Hesitation / lack of power around 5k rpm

I have had a nagging problem with my Z650-f4 (1984). It looses power at about 5000rpm. It accelerates okay to that point but as soon as I reach about 5000rpm it starts to hesitate and loose power. Power drops off a cliff if i put it into top and even worse if I hit a hill or into a stiff breeze. If I thrash it through 5000rpm it seems to pick up again. It is also fitted with a Harris 4-1 exhaust. Garage checked compression and said it is okay and good for age. It has had a set of new green 3.0ohm coils fitted before I got it.

Things I have tried.
1) New carb/engine rubbers.
2) Cleaned out carbs, tank filter and petcock.
3) New air filter.
4) New plugs and leads.
5) New carbd diaphams
5) Set and balanced carbs using Colour tune and Carbtune tools
6) Upsized main jets (from standard 102.5) to 107.5
7) Experimented with different needle heights (currently on mid position.

Any suggestions


Sorry, should add that the problem is worst on full throttle. Ease off a bit and it improves, but still low on power.

Thanks in advance

Stewart

Sorry, something else, when changing the jet needle position I noticed that there is a small washer on the needle just below the clip. Is this standard or is it a previous half way fix between two clip positions??

Seems you've checked out some things already.
There's a similar post (or more) on kz650info at the moment:
forums.kz650.info/index.php?topic=3729.0

Spark plug reading?
--- after throttle chop at problem rpm.

Valve clearances to specs?

Ignition timing advancer functioning properly?
--- watch action under strobe timing light.

Fuel starvation issue?
--- from any of several possible causes.
--- Might be simply a clogged vent in fuel cap.
--- Would test run with loose fuel cap.
--- In any event, need ample clean fuel supply to carbs.

Clear tube test?
--- otherwise perfect carbs suffer from incorrect fuel levels.

Good Fortune!

Might be crud inside fuel tank, suspended and hovering about petcock pick-up tubes inside tank, and obstructing pick-up screens at higher throttle.

Could remove tank, shake, and fast-dump fuel out open filler cap. Add some fresh gas, and repeat shake and fast-dump.

Good Fortune!

Set your fuel heights perfectly, then go back to the stock mains. Increasing the fuel, decreased the power.

Check the exhaust for an ubstruction, like a nest.

Patton wrote:

Spark plug reading?
--- after throttle chop at problem rpm. DONE THAT. SEEMS OKAY

Valve clearances to specs? DONT KNOW. REPLACED GASKETS LAST YEAR AND SET CORRECT THEN.

Ignition timing advancer functioning properly?
--- watch action under strobe timing light. I HAVE HAD IT OFF AND CHECKED OPERATION BUT NOT UNDER STROBE.

Fuel starvation issue?
--- from any of several possible causes.
--- Might be simply a clogged vent in fuel cap.
--- Would test run with loose fuel cap. DONE
--- In any event, need ample clean fuel supply to carbs.

Clear tube test?
--- otherwise perfect carbs suffer from incorrect fuel levels. NOT SURE WHAT YOU MEAN. DO YOU MEAN CHECKING FLOAT LEVELS. CHECKED THESE AND ARE CORRECT.

Good Fortune!

I have checked most of these out. Would going up to 110 main jet help.

Patton wrote:

Might be crud inside fuel tank, suspended and hovering about petcock pick-up tubes inside tank, and obstructing pick-up screens at higher throttle.

Could remove tank, shake, and fast-dump fuel out open filler cap. Add some fresh gas, and repeat shake and fast-dump.

Good Fortune!

Have had this off (at least 3 times) and checked for debris. The tank filter was pretty clean.

TeK9iNe wrote:

Set your fuel heights perfectly, then go back to the stock mains. Increasing the fuel, decreased the power.

Check the exhaust for an ubstruction, like a nest.

I have had the carbs off (at least 3 times) and checked the float levels seem okay. 18.6mm in the Clymer manual?

Fuel heights. Do you need a special tool for that or can I measure it with a tube from the drain pipe.

Can anyone also confirm the spark plug gap. On searching the internet there seems to be a bigger gap recommended for 3ohm coils??

Thanks

Stewart

You don't say if it ran right before the 4 into 1, these require a bigger step up in main jetting, but fist you must make sure your not having another issue. First as suggested make sure your floats are all set to the correct level, as if low this will lean your carb's and high rich.
As far as the washer at the needle position, you can check your manual to be sure of it's inclusion or position. When tuning carb's try to get a plug reading when operated under the same condition, bring you plug wrench and pull over and pull them all out after running at the problem area. Motors are Air Pumps, 4 into 1 means it will pump more air, so more fuel needed to keep the correct air/ fuel ratio. Sounds simple right? It may take 1 adjustment of jets or it may take several, be patient.

Based on your post symptoms and reporting...

"Upsized main jets (from standard 102.5) to 107.5"...
Why was this done for this is a fairly large jump in mains jets...

The symptom you're describing seems the term "Falling on it's face" when describing carb issues. Based on the Jett charting graph below... running Lean! @ 5Krpm, this would be ~ 3/4 range right between needle and main operations.

As mentioned, go back to 102 jet and take for a test ride, hit the 5Krpm range and make note of performance. If all good, then tackle mid rpm range needle clip position/jet.

Good luck...

OMR

Lean # 3....