Topic: #1-4 ignition timing inconsistent

[huyzer]

Hey All,

I wanted to see if anyone has seen this and what they did to remedy the situation. 

So what I have:
1978 K8 CB750

What I've done/checked.

Valves: tappet gap checked and all are good
Points gaps: checked and set to specs
New plug caps
Cam Tensioner adjusted...done yesterday. 
Carbs rebuilt and balanced.
Running Pods (yes I know the contraversy  )
slow jet #40
Main 122.5.
Bike runs well and will idle at around 1200 RPM and timing checks out.  More to come on this below.

now...for what I find to be a problem:
So below 1200 rpm...specifically at 1000RPM, the bike seems to be a little rough.  It will still idle but when I check how the bike is firing/timing is where I see an issue. 

Firing of 1-4 jumps from being spot on to being retarded (not in the bad mental impairment type..you guys know what I mean) to sometime slightly advanced.  Basically all over the place....with half the time being good. 
Checked out #2-3 timing and its spot on every time.  Smooth.

Advance spark at high RPM is good too.  for both the 1/4 and 2/3.   

now, aside from tearing the motor apart, which isn't feasible for me at this time...anything else I can do? check?

possible causes???

Anyone have experience with the Pamco electronic ignition? is it worth it...I would just get the basic model and reuse the ignition coils. 

Thanks guys. 

[pamcopete]

There is a phenomena with dual output "wasted spark" coils and that is that the dual output coil produces a positive voltage on one plug wire and a negative on the other. This is true for either points or an electronic ignition.

Most timing lights work best on the negative wire, but there is no easy way to determine which that is,  so if you are using #4 spark plug wire and it just happens to be the positive wire, then you may experience erratic behavior from the timing light, including the symptoms you are experiencing. In the worst case, the timing light won't work at all on the positive wire. 

So, just try switching the timing light to the other spark plug wire that is fed from the 1-4 coil. 

The fact that 2-3 does not have this problem should eliminate the advancer as it's the same advancer for all cylinders, but worth checking out anyway.

[Deltarider]

Most timing lights work best on the negative wire, but there is no easy way to determine which that is,  so if you are using #4 spark plug wire and it just happens to be the positive wire, then you may experience erratic behavior from the timing light, including the symptoms you are experiencing. In the worst case, the timing light won't work at all on the positive wire. 

So, just try switching the timing light to the other spark plug wire that is fed from the 1-4 coil.

Also make sure the HTlead is clean and free of grease where the pick-up sits. Some timinglights work best on an auxilary battery.

[Davez134]

One more easy thing to check, measure the runout of the advancer shaft. If it is not true, the advancer unit will move around slightly causing inconsistent timing.

[huyzer]

Thanks guys. Those are all really good points.

Currently the bike is running the original, or at least I think try are, points.

I'll probably start with checking the advancer unit run off. Maybe the weights are not returning to the spot on slow idle RPMS and slightly throwing off the timing.  Maybe a simple clean and lube of the unit will do some wonders.

Makes a little sense because on high RPMS the advance spark timing is spot on everytime.

[Davez134]

Sorry for the extra posts, just want to clarify...the runout I was talking about was for the advance "shaft" not advancer unit. If not trued, it will wobble a very small amount, but it can be enough to throw timing off. Measured in thousandths of an inch with dial gauge.

[enwri]

Can also swap the condenser leads on the points and see if it follows to 2-3, at least to rule them out.
Run it in the dark with the points cover off to see if it's arcing at the 1-4  points. Clean them with alcohol and lint free cloth or something

If yours are new and hopefully have perfectly parallel faces, it only takes the tiniest particle sticking up from the surface to stop them closing reliably and/or giving a place for an arc to jump from occasionally, delaying the current cutoff and the timing.
If you're really unlucky it can happen the first time they open, (maybe the condenser is dead or not connected)  you hear and see a little splat and spark from the points. welding a tiny point (usually a very hard material) on its opposite face. 
 
If points have ever been cleaned with abrasives like sandpaper, grit can become embedded and cause problems with arcing. Clean them with alcohol and lint free cloth or something afterwards.
Good diamond points files don't sacrifice their grit as readily as sandpaper.