified by my Haynes manual.

3. Carb diaphramgs - no holes or tears and they're sitting properly.

4. Carb sync - close as I can get them. Needed the assistance of my father's because the sync screws are a pain in the a*s to get to on this bike.

5. Mixtures - definitely a tad bit rich but no major soot build up or oil. During the mixtures richer does slightly improve the issue, but not by much

6. Spark test - four brand new plugs all showing good spark. Tested my coils as well and there seems to be no problem there.

7. Jets - I haven't touched them. And the bike was running fine last year. No gunk buildup or debris and I sprayed them out well.

8. Vacuum fuel pump - the old one looked grodie so I swapped it out for a new one but no change.

9. Inline fuel filter - swapped to a more free flowing one but no change.

10. Airbox/filter - removed, cleaned and reinstalled. No change.

11. Tank and petcock. Can't be the issue, as the problem persists with both the original tank and the auxiliary one I've been running the engine off of.

12. Intake boots. Brand new, no tears or cracks.

13. Vacuum leaks. Cannot hear nor find any.

I suspect fuel starvation of some kind. But I don't know what's causing it.

If I rev it up to 7000 then the bike stutters and backfires. But if I try to do it again immediately after then the bike will not even get up to 3000. The float bowls are filling up, but the carbs cannot suck up fuel fast enough to sustain the high RPMs.

My father recommended re-checking my valves clearances. Clearly I'm more amateur than i thought, because despite triple checking my work I was way off. Three of the intake valves have clearances which are too loose. And one of the exhaust valves was a little tight.

To be more specific:

Intake range: .11 - .15 mm (0.004 - 0.006 inchs)

Exhaust range: .21 - .25 mm (0.008 - 0.010 inches)

Intake valves for cylinders 1, 3, & 4 were loose. A .28 mm feeler gage could fit into #2, for context.

Exhaust valve for cylinder 2 was loose.

Part of me wonders if the valve clearances could be too blame. But I can't rule out the carbs just yet.

Another possibility:

The carb diaphragms, or in layman's terms the sliding parts that open and close in response to vacuum; do not appear to be moving at all when I rev the engine.

Before my current issues, I had an issue with them sticking. When I push them with my finger now they slide up and down as they should. But I do not see them moving in response to changes in throttle.

Any ideas? Thanks.

 

if you had the carb hats off , then did you make doubly sure that the diaphragms are seated and sealed correctly, i usually stick the rubber into the grove with permatex gasket adhesive, any leak there would cause the slide not to lift.
you can check this (with the carbs off the bike) by blowing down the kidney shaped passage on the airbox side of the carb intake

 

Run the bike with the airbox off - you should be abale to see the sliders move as you open the throttle (operates teh butterfly) and more air is sucked in
If these aren't moving or aren't returning then this is part/all of the problem - you remembered to refit the slide springs?