1978 KZ650 High RPM

Is there combustion on all four cylinders?

Shortly after starting cold engine, would spray water mist onto the exhaust headers and look for it to immediately sizzle and vaporize. If any water drips down the header, the cylinder isn't combusting (for whatever reason).

Also, has spark quality been visibly observed with plugs removed to be a fat blue spark?

Good Fortune!

A KZ650 that can't run @ 70mph :blink: .... That damn thing should be able to hit 100+ no prob... Hell a little 250cc does 65-70mph so you have some serious crap going on no doubt.....

Ignition plate module.... Something odd in that image, maybe just the angle but..... Under the crank bolt, is the rotor/magnet off centered?:blink:

Out of curisoity.... In line fuel filter or gas cap issues?

Just some thoughts on what you need to check...
NOTE, you've mentioned you've already done some of these ok... but unless you truly know the history of this beauty go through a complete measurement, test, adjustment etc....

1) Compression and leak down measurement/testing.
2) Cams are timed to spec...
3) All Electrical test & measurements (dynamo/rectifier/regulator) are accomplished and are in spec.... Voltage measurements to coils in spec... All electical connections solid, not frayed, oxidized burned up etc....
4) Ignition modules or Points & condensers are dialed in.
5) Coils in good condition (cracks, conductor connections), plug wires & cap in spec'd condition.... Engine grounded to spec.....
6) Spark plug conditions dialed in or fouled, carbon, wet... Plug wires stiff, brittle, cracked or soft...
7) Carbs are sync'd and you don't have any vacuum leaks, air filters good, fuel filters good etc....

Since you mentioned good pull & operations up to 3/4 rpm range... Either Needle/clip position changes required (falls flat on face) or Main jets the cause.... Could be clogged...

If it was my 650... I'd be doing all the above.... When in doubt, throw it out.... Meaning if you're border line in the measurement test, get a new replacement...

example, coils are cracked, resistance measurement is right on the edge of being out of spec, replace it....

Plug wires hard, non pliable, cracked been there for 30 years.... replace them....

It just might be a combination of factors causing your poor old girls performance issues for one thing is certain, a stock KZ650 should be able to hit 120mph when dialed in....

Take a weekend & go through the above mentioned, don't skimp on the tasks or measurements... Be prepared to throw a couple hunskies at her if required...

Good luck....

OMR

Ok, I THINK I got it! Did the header spray test as per Patton and the 3rd cylinder wasn't firing! Took the plug out and it was oil soaked. Put all 4 new plugs in and idle sounds great! Didn't notice the difference till after the new plugs I guess. Spray test again....3rd cylinder is firing! Woohoo. take the KZ for a spin and it feels great! No popping low 1st now. Accel is much stronger (again didn't notice till after the swap). Heres the bad part, start feeling a wobble in my back end, look back and the tires almost flat! I will post this problem over in the wheel section, don't want to cross post. But I didnt get above 50 before the wobble, hence I THINK it's fixed. Thanks all!