Engine cutting in and out baddddd above 3500 rpm's

Hey guys, I have a 78 KZ650 that cuts in and out very bad above 3500 rpm's. There have been a (few) times that it has ran normal. Doesn't make sense. Everything is either new or checks out normal. I've been trying to figure this out for a long time. I just don't know what else to do besides go internally. Engine compression is around 170 in all cylinders. This is my second set of carbs. I've been through all the jetting and adjustments over and over. Valves have been adjusted. I've used dyna ignition and I've used points with no change. I have new dyna coils. I have 11.8 volts at the coils. I also have a stumble off idle when warm. Any suggestions before I give up ??

is the advance on your ignition working?put timing light on and check would be the easiest starting point for me

Ignition advance is working perfectly.

hang on my oldbraindont work that fast anymore!:laugh:

it can also be wires coming off the ignition pickup coils or points.
My old kz750 did this on 4 of the ignition pickup coils i purchased or recieved gracefully from other KZ owners.
I finally removed & replaced the wires off one, and it ran great afterwards.

is it getting enough fuel ?

There is one thing I haven't checked. The exhaust gaskets. I don't hear any leaks but I've noticed that after a vehicle in the other lane goes past me the engine really looses power even more.

Totally getting fuel.

Kawman wrote:

... 78 KZ650 that cuts in and out very bad above 3500 rpm's. There have been a (few) times that it has ran normal. Doesn't make sense... also have a stumble off idle when warm....

Each coil of course requires a continuous uninterrupted voltage supply.

Could test for possible intermittant wiring short supplying voltage to coils -- by running a temporary new wire direct from positive battery terminal to coil primaries. Then test ride for effect. Don't leave temporary wire connected, as that's the same as leaving ignition switch turned ON. This test is akin to the wg coil mod.

There's likely a connector under the tank (1 wire in, 2 wires out) providing positive voltage to the coils. It's possible to have a non-visible internally frayed wire causing intermittant voltage to the coil inside the wire from the connector to the coil. Could test with ohmmeter for continuity or test with engine running by manipulating and wiggling the wire.

As to the stumble off idle when warm -- often caused by less than perfect pilot circuit in carbs (air or fuel passage or both partally obstructed, or over-rich mixture due to a defective pilot jet, or too large pilot jet). Could also be caused by enrichener circuit failing to completely close when choke is OFF (for whatever reason).

Would assure air filter is clean, clear and unobstructed, and not over-oiled. Could test run without air filter to determine if stumble off idle persists .

Could also try leaning the pilot circuit. Side located air adjustment screws would be turned out counter-clockwise to allow more air. Bottom-located mixture screws would be turned in clockwise to reduce mixture volume.

What plug wires and caps are being used and what is their condition? Hope new Dyna solid wire core plug wires with built-in caps are already in place.

No specific mention yet of whether using brand-new spark plugs of correct size. Thinking NGKB8ES or maybe 7ES.

Has plug reading been done? How do they look?

Just thoughts. Good Luck!

in addition to things mentioned, have you checked the screen in the tank, may have some crap on it that is not letting enough fuel flow

If you are using a fuel filter, replace it and make sure it is installed correctly this time. Sounds like you are using one and it is installed backwards. May also be a vacuum lock. Next time the bike starts cutting out, pop the gas cap open. If the bike quits cutting out, you have a cap venting issue. If these things fail to resolve the issue, start looking at the ignition system.

Thanks for all your tips george. My fuel filter is fine and installed correctly. I did change out some of my air screws and it ran fine. Is that not weird. I'll ride it some more and let you guys know what happens. Thanks