Poor running CB650 -80?

[Rc03]

Hello!

My problem is that my bike runs great when on high rpm:s but poor during idling. It seems that cyl 2 and 3 deactivates when the rpm gets low.

My thoughts went of course to the right side coil which serve these cylinders but when I mounted a new one I got the same result. The carbs are cleaned and seems well functioning; I´ve also tried with two other (similar) carb sets and got the same result.

The spark on plug 2 and 3 seems ok. Does anyone have an idea of what my next step should be?

Best regards!

Rc03
Sweden

[Mainerider]

Doesn't sound like a coil issue; a weak coil breaks down under load not at idle so your symptoms would be the opposite of what you have. How are you arriving at the conclusion that 2 and 3 aren't pulling their weight? Are your pipes cool on those two? To check for misfires, you can use an inductive timing light as a poor man's ignition scope. Hook up the gun to your batt and then clamp one of your ignition plug wires. Start the bike and let it warm up, then hold the timing light towards your face or point it towards a wall that reflects. The human eye is quick enough to detect any snycopation (un-even beat) in the ignition firing. Try it on each of the 4 plug wires in turn. This will help you in your diagnosis of whether it is electrical or fuel-related.

But it sounds more like a fueling issue, could be your idle/low speed circuits are the problem since it only occurs at or near idle and clears up when your carbs move to the main circuit. Also, have they been synched? Have the other 2 sets been synched? Carb float levels in spec? Sure sounds like a fuel delivery problem...

[Volker_P]

Welcome here!
2+3 failure sounds like ignition. I would first check out ignition timing and moreover the pulser gap beneath the right side crankshaft cover.
As the ignition is series, one bad plug may cause this, too. One may swap 2+3 vs 1+4 plugs to verify.
Also the ignitors (CDI's) may be swapped and checked by this method.

[Rc03]

Great thanks for good and useful advices and for a great site! I will spend some of the easter-weekend in my garage trying out some of them.

The pipes on 2 and 3 are not cold but a lot colder than 1 and 4. None of the carb sets has been synched why that would be my next step.

I´ll be back!

BR

Rc03

[Volker_P]

Carb synchronization with vacuum gauges is fine tuning and moreover only makes sense if everything else is already all right.
To run and idle nice at all four, a visual synchronization should be sufficient. This means all four throttle pistons are set to close to the same very small gap at idle. For CV carbs one has to care for the throttle flaps gap instead.

[arcangel]

I would check the spark on the colder pipes your pulser gens can cause a colder or erratic spark causing the bike to miss and run colder on 2 cylinders

The pulse gens should be around 550 ohms at 60 deg but check them after the bike heats up and see it you get close to the 550 + - 200 ohms hot
check both the blue wires then both the yellow wires,

mine would drop to 1 ohm after heating up on 2 cylinders and cause them not to fire correctly while the other two worked correctly.