SOHC.co.uk Forums > CB500/550

Misfire and running on 2 cylinders when hot?

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Hudson.:
I recently got my 78' 550 road worthy. Recent work includes a total carb rebuild (floats are all set, everything clean and clear), pod filters (I know!), larger main jets, valve clearences, cam chain tension, vacuum sync.

Bike easily starts without choke (too rich?), but idles very nicely at 1000rpm. Theres some slight spluttering at part throttle which im sure is down to fine tuning of the jets and needle positions.

However, after a 30min ride last night, most of which it was running great, i opened the bike up in 3rd gear when joining the motorway, revs up to approx 8k and the bike spluttered. I backed off the throttle and the bike misfired 8 miles home on what seemed like 2 cylinders. Id been at WOT a few times before hand with no issues and it pulled great.

Sure enough when i got home I took out the plugs to find plugs in 2 and 3 a lot more coked up than 1 and 4, although not wet, just black in comparison to the biscuit colour of 1 and 4.

Im still learning all this, but am I right in thinking this is an ignition issue? Where should I start looking?

Dont have much time to work on the bike at the moment and really need it for next weekend, im tempted to buy new coilpack and points but itd be nice to diagnose the issue instead of just throwing money at it?

hairygit:
Ignition fault would be my first thought, so check the condition and security of the points, the condensors and all the wiring leading from them. Some Honda fours, 550's I think were prone to the yellow and blue wires working loose where the snap connectors join in the rubber shield near the back brake pedal.

Bryanj:
If its a 78K with PD carb and starts without choke then the carbs just aint set right

Johnwebley:
check the resistor plug caps,

 they normally have 5k resistance,but over time,when hot ,they fail,

I had one go 10 days ago,new cap,smooth as silk again ( I hope,well it run sweetly now)

does anyone understand electricity and resistance?

 with electronic ignition,I get a FAT BIG spark,so I run 33thou plug gap,how does 5K resistor plug caps affect them,would the bigger gap
cause the cap to fail sooner ??

mike the bike:
I does sound like an ignition problem.   There's two separate, independent ignition systems, one firing 1 and 4 and the other firing 2 and 3.
Try to determine which by feeling the temperatures of the header pipes.
What ignition set up have you got;  is it the original points or have you gone electronic?

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