Topic: 750K3 runs really rough/misfires after warmed up

[CBRat]

My 73 CB750 has been running great all summer, but just started misfiring bad, cutting in and out on a couple cylinders after riding for a dozen miles or so.  I parked it and tried again in a couple of days and it ran great for a bit then the same thing.  I adjusted the point gaps and threw in some new plugs, but nothing.  What would cause this misfiring when warmed up?  Coil failing?  Any tips would be great. 

[rlarkin70]

I once had a condenser go bad and it presented similar to that. It would work when the bike was cold, but produced a very weak spark when the bike got warmed up to running temperature. This was on a 74 CB450, but I have heard of the same thing happening on other bikes.

Have you checked for spark at each cylinder when it's warm and starts misfiring?

[750essess]

Also, valve adjustments to tight, coil, spark plug caps, wires. These kind of problems can be a bear sometimes. Start with the condensers and go from there.

[TwoTired]

How old/dirty is your air filter?

[CBRat]

Yeah, i have a new set of points/condensers laying around.  I'll replace 'em and see if that solves it.  Thanks for the help. 

on preview:  yes the air filter is dirty and old I've been putting off replacing it as it is a bit of a pain to get the airbox on and off.  I was hoping to make it another season with the old one but the time could be nigh  I'll give that a go as well, but would that not affect it when cold too?  Thanks again

[BobbyR]

TT may have hit on something. You may also want to see if your choke has any effect better or worse. Perhaps the butterflies are not opening all the way. This is one of those point by point elimination exercises. Start with the easy stuff and work toward the more difficult.

[Patrick]

The air filter is relatively easy to change if you just take off the bottom of the airbox and leave the top on the bike. Unscrew the bolts with the airbox on the bike, drop the bottom of the airbox and pull it out of the left side.

Patrick

[TwoTired]

As the air filter traps dirt, it becomes more restrictive.  Like having partial choke applied.  As cold engines don't atomized fuel well, they tolerate a rich mixture much better than a hot one.

It is gratifying that these old machines respond so well to a simple tune up check list.


Cheers,